treewide: parallel tests

Most tests already had no global state, however parallel was never enabled. This change enables it for all applicable tests. Signed-off-by: Ophestra <cat@gensokyo.uk>
helper/stub: write ready byte late
2025-10-13 04:38:48 +09:00 · 2025-10-13 01:55:44 +09:00 · 2025-10-13 01:12:44 +09:00 · 2025-10-13 01:06:45 +09:00 · 2025-10-12 05:03:14 +09:00 · 2025-10-12 03:58:53 +09:00
247 changed files with 16186 additions and 10092 deletions
--- a/cmd/hakurei/command.go
+++ b/cmd/hakurei/command.go
@@ -2,77 +2,91 @@ package main

 import (
 	"context"
+	"encoding/json"
 	"fmt"
 	"io"
 	"log"
 	"os"
-	"os/signal"
+	"os/exec"
 	"os/user"
 	"strconv"
 	"sync"
-	"syscall"
 	"time"
+	_ "unsafe"

 	"hakurei.app/command"
-	"hakurei.app/container"
+	"hakurei.app/container/check"
+	"hakurei.app/container/fhs"
 	"hakurei.app/hst"
 	"hakurei.app/internal"
 	"hakurei.app/internal/app"
 	"hakurei.app/internal/app/state"
-	"hakurei.app/internal/hlog"
-	"hakurei.app/system"
+	"hakurei.app/message"
 	"hakurei.app/system/dbus"
 )

-func buildCommand(out io.Writer) command.Command {
+//go:linkname optionalErrorUnwrap hakurei.app/container.optionalErrorUnwrap
+func optionalErrorUnwrap(_ error) error
+
+func buildCommand(ctx context.Context, msg message.Msg, early *earlyHardeningErrs, out io.Writer) command.Command {
 	var (
 		flagVerbose bool
 		flagJSON    bool
 	)
-	c := command.New(out, log.Printf, "hakurei", func([]string) error { internal.InstallOutput(flagVerbose); return nil }).
+	c := command.New(out, log.Printf, "hakurei", func([]string) error {
+		msg.SwapVerbose(flagVerbose)
+
+		if early.yamaLSM != nil {
+			msg.Verbosef("cannot enable ptrace protection via Yama LSM: %v", early.yamaLSM)
+			// not fatal
+		}
+
+		if early.dumpable != nil {
+			log.Printf("cannot set SUID_DUMP_DISABLE: %s", early.dumpable)
+			// not fatal
+		}
+
+		return nil
+	}).
 		Flag(&flagVerbose, "v", command.BoolFlag(false), "Increase log verbosity").
 		Flag(&flagJSON, "json", command.BoolFlag(false), "Serialise output in JSON when applicable")

 	c.Command("shim", command.UsageInternal, func([]string) error { app.ShimMain(); return errSuccess })

-	c.Command("app", "Load app from configuration file", func(args []string) error {
+	c.Command("app", "Load and start container from configuration file", func(args []string) error {
 		if len(args) < 1 {
 			log.Fatal("app requires at least 1 argument")
 		}

 		// config extraArgs...
-		config := tryPath(args[0])
-		config.Args = append(config.Args, args[1:]...)
+		config := tryPath(msg, args[0])
+		if config != nil && config.Container != nil {
+			config.Container.Args = append(config.Container.Args, args[1:]...)
+		}

-		runApp(config)
+		app.Main(ctx, msg, config)
 		panic("unreachable")
 	})

 	{
 		var (
-			dbusConfigSession string
-			dbusConfigSystem  string
-			mpris             bool
-			dbusVerbose       bool
+			flagDBusConfigSession string
+			flagDBusConfigSystem  string
+			flagDBusMpris         bool
+			flagDBusVerbose       bool

-			fid      string
-			aid      int
-			groups   command.RepeatableFlag
-			homeDir  string
-			userName string
+			flagID       string
+			flagIdentity int
+			flagGroups   command.RepeatableFlag
+			flagHomeDir  string
+			flagUserName string

-			wayland, x11, dBus, pulse bool
+			flagWayland, flagX11, flagDBus, flagPulse bool
 		)

-		c.NewCommand("run", "Configure and start a permissive default sandbox", func(args []string) error {
-			// initialise config from flags
-			config := &hst.Config{
-				ID:   fid,
-				Args: args,
-			}
-
-			if aid < 0 || aid > 9999 {
-				log.Fatalf("aid %d out of range", aid)
+		c.NewCommand("run", "Configure and start a permissive container", func(args []string) error {
+			if flagIdentity < hst.IdentityMin || flagIdentity > hst.IdentityMax {
+				log.Fatalf("identity %d out of range", flagIdentity)
 			}

 			// resolve home/username from os when flag is unset
@@ -80,22 +94,15 @@ func buildCommand(out io.Writer) command.Command {
 				passwd     *user.User
 				passwdOnce sync.Once
 				passwdFunc = func() {
-					var us string
-					if uid, err := std.Uid(aid); err != nil {
-						hlog.PrintBaseError(err, "cannot obtain uid from setuid wrapper:")
-						os.Exit(1)
-					} else {
-						us = strconv.Itoa(uid)
-					}
-
+					us := strconv.Itoa(app.HsuUid(new(app.Hsu).MustIDMsg(msg), flagIdentity))
 					if u, err := user.LookupId(us); err != nil {
-						hlog.Verbosef("cannot look up uid %s", us)
+						msg.Verbosef("cannot look up uid %s", us)
 						passwd = &user.User{
 							Uid:      us,
 							Gid:      us,
 							Username: "chronos",
 							Name:     "Hakurei Permissive Default",
-							HomeDir:  container.FHSVarEmpty,
+							HomeDir:  fhs.VarEmpty,
 						}
 					} else {
 						passwd = u
@@ -103,164 +110,211 @@ func buildCommand(out io.Writer) command.Command {
 				}
 			)

-			if homeDir == "os" {
+			// paths are identical, resolve inner shell and program path
+			shell := fhs.AbsRoot.Append("bin", "sh")
+			if a, err := check.NewAbs(os.Getenv("SHELL")); err == nil {
+				shell = a
+			}
+			progPath := shell
+			if len(args) > 0 {
+				if p, err := exec.LookPath(args[0]); err != nil {
+					log.Fatal(optionalErrorUnwrap(err))
+					return err
+				} else if progPath, err = check.NewAbs(p); err != nil {
+					log.Fatal(err.Error())
+					return err
+				}
+			}
+
+			var et hst.Enablement
+			if flagWayland {
+				et |= hst.EWayland
+			}
+			if flagX11 {
+				et |= hst.EX11
+			}
+			if flagDBus {
+				et |= hst.EDBus
+			}
+			if flagPulse {
+				et |= hst.EPulse
+			}
+
+			config := &hst.Config{
+				ID:          flagID,
+				Identity:    flagIdentity,
+				Groups:      flagGroups,
+				Enablements: hst.NewEnablements(et),
+
+				Container: &hst.ContainerConfig{
+					Userns:       true,
+					HostNet:      true,
+					Tty:          true,
+					HostAbstract: true,
+
+					Filesystem: []hst.FilesystemConfigJSON{
+						// autoroot, includes the home directory
+						{FilesystemConfig: &hst.FSBind{
+							Target:  fhs.AbsRoot,
+							Source:  fhs.AbsRoot,
+							Write:   true,
+							Special: true,
+						}},
+					},
+
+					Username: flagUserName,
+					Shell:    shell,
+
+					Path: progPath,
+					Args: args,
+				},
+			}
+
+			// bind GPU stuff
+			if et&(hst.EX11|hst.EWayland) != 0 {
+				config.Container.Filesystem = append(config.Container.Filesystem, hst.FilesystemConfigJSON{FilesystemConfig: &hst.FSBind{
+					Source:   fhs.AbsDev.Append("dri"),
+					Device:   true,
+					Optional: true,
+				}})
+			}
+
+			config.Container.Filesystem = append(config.Container.Filesystem,
+				// opportunistically bind kvm
+				hst.FilesystemConfigJSON{FilesystemConfig: &hst.FSBind{
+					Source:   fhs.AbsDev.Append("kvm"),
+					Device:   true,
+					Optional: true,
+				}},
+
+				// do autoetc last
+				hst.FilesystemConfigJSON{FilesystemConfig: &hst.FSBind{
+					Target:  fhs.AbsEtc,
+					Source:  fhs.AbsEtc,
+					Special: true,
+				}},
+			)
+
+			if config.Container.Username == "chronos" {
 				passwdOnce.Do(passwdFunc)
-				homeDir = passwd.HomeDir
+				config.Container.Username = passwd.Username
 			}

-			if userName == "chronos" {
-				passwdOnce.Do(passwdFunc)
-				userName = passwd.Username
+			{
+				homeDir := flagHomeDir
+				if homeDir == "os" {
+					passwdOnce.Do(passwdFunc)
+					homeDir = passwd.HomeDir
+				}
+				if a, err := check.NewAbs(homeDir); err != nil {
+					log.Fatal(err.Error())
+					return err
+				} else {
+					config.Container.Home = a
+				}
 			}

-			config.Identity = aid
-			config.Groups = groups
-			config.Username = userName
-
-			if a, err := container.NewAbs(homeDir); err != nil {
-				log.Fatal(err.Error())
-				return err
-			} else {
-				config.Home = a
-			}
-
-			var e system.Enablement
-			if wayland {
-				e |= system.EWayland
-			}
-			if x11 {
-				e |= system.EX11
-			}
-			if dBus {
-				e |= system.EDBus
-			}
-			if pulse {
-				e |= system.EPulse
-			}
-			config.Enablements = hst.NewEnablements(e)
-
 			// parse D-Bus config file from flags if applicable
-			if dBus {
-				if dbusConfigSession == "builtin" {
-					config.SessionBus = dbus.NewConfig(fid, true, mpris)
+			if flagDBus {
+				if flagDBusConfigSession == "builtin" {
+					config.SessionBus = dbus.NewConfig(flagID, true, flagDBusMpris)
 				} else {
-					if conf, err := dbus.NewConfigFromFile(dbusConfigSession); err != nil {
-						log.Fatalf("cannot load session bus proxy config from %q: %s", dbusConfigSession, err)
-					} else {
-						config.SessionBus = conf
+					if f, err := os.Open(flagDBusConfigSession); err != nil {
+						log.Fatal(err.Error())
+					} else if err = json.NewDecoder(f).Decode(&config.SessionBus); err != nil {
+						log.Fatalf("cannot load session bus proxy config from %q: %s", flagDBusConfigSession, err)
 					}
 				}

 				// system bus proxy is optional
-				if dbusConfigSystem != "nil" {
-					if conf, err := dbus.NewConfigFromFile(dbusConfigSystem); err != nil {
-						log.Fatalf("cannot load system bus proxy config from %q: %s", dbusConfigSystem, err)
-					} else {
-						config.SystemBus = conf
+				if flagDBusConfigSystem != "nil" {
+					if f, err := os.Open(flagDBusConfigSystem); err != nil {
+						log.Fatal(err.Error())
+					} else if err = json.NewDecoder(f).Decode(&config.SystemBus); err != nil {
+						log.Fatalf("cannot load system bus proxy config from %q: %s", flagDBusConfigSystem, err)
 					}
 				}

 				// override log from configuration
-				if dbusVerbose {
-					config.SessionBus.Log = true
-					config.SystemBus.Log = true
+				if flagDBusVerbose {
+					if config.SessionBus != nil {
+						config.SessionBus.Log = true
+					}
+					if config.SystemBus != nil {
+						config.SystemBus.Log = true
+					}
 				}
 			}

-			// invoke app
-			runApp(config)
+			app.Main(ctx, msg, config)
 			panic("unreachable")
 		}).
-			Flag(&dbusConfigSession, "dbus-config", command.StringFlag("builtin"),
+			Flag(&flagDBusConfigSession, "dbus-config", command.StringFlag("builtin"),
 				"Path to session bus proxy config file, or \"builtin\" for defaults").
-			Flag(&dbusConfigSystem, "dbus-system", command.StringFlag("nil"),
+			Flag(&flagDBusConfigSystem, "dbus-system", command.StringFlag("nil"),
 				"Path to system bus proxy config file, or \"nil\" to disable").
-			Flag(&mpris, "mpris", command.BoolFlag(false),
+			Flag(&flagDBusMpris, "mpris", command.BoolFlag(false),
 				"Allow owning MPRIS D-Bus path, has no effect if custom config is available").
-			Flag(&dbusVerbose, "dbus-log", command.BoolFlag(false),
+			Flag(&flagDBusVerbose, "dbus-log", command.BoolFlag(false),
 				"Force buffered logging in the D-Bus proxy").
-			Flag(&fid, "id", command.StringFlag(""),
+			Flag(&flagID, "id", command.StringFlag(""),
 				"Reverse-DNS style Application identifier, leave empty to inherit instance identifier").
-			Flag(&aid, "a", command.IntFlag(0),
+			Flag(&flagIdentity, "a", command.IntFlag(0),
 				"Application identity").
-			Flag(nil, "g", &groups,
+			Flag(nil, "g", &flagGroups,
 				"Groups inherited by all container processes").
-			Flag(&homeDir, "d", command.StringFlag("os"),
+			Flag(&flagHomeDir, "d", command.StringFlag("os"),
 				"Container home directory").
-			Flag(&userName, "u", command.StringFlag("chronos"),
+			Flag(&flagUserName, "u", command.StringFlag("chronos"),
 				"Passwd user name within sandbox").
-			Flag(&wayland, "wayland", command.BoolFlag(false),
+			Flag(&flagWayland, "wayland", command.BoolFlag(false),
 				"Enable connection to Wayland via security-context-v1").
-			Flag(&x11, "X", command.BoolFlag(false),
+			Flag(&flagX11, "X", command.BoolFlag(false),
 				"Enable direct connection to X11").
-			Flag(&dBus, "dbus", command.BoolFlag(false),
+			Flag(&flagDBus, "dbus", command.BoolFlag(false),
 				"Enable proxied connection to D-Bus").
-			Flag(&pulse, "pulse", command.BoolFlag(false),
+			Flag(&flagPulse, "pulse", command.BoolFlag(false),
 				"Enable direct connection to PulseAudio")
 	}

-	var showFlagShort bool
-	c.NewCommand("show", "Show live or local app configuration", func(args []string) error {
-		switch len(args) {
-		case 0: // system
-			printShowSystem(os.Stdout, showFlagShort, flagJSON)
+	{
+		var flagShort bool
+		c.NewCommand("show", "Show live or local app configuration", func(args []string) error {
+			switch len(args) {
+			case 0: // system
+				printShowSystem(os.Stdout, flagShort, flagJSON)

-		case 1: // instance
-			name := args[0]
-			config, entry := tryShort(name)
-			if config == nil {
-				config = tryPath(name)
+			case 1: // instance
+				name := args[0]
+				config, entry := tryShort(msg, name)
+				if config == nil {
+					config = tryPath(msg, name)
+				}
+				if !printShowInstance(os.Stdout, time.Now().UTC(), entry, config, flagShort, flagJSON) {
+					os.Exit(1)
+				}
+
+			default:
+				log.Fatal("show requires 1 argument")
 			}
-			printShowInstance(os.Stdout, time.Now().UTC(), entry, config, showFlagShort, flagJSON)
+			return errSuccess
+		}).Flag(&flagShort, "short", command.BoolFlag(false), "Omit filesystem information")
+	}

-		default:
-			log.Fatal("show requires 1 argument")
-		}
-		return errSuccess
-	}).Flag(&showFlagShort, "short", command.BoolFlag(false), "Omit filesystem information")
+	{
+		var flagShort bool
+		c.NewCommand("ps", "List active instances", func(args []string) error {
+			var sc hst.Paths
+			app.CopyPaths().Copy(&sc, new(app.Hsu).MustID())
+			printPs(os.Stdout, time.Now().UTC(), state.NewMulti(msg, sc.RunDirPath.String()), flagShort, flagJSON)
+			return errSuccess
+		}).Flag(&flagShort, "short", command.BoolFlag(false), "Print instance id")
+	}

-	var psFlagShort bool
-	c.NewCommand("ps", "List active instances", func(args []string) error {
-		printPs(os.Stdout, time.Now().UTC(), state.NewMulti(std.Paths().RunDirPath.String()), psFlagShort, flagJSON)
-		return errSuccess
-	}).Flag(&psFlagShort, "short", command.BoolFlag(false), "Print instance id")
-
-	c.Command("version", "Display version information", func(args []string) error {
-		fmt.Println(internal.Version())
-		return errSuccess
-	})
-
-	c.Command("license", "Show full license text", func(args []string) error {
-		fmt.Println(license)
-		return errSuccess
-	})
-
-	c.Command("template", "Produce a config template", func(args []string) error {
-		printJSON(os.Stdout, false, hst.Template())
-		return errSuccess
-	})
-
-	c.Command("help", "Show this help message", func([]string) error {
-		c.PrintHelp()
-		return errSuccess
-	})
+	c.Command("version", "Display version information", func(args []string) error { fmt.Println(internal.Version()); return errSuccess })
+	c.Command("license", "Show full license text", func(args []string) error { fmt.Println(license); return errSuccess })
+	c.Command("template", "Produce a config template", func(args []string) error { printJSON(os.Stdout, false, hst.Template()); return errSuccess })
+	c.Command("help", "Show this help message", func([]string) error { c.PrintHelp(); return errSuccess })

 	return c
 }
-
-func runApp(config *hst.Config) {
-	ctx, stop := signal.NotifyContext(context.Background(),
-		syscall.SIGINT, syscall.SIGTERM)
-	defer stop() // unreachable
-	a := app.MustNew(ctx, std)
-
-	rs := new(app.RunState)
-	if sa, err := a.Seal(config); err != nil {
-		hlog.PrintBaseError(err, "cannot seal app:")
-		internal.Exit(1)
-	} else {
-		internal.Exit(app.PrintRunStateErr(rs, sa.Run(rs)))
-	}
-
-	*(*int)(nil) = 0 // not reached
-}
--- a/cmd/hakurei/command_test.go
+++ b/cmd/hakurei/command_test.go
@@ -7,9 +7,12 @@ import (
 	"testing"

 	"hakurei.app/command"
+	"hakurei.app/message"
 )

 func TestHelp(t *testing.T) {
+	t.Parallel()
+
 	testCases := []struct {
 		name string
 		args []string
@@ -20,8 +23,8 @@ func TestHelp(t *testing.T) {
 Usage:	hakurei [-h | --help] [-v] [--json] COMMAND [OPTIONS]

 Commands:
-    app         Load app from configuration file
-    run         Configure and start a permissive default sandbox
+    app         Load and start container from configuration file
+    run         Configure and start a permissive container
    show        Show live or local app configuration
    ps          List active instances
    version     Display version information
@@ -67,8 +70,10 @@ Flags:
 	}
 	for _, tc := range testCases {
 		t.Run(tc.name, func(t *testing.T) {
+			t.Parallel()
+
 			out := new(bytes.Buffer)
-			c := buildCommand(out)
+			c := buildCommand(t.Context(), message.NewMsg(nil), new(earlyHardeningErrs), out)
 			if err := c.Parse(tc.args); !errors.Is(err, command.ErrHelp) && !errors.Is(err, flag.ErrHelp) {
 				t.Errorf("Parse: error = %v; want %v",
 					err, command.ErrHelp)
--- a/cmd/hakurei/main.go
+++ b/cmd/hakurei/main.go
@@ -4,15 +4,16 @@ package main
 //go:generate cp ../../LICENSE .

 import (
+	"context"
 	_ "embed"
 	"errors"
 	"log"
 	"os"
+	"os/signal"
+	"syscall"

 	"hakurei.app/container"
-	"hakurei.app/internal"
-	"hakurei.app/internal/hlog"
-	"hakurei.app/internal/sys"
+	"hakurei.app/message"
 )

 var (
@@ -22,32 +23,34 @@ var (
 	license string
 )

-func init() { hlog.Prepare("hakurei") }
-
-var std sys.State = new(sys.Std)
+// earlyHardeningErrs are errors collected while setting up early hardening feature.
+type earlyHardeningErrs struct{ yamaLSM, dumpable error }

 func main() {
 	// early init path, skips root check and duplicate PR_SET_DUMPABLE
-	container.TryArgv0(hlog.Output{}, hlog.Prepare, internal.InstallOutput)
+	container.TryArgv0(nil)

-	if err := container.SetPtracer(0); err != nil {
-		hlog.Verbosef("cannot enable ptrace protection via Yama LSM: %v", err)
-		// not fatal: this program runs as the privileged user
-	}
+	log.SetPrefix("hakurei: ")
+	log.SetFlags(0)
+	msg := message.NewMsg(log.Default())

-	if err := container.SetDumpable(container.SUID_DUMP_DISABLE); err != nil {
-		log.Printf("cannot set SUID_DUMP_DISABLE: %s", err)
-		// not fatal: this program runs as the privileged user
+	early := earlyHardeningErrs{
+		yamaLSM:  container.SetPtracer(0),
+		dumpable: container.SetDumpable(container.SUID_DUMP_DISABLE),
 	}

 	if os.Geteuid() == 0 {
 		log.Fatal("this program must not run as root")
 	}

-	buildCommand(os.Stderr).MustParse(os.Args[1:], func(err error) {
-		hlog.Verbosef("command returned %v", err)
+	ctx, stop := signal.NotifyContext(context.Background(),
+		syscall.SIGINT, syscall.SIGTERM)
+	defer stop() // unreachable
+
+	buildCommand(ctx, msg, &early, os.Stderr).MustParse(os.Args[1:], func(err error) {
+		msg.Verbosef("command returned %v", err)
 		if errors.Is(err, errSuccess) {
-			hlog.BeforeExit()
+			msg.BeforeExit()
 			os.Exit(0)
 		}
 		// this catches faulty command handlers that fail to return before this point
--- a/cmd/hakurei/parse.go
+++ b/cmd/hakurei/parse.go
@@ -11,18 +11,19 @@ import (
 	"syscall"

 	"hakurei.app/hst"
+	"hakurei.app/internal/app"
 	"hakurei.app/internal/app/state"
-	"hakurei.app/internal/hlog"
+	"hakurei.app/message"
 )

-func tryPath(name string) (config *hst.Config) {
+func tryPath(msg message.Msg, name string) (config *hst.Config) {
 	var r io.Reader
 	config = new(hst.Config)

 	if name != "-" {
-		r = tryFd(name)
+		r = tryFd(msg, name)
 		if r == nil {
-			hlog.Verbose("load configuration from file")
+			msg.Verbose("load configuration from file")

 			if f, err := os.Open(name); err != nil {
 				log.Fatalf("cannot access configuration file %q: %s", name, err)
@@ -48,14 +49,14 @@ func tryPath(name string) (config *hst.Config) {
 	return
 }

-func tryFd(name string) io.ReadCloser {
+func tryFd(msg message.Msg, name string) io.ReadCloser {
 	if v, err := strconv.Atoi(name); err != nil {
 		if !errors.Is(err, strconv.ErrSyntax) {
-			hlog.Verbosef("name cannot be interpreted as int64: %v", err)
+			msg.Verbosef("name cannot be interpreted as int64: %v", err)
 		}
 		return nil
 	} else {
-		hlog.Verbosef("trying config stream from %d", v)
+		msg.Verbosef("trying config stream from %d", v)
 		fd := uintptr(v)
 		if _, _, errno := syscall.Syscall(syscall.SYS_FCNTL, fd, syscall.F_GETFD, 0); errno != 0 {
 			if errors.Is(errno, syscall.EBADF) {
@@ -67,7 +68,7 @@ func tryFd(name string) io.ReadCloser {
 	}
 }

-func tryShort(name string) (config *hst.Config, entry *state.State) {
+func tryShort(msg message.Msg, name string) (config *hst.Config, entry *state.State) {
 	likePrefix := false
 	if len(name) <= 32 {
 		likePrefix = true
@@ -85,9 +86,11 @@ func tryShort(name string) (config *hst.Config, entry *state.State) {

 	// try to match from state store
 	if likePrefix && len(name) >= 8 {
-		hlog.Verbose("argument looks like prefix")
+		msg.Verbose("argument looks like prefix")

-		s := state.NewMulti(std.Paths().RunDirPath.String())
+		var sc hst.Paths
+		app.CopyPaths().Copy(&sc, new(app.Hsu).MustID())
+		s := state.NewMulti(msg, sc.RunDirPath.String())
 		if entries, err := state.Join(s); err != nil {
 			log.Printf("cannot join store: %v", err)
 			// drop to fetch from file
@@ -101,7 +104,7 @@ func tryShort(name string) (config *hst.Config, entry *state.State) {
 					break
 				}

-				hlog.Verbosef("instance %s skipped", v)
+				msg.Verbosef("instance %s skipped", v)
 			}
 		}
 	}
--- a/cmd/hakurei/print.go
+++ b/cmd/hakurei/print.go
@@ -5,7 +5,6 @@ import (
 	"fmt"
 	"io"
 	"log"
-	"os"
 	"slices"
 	"strconv"
 	"strings"
@@ -13,24 +12,17 @@ import (
 	"time"

 	"hakurei.app/hst"
+	"hakurei.app/internal/app"
 	"hakurei.app/internal/app/state"
-	"hakurei.app/internal/hlog"
-	"hakurei.app/system/dbus"
+	"hakurei.app/message"
 )

 func printShowSystem(output io.Writer, short, flagJSON bool) {
 	t := newPrinter(output)
 	defer t.MustFlush()

-	info := &hst.Info{Paths: std.Paths()}
-
-	// get hid by querying uid of identity 0
-	if uid, err := std.Uid(0); err != nil {
-		hlog.PrintBaseError(err, "cannot obtain uid from setuid wrapper:")
-		os.Exit(1)
-	} else {
-		info.User = (uid / 10000) - 100
-	}
+	info := &hst.Info{User: new(app.Hsu).MustID()}
+	app.CopyPaths().Copy(&info.Paths, info.User)

 	if flagJSON {
 		printJSON(output, short, info)
@@ -47,7 +39,9 @@ func printShowSystem(output io.Writer, short, flagJSON bool) {
 func printShowInstance(
 	output io.Writer, now time.Time,
 	instance *state.State, config *hst.Config,
-	short, flagJSON bool) {
+	short, flagJSON bool) (valid bool) {
+	valid = true
+
 	if flagJSON {
 		if instance != nil {
 			printJSON(output, short, instance)
@@ -60,8 +54,11 @@ func printShowInstance(
 	t := newPrinter(output)
 	defer t.MustFlush()

-	if config.Container == nil {
-		mustPrint(output, "Warning: this configuration uses permissive defaults!\n\n")
+	if err := config.Validate(); err != nil {
+		valid = false
+		if m, ok := message.GetMessage(err); ok {
+			mustPrint(output, "Error: "+m+"!\n\n")
+		}
 	}

 	if instance != nil {
@@ -81,11 +78,11 @@ func printShowInstance(
 	if len(config.Groups) > 0 {
 		t.Printf(" Groups:\t%s\n", strings.Join(config.Groups, ", "))
 	}
-	if config.Home != nil {
-		t.Printf(" Home:\t%s\n", config.Home)
-	}
 	if config.Container != nil {
 		params := config.Container
+		if params.Home != nil {
+			t.Printf(" Home:\t%s\n", params.Home)
+		}
 		if params.Hostname != "" {
 			t.Printf(" Hostname:\t%s\n", params.Hostname)
 		}
@@ -108,12 +105,12 @@ func printShowInstance(
 		}
 		t.Printf(" Flags:\t%s\n", strings.Join(flags, " "))

-		if config.Path != nil {
-			t.Printf(" Path:\t%s\n", config.Path)
+		if params.Path != nil {
+			t.Printf(" Path:\t%s\n", params.Path)
+		}
+		if len(params.Args) > 0 {
+			t.Printf(" Arguments:\t%s\n", strings.Join(params.Args, " "))
 		}
-	}
-	if len(config.Args) > 0 {
-		t.Printf(" Arguments:\t%s\n", strings.Join(config.Args, " "))
 	}
 	t.Printf("\n")

@@ -122,6 +119,7 @@ func printShowInstance(
 			t.Printf("Filesystem\n")
 			for _, f := range config.Container.Filesystem {
 				if !f.Valid() {
+					valid = false
 					t.Println(" <invalid>")
 					continue
 				}
@@ -141,7 +139,7 @@ func printShowInstance(
 		}
 	}

-	printDBus := func(c *dbus.Config) {
+	printDBus := func(c *hst.BusConfig) {
 		t.Printf(" Filter:\t%v\n", c.Filter)
 		if len(c.See) > 0 {
 			t.Printf(" See:\t%q\n", c.See)
@@ -169,6 +167,8 @@ func printShowInstance(
 		printDBus(config.SystemBus)
 		t.Printf("\n")
 	}
+
+	return
 }

 func printPs(output io.Writer, now time.Time, s state.Store, short, flagJSON bool) {
--- a/cmd/hakurei/print_test.go
+++ b/cmd/hakurei/print_test.go
@@ -7,7 +7,6 @@ import (

 	"hakurei.app/hst"
 	"hakurei.app/internal/app/state"
-	"hakurei.app/system/dbus"
 )

 var (
@@ -27,13 +26,16 @@ var (
 	testAppTime = time.Unix(0, 9).UTC()
 )

-func Test_printShowInstance(t *testing.T) {
+func TestPrintShowInstance(t *testing.T) {
+	t.Parallel()
+
 	testCases := []struct {
 		name        string
 		instance    *state.State
 		config      *hst.Config
 		short, json bool
 		want        string
+		valid       bool
 	}{
 		{"config", nil, hst.Template(), false, false, `App
 Identity:       9 (org.chromium.Chromium)
@@ -49,8 +51,7 @@ Filesystem
 autoroot:w:/var/lib/hakurei/base/org.debian
 autoetc:/etc/
 w+ephemeral(-rwxr-xr-x):/tmp/
- w*/nix/store:/mnt-root/nix/.rw-store/upper:/mnt-root/nix/.rw-store/work:/mnt-root/nix/.ro-store
- */nix/store
+ w*/nix/store:/var/lib/hakurei/nix/u0/org.chromium.Chromium/rw-store/upper:/var/lib/hakurei/nix/u0/org.chromium.Chromium/rw-store/work:/var/lib/hakurei/base/org.nixos/ro-store
 /run/current-system@
 /run/opengl-driver@
 w-/var/lib/hakurei/u0/org.chromium.Chromium:/data/data/org.chromium.Chromium
@@ -71,21 +72,25 @@ System bus
 Filter:    true
 Talk:      ["org.bluez" "org.freedesktop.Avahi" "org.freedesktop.UPower"]

-`},
-		{"config pd", nil, new(hst.Config), false, false, `Warning: this configuration uses permissive defaults!
+`, true},
+		{"config pd", nil, new(hst.Config), false, false, `Error: configuration missing container state!

 App
 Identity:       0
 Enablements:    (no enablements)

-`},
-		{"config flag none", nil, &hst.Config{Container: new(hst.ContainerConfig)}, false, false, `App
+`, false},
+		{"config flag none", nil, &hst.Config{Container: new(hst.ContainerConfig)}, false, false, `Error: container configuration missing path to home directory!
+
+App
 Identity:       0
 Enablements:    (no enablements)
 Flags:          none

-`},
-		{"config nil entries", nil, &hst.Config{Container: &hst.ContainerConfig{Filesystem: make([]hst.FilesystemConfigJSON, 1)}, ExtraPerms: make([]*hst.ExtraPermConfig, 1)}, false, false, `App
+`, false},
+		{"config nil entries", nil, &hst.Config{Container: &hst.ContainerConfig{Filesystem: make([]hst.FilesystemConfigJSON, 1)}, ExtraPerms: make([]*hst.ExtraPermConfig, 1)}, false, false, `Error: container configuration missing path to home directory!
+
+App
 Identity:       0
 Enablements:    (no enablements)
 Flags:          none
@@ -95,8 +100,8 @@ Filesystem

 Extra ACL

-`},
-		{"config pd dbus see", nil, &hst.Config{SessionBus: &dbus.Config{See: []string{"org.example.test"}}}, false, false, `Warning: this configuration uses permissive defaults!
+`, false},
+		{"config pd dbus see", nil, &hst.Config{SessionBus: &hst.BusConfig{See: []string{"org.example.test"}}}, false, false, `Error: configuration missing container state!

 App
 Identity:       0
@@ -106,7 +111,7 @@ Session bus
 Filter:    false
 See:       ["org.example.test"]

-`},
+`, false},

 		{"instance", testState, hst.Template(), false, false, `State
 Instance:    8e2c76b066dabe574cf073bdb46eb5c1 (3735928559)
@@ -126,8 +131,7 @@ Filesystem
 autoroot:w:/var/lib/hakurei/base/org.debian
 autoetc:/etc/
 w+ephemeral(-rwxr-xr-x):/tmp/
- w*/nix/store:/mnt-root/nix/.rw-store/upper:/mnt-root/nix/.rw-store/work:/mnt-root/nix/.ro-store
- */nix/store
+ w*/nix/store:/var/lib/hakurei/nix/u0/org.chromium.Chromium/rw-store/upper:/var/lib/hakurei/nix/u0/org.chromium.Chromium/rw-store/work:/var/lib/hakurei/base/org.nixos/ro-store
 /run/current-system@
 /run/opengl-driver@
 w-/var/lib/hakurei/u0/org.chromium.Chromium:/data/data/org.chromium.Chromium
@@ -148,8 +152,8 @@ System bus
 Filter:    true
 Talk:      ["org.bluez" "org.freedesktop.Avahi" "org.freedesktop.UPower"]

-`},
-		{"instance pd", testState, new(hst.Config), false, false, `Warning: this configuration uses permissive defaults!
+`, true},
+		{"instance pd", testState, new(hst.Config), false, false, `Error: configuration missing container state!

 State
 Instance:    8e2c76b066dabe574cf073bdb46eb5c1 (3735928559)
@@ -159,10 +163,10 @@ App
 Identity:       0
 Enablements:    (no enablements)

-`},
+`, false},

 		{"json nil", nil, nil, false, true, `null
-`},
+`, true},
 		{"json instance", testState, nil, false, true, `{
  "instance": [
    142,
@@ -185,14 +189,6 @@ App
  "pid": 3735928559,
  "config": {
    "id": "org.chromium.Chromium",
-    "path": "/run/current-system/sw/bin/chromium",
-    "args": [
-      "chromium",
-      "--ignore-gpu-blocklist",
-      "--disable-smooth-scrolling",
-      "--enable-features=UseOzonePlatform",
-      "--ozone-platform=wayland"
-    ],
    "enablements": {
      "wayland": true,
      "dbus": true,
@@ -234,9 +230,6 @@ App
      "broadcast": null,
      "filter": true
    },
-    "username": "chronos",
-    "shell": "/run/current-system/sw/bin/zsh",
-    "home": "/data/data/org.chromium.Chromium",
    "extra_perms": [
      {
        "ensure": true,
@@ -259,8 +252,6 @@ App
    "container": {
      "hostname": "localhost",
      "wait_delay": -1,
-      "seccomp_flags": 1,
-      "seccomp_presets": 1,
      "seccomp_compat": true,
      "devel": true,
      "userns": true,
@@ -299,14 +290,10 @@ App
          "type": "overlay",
          "dst": "/nix/store",
          "lower": [
-            "/mnt-root/nix/.ro-store"
+            "/var/lib/hakurei/base/org.nixos/ro-store"
          ],
-          "upper": "/mnt-root/nix/.rw-store/upper",
-          "work": "/mnt-root/nix/.rw-store/work"
-        },
-        {
-          "type": "bind",
-          "src": "/nix/store"
+          "upper": "/var/lib/hakurei/nix/u0/org.chromium.Chromium/rw-store/upper",
+          "work": "/var/lib/hakurei/nix/u0/org.chromium.Chromium/rw-store/work"
        },
        {
          "type": "link",
@@ -333,22 +320,25 @@ App
          "dev": true,
          "optional": true
        }
+      ],
+      "username": "chronos",
+      "shell": "/run/current-system/sw/bin/zsh",
+      "home": "/data/data/org.chromium.Chromium",
+      "path": "/run/current-system/sw/bin/chromium",
+      "args": [
+        "chromium",
+        "--ignore-gpu-blocklist",
+        "--disable-smooth-scrolling",
+        "--enable-features=UseOzonePlatform",
+        "--ozone-platform=wayland"
      ]
    }
  },
  "time": "1970-01-01T00:00:00.000000009Z"
 }
-`},
+`, true},
 		{"json config", nil, hst.Template(), false, true, `{
  "id": "org.chromium.Chromium",
-  "path": "/run/current-system/sw/bin/chromium",
-  "args": [
-    "chromium",
-    "--ignore-gpu-blocklist",
-    "--disable-smooth-scrolling",
-    "--enable-features=UseOzonePlatform",
-    "--ozone-platform=wayland"
-  ],
  "enablements": {
    "wayland": true,
    "dbus": true,
@@ -390,9 +380,6 @@ App
    "broadcast": null,
    "filter": true
  },
-  "username": "chronos",
-  "shell": "/run/current-system/sw/bin/zsh",
-  "home": "/data/data/org.chromium.Chromium",
  "extra_perms": [
    {
      "ensure": true,
@@ -415,8 +402,6 @@ App
  "container": {
    "hostname": "localhost",
    "wait_delay": -1,
-    "seccomp_flags": 1,
-    "seccomp_presets": 1,
    "seccomp_compat": true,
    "devel": true,
    "userns": true,
@@ -455,14 +440,10 @@ App
        "type": "overlay",
        "dst": "/nix/store",
        "lower": [
-          "/mnt-root/nix/.ro-store"
+          "/var/lib/hakurei/base/org.nixos/ro-store"
        ],
-        "upper": "/mnt-root/nix/.rw-store/upper",
-        "work": "/mnt-root/nix/.rw-store/work"
-      },
-      {
-        "type": "bind",
-        "src": "/nix/store"
+        "upper": "/var/lib/hakurei/nix/u0/org.chromium.Chromium/rw-store/upper",
+        "work": "/var/lib/hakurei/nix/u0/org.chromium.Chromium/rw-store/work"
      },
      {
        "type": "link",
@@ -489,26 +470,43 @@ App
        "dev": true,
        "optional": true
      }
+    ],
+    "username": "chronos",
+    "shell": "/run/current-system/sw/bin/zsh",
+    "home": "/data/data/org.chromium.Chromium",
+    "path": "/run/current-system/sw/bin/chromium",
+    "args": [
+      "chromium",
+      "--ignore-gpu-blocklist",
+      "--disable-smooth-scrolling",
+      "--enable-features=UseOzonePlatform",
+      "--ozone-platform=wayland"
    ]
  }
 }
-`},
+`, true},
 	}

 	for _, tc := range testCases {
 		t.Run(tc.name, func(t *testing.T) {
+			t.Parallel()
+
 			output := new(strings.Builder)
-			printShowInstance(output, testTime, tc.instance, tc.config, tc.short, tc.json)
+			gotValid := printShowInstance(output, testTime, tc.instance, tc.config, tc.short, tc.json)
 			if got := output.String(); got != tc.want {
-				t.Errorf("printShowInstance: got\n%s\nwant\n%s",
-					got, tc.want)
+				t.Errorf("printShowInstance: \n%s\nwant\n%s", got, tc.want)
 				return
 			}
+			if gotValid != tc.valid {
+				t.Errorf("printShowInstance: valid = %v, want %v", gotValid, tc.valid)
+			}
 		})
 	}
 }

-func Test_printPs(t *testing.T) {
+func TestPrintPs(t *testing.T) {
+	t.Parallel()
+
 	testCases := []struct {
 		name        string
 		entries     state.Entries
@@ -551,14 +549,6 @@ func Test_printPs(t *testing.T) {
    "pid": 3735928559,
    "config": {
      "id": "org.chromium.Chromium",
-      "path": "/run/current-system/sw/bin/chromium",
-      "args": [
-        "chromium",
-        "--ignore-gpu-blocklist",
-        "--disable-smooth-scrolling",
-        "--enable-features=UseOzonePlatform",
-        "--ozone-platform=wayland"
-      ],
      "enablements": {
        "wayland": true,
        "dbus": true,
@@ -600,9 +590,6 @@ func Test_printPs(t *testing.T) {
        "broadcast": null,
        "filter": true
      },
-      "username": "chronos",
-      "shell": "/run/current-system/sw/bin/zsh",
-      "home": "/data/data/org.chromium.Chromium",
      "extra_perms": [
        {
          "ensure": true,
@@ -625,8 +612,6 @@ func Test_printPs(t *testing.T) {
      "container": {
        "hostname": "localhost",
        "wait_delay": -1,
-        "seccomp_flags": 1,
-        "seccomp_presets": 1,
        "seccomp_compat": true,
        "devel": true,
        "userns": true,
@@ -665,14 +650,10 @@ func Test_printPs(t *testing.T) {
            "type": "overlay",
            "dst": "/nix/store",
            "lower": [
-              "/mnt-root/nix/.ro-store"
+              "/var/lib/hakurei/base/org.nixos/ro-store"
            ],
-            "upper": "/mnt-root/nix/.rw-store/upper",
-            "work": "/mnt-root/nix/.rw-store/work"
-          },
-          {
-            "type": "bind",
-            "src": "/nix/store"
+            "upper": "/var/lib/hakurei/nix/u0/org.chromium.Chromium/rw-store/upper",
+            "work": "/var/lib/hakurei/nix/u0/org.chromium.Chromium/rw-store/work"
          },
          {
            "type": "link",
@@ -699,6 +680,17 @@ func Test_printPs(t *testing.T) {
            "dev": true,
            "optional": true
          }
+        ],
+        "username": "chronos",
+        "shell": "/run/current-system/sw/bin/zsh",
+        "home": "/data/data/org.chromium.Chromium",
+        "path": "/run/current-system/sw/bin/chromium",
+        "args": [
+          "chromium",
+          "--ignore-gpu-blocklist",
+          "--disable-smooth-scrolling",
+          "--enable-features=UseOzonePlatform",
+          "--ozone-platform=wayland"
        ]
      }
    },
@@ -712,6 +704,8 @@ func Test_printPs(t *testing.T) {

 	for _, tc := range testCases {
 		t.Run(tc.name, func(t *testing.T) {
+			t.Parallel()
+
 			output := new(strings.Builder)
 			printPs(output, testTime, stubStore(tc.entries), tc.short, tc.json)
 			if got := output.String(); got != tc.want {
--- a/cmd/hpkg/app.go
+++ b/cmd/hpkg/app.go
@@ -5,10 +5,9 @@ import (
 	"log"
 	"os"

-	"hakurei.app/container"
-	"hakurei.app/container/seccomp"
+	"hakurei.app/container/check"
+	"hakurei.app/container/fhs"
 	"hakurei.app/hst"
-	"hakurei.app/system/dbus"
 )

 type appInfo struct {
@@ -38,9 +37,9 @@ type appInfo struct {
 	// passed through to [hst.Config]
 	DirectWayland bool `json:"direct_wayland,omitempty"`
 	// passed through to [hst.Config]
-	SystemBus *dbus.Config `json:"system_bus,omitempty"`
+	SystemBus *hst.BusConfig `json:"system_bus,omitempty"`
 	// passed through to [hst.Config]
-	SessionBus *dbus.Config `json:"session_bus,omitempty"`
+	SessionBus *hst.BusConfig `json:"session_bus,omitempty"`
 	// passed through to [hst.Config]
 	Enablements *hst.Enablements `json:"enablements,omitempty"`

@@ -56,30 +55,23 @@ type appInfo struct {
 	// store path to nixGL source
 	NixGL string `json:"nix_gl,omitempty"`
 	// store path to activate-and-exec script
-	Launcher *container.Absolute `json:"launcher"`
+	Launcher *check.Absolute `json:"launcher"`
 	// store path to /run/current-system
-	CurrentSystem *container.Absolute `json:"current_system"`
+	CurrentSystem *check.Absolute `json:"current_system"`
 	// store path to home-manager activation package
 	ActivationPackage string `json:"activation_package"`
 }

-func (app *appInfo) toHst(pathSet *appPathSet, pathname *container.Absolute, argv []string, flagDropShell bool) *hst.Config {
+func (app *appInfo) toHst(pathSet *appPathSet, pathname *check.Absolute, argv []string, flagDropShell bool) *hst.Config {
 	config := &hst.Config{
 		ID: app.ID,

-		Path: pathname,
-		Args: argv,
-
 		Enablements: app.Enablements,

 		SystemBus:     app.SystemBus,
 		SessionBus:    app.SessionBus,
 		DirectWayland: app.DirectWayland,

-		Username: "hakurei",
-		Shell:    pathShell,
-		Home:     pathDataData.Append(app.ID),
-
 		Identity: app.Identity,
 		Groups:   app.Groups,

@@ -92,33 +84,35 @@ func (app *appInfo) toHst(pathSet *appPathSet, pathname *container.Absolute, arg
 			Device:       app.Device,
 			Tty:          app.Tty || flagDropShell,
 			MapRealUID:   app.MapRealUID,
+			Multiarch:    app.Multiarch,
 			Filesystem: []hst.FilesystemConfigJSON{
-				{FilesystemConfig: &hst.FSBind{Target: container.AbsFHSEtc, Source: pathSet.cacheDir.Append("etc"), Special: true}},
+				{FilesystemConfig: &hst.FSBind{Target: fhs.AbsEtc, Source: pathSet.cacheDir.Append("etc"), Special: true}},
 				{FilesystemConfig: &hst.FSBind{Source: pathSet.nixPath.Append("store"), Target: pathNixStore}},
 				{FilesystemConfig: &hst.FSLink{Target: pathCurrentSystem, Linkname: app.CurrentSystem.String()}},
 				{FilesystemConfig: &hst.FSLink{Target: pathBin, Linkname: pathSwBin.String()}},
-				{FilesystemConfig: &hst.FSLink{Target: container.AbsFHSUsrBin, Linkname: pathSwBin.String()}},
-				{FilesystemConfig: &hst.FSBind{Source: pathSet.metaPath, Target: hst.AbsTmp.Append("app")}},
-				{FilesystemConfig: &hst.FSBind{Source: container.AbsFHSEtc.Append("resolv.conf"), Optional: true}},
-				{FilesystemConfig: &hst.FSBind{Source: container.AbsFHSSys.Append("block"), Optional: true}},
-				{FilesystemConfig: &hst.FSBind{Source: container.AbsFHSSys.Append("bus"), Optional: true}},
-				{FilesystemConfig: &hst.FSBind{Source: container.AbsFHSSys.Append("class"), Optional: true}},
-				{FilesystemConfig: &hst.FSBind{Source: container.AbsFHSSys.Append("dev"), Optional: true}},
-				{FilesystemConfig: &hst.FSBind{Source: container.AbsFHSSys.Append("devices"), Optional: true}},
+				{FilesystemConfig: &hst.FSLink{Target: fhs.AbsUsrBin, Linkname: pathSwBin.String()}},
+				{FilesystemConfig: &hst.FSBind{Source: pathSet.metaPath, Target: hst.AbsPrivateTmp.Append("app")}},
+				{FilesystemConfig: &hst.FSBind{Source: fhs.AbsEtc.Append("resolv.conf"), Optional: true}},
+				{FilesystemConfig: &hst.FSBind{Source: fhs.AbsSys.Append("block"), Optional: true}},
+				{FilesystemConfig: &hst.FSBind{Source: fhs.AbsSys.Append("bus"), Optional: true}},
+				{FilesystemConfig: &hst.FSBind{Source: fhs.AbsSys.Append("class"), Optional: true}},
+				{FilesystemConfig: &hst.FSBind{Source: fhs.AbsSys.Append("dev"), Optional: true}},
+				{FilesystemConfig: &hst.FSBind{Source: fhs.AbsSys.Append("devices"), Optional: true}},
 				{FilesystemConfig: &hst.FSBind{Target: pathDataData.Append(app.ID), Source: pathSet.homeDir, Write: true, Ensure: true}},
 			},
+
+			Username: "hakurei",
+			Shell:    pathShell,
+			Home:     pathDataData.Append(app.ID),
+
+			Path: pathname,
+			Args: argv,
 		},
 		ExtraPerms: []*hst.ExtraPermConfig{
 			{Path: dataHome, Execute: true},
 			{Ensure: true, Path: pathSet.baseDir, Read: true, Write: true, Execute: true},
 		},
 	}
-	if app.Multiarch {
-		config.Container.SeccompFlags |= seccomp.AllowMultiarch
-	}
-	if app.Bluetooth {
-		config.Container.SeccompFlags |= seccomp.AllowBluetooth
-	}
 	return config
 }

--- a/cmd/hpkg/main.go
+++ b/cmd/hpkg/main.go
@@ -11,24 +11,25 @@ import (
 	"syscall"

 	"hakurei.app/command"
-	"hakurei.app/container"
+	"hakurei.app/container/check"
+	"hakurei.app/container/fhs"
 	"hakurei.app/hst"
-	"hakurei.app/internal"
-	"hakurei.app/internal/hlog"
+	"hakurei.app/message"
 )

 var (
 	errSuccess = errors.New("success")
 )

-func init() {
-	hlog.Prepare("hpkg")
+func main() {
+	log.SetPrefix("hpkg: ")
+	log.SetFlags(0)
+	msg := message.NewMsg(log.Default())
+
 	if err := os.Setenv("SHELL", pathShell.String()); err != nil {
 		log.Fatalf("cannot set $SHELL: %v", err)
 	}
-}

-func main() {
 	if os.Geteuid() == 0 {
 		log.Fatal("this program must not run as root")
 	}
@@ -41,7 +42,7 @@ func main() {
 		flagVerbose   bool
 		flagDropShell bool
 	)
-	c := command.New(os.Stderr, log.Printf, "hpkg", func([]string) error { internal.InstallOutput(flagVerbose); return nil }).
+	c := command.New(os.Stderr, log.Printf, "hpkg", func([]string) error { msg.SwapVerbose(flagVerbose); return nil }).
 		Flag(&flagVerbose, "v", command.BoolFlag(false), "Print debug messages to the console").
 		Flag(&flagDropShell, "s", command.BoolFlag(false), "Drop to a shell in place of next hakurei action")

@@ -80,22 +81,22 @@ func main() {
 				Extract package and set up for cleanup.
 			*/

-			var workDir *container.Absolute
+			var workDir *check.Absolute
 			if p, err := os.MkdirTemp("", "hpkg.*"); err != nil {
 				log.Printf("cannot create temporary directory: %v", err)
 				return err
-			} else if workDir, err = container.NewAbs(p); err != nil {
+			} else if workDir, err = check.NewAbs(p); err != nil {
 				log.Printf("invalid temporary directory: %v", err)
 				return err
 			}
 			cleanup := func() {
 				// should be faster than a native implementation
-				mustRun(chmod, "-R", "+w", workDir.String())
-				mustRun(rm, "-rf", workDir.String())
+				mustRun(msg, chmod, "-R", "+w", workDir.String())
+				mustRun(msg, rm, "-rf", workDir.String())
 			}
 			beforeRunFail.Store(&cleanup)

-			mustRun(tar, "-C", workDir.String(), "-xf", pkgPath)
+			mustRun(msg, tar, "-C", workDir.String(), "-xf", pkgPath)

 			/*
 				Parse bundle and app metadata, do pre-install checks.
@@ -148,10 +149,10 @@ func main() {
 				}

 				// sec: should compare version string
-				hlog.Verbosef("installing application %q version %q over local %q",
+				msg.Verbosef("installing application %q version %q over local %q",
 					bundle.ID, bundle.Version, a.Version)
 			} else {
-				hlog.Verbosef("application %q clean installation", bundle.ID)
+				msg.Verbosef("application %q clean installation", bundle.ID)
 				// sec: should install credentials
 			}

@@ -159,9 +160,9 @@ func main() {
 				Setup steps for files owned by the target user.
 			*/

-			withCacheDir(ctx, "install", []string{
+			withCacheDir(ctx, msg, "install", []string{
 				// export inner bundle path in the environment
-				"export BUNDLE=" + hst.Tmp + "/bundle",
+				"export BUNDLE=" + hst.PrivateTmp + "/bundle",
 				// replace inner /etc
 				"mkdir -p etc",
 				"chmod -R +w etc",
@@ -181,7 +182,7 @@ func main() {
 			}, workDir, bundle, pathSet, flagDropShell, cleanup)

 			if bundle.GPU {
-				withCacheDir(ctx, "mesa-wrappers", []string{
+				withCacheDir(ctx, msg, "mesa-wrappers", []string{
 					// link nixGL mesa wrappers
 					"mkdir -p nix/.nixGL",
 					"ln -s " + bundle.Mesa + "/bin/nixGLIntel nix/.nixGL/nixGL",
@@ -193,7 +194,7 @@ func main() {
 				Activate home-manager generation.
 			*/

-			withNixDaemon(ctx, "activate", []string{
+			withNixDaemon(ctx, msg, "activate", []string{
 				// clean up broken links
 				"mkdir -p .local/state/{nix,home-manager}",
 				"chmod -R +w .local/state/{nix,home-manager}",
@@ -261,7 +262,7 @@ func main() {
 			*/

 			if a.GPU && flagAutoDrivers {
-				withNixDaemon(ctx, "nix-gl", []string{
+				withNixDaemon(ctx, msg, "nix-gl", []string{
 					"mkdir -p /nix/.nixGL/auto",
 					"rm -rf /nix/.nixGL/auto",
 					"export NIXPKGS_ALLOW_UNFREE=1",
@@ -275,12 +276,12 @@ func main() {
 						"path:" + a.NixGL + "#nixVulkanNvidia",
 				}, true, func(config *hst.Config) *hst.Config {
 					config.Container.Filesystem = append(config.Container.Filesystem, []hst.FilesystemConfigJSON{
-						{FilesystemConfig: &hst.FSBind{Source: container.AbsFHSEtc.Append("resolv.conf"), Optional: true}},
-						{FilesystemConfig: &hst.FSBind{Source: container.AbsFHSSys.Append("block"), Optional: true}},
-						{FilesystemConfig: &hst.FSBind{Source: container.AbsFHSSys.Append("bus"), Optional: true}},
-						{FilesystemConfig: &hst.FSBind{Source: container.AbsFHSSys.Append("class"), Optional: true}},
-						{FilesystemConfig: &hst.FSBind{Source: container.AbsFHSSys.Append("dev"), Optional: true}},
-						{FilesystemConfig: &hst.FSBind{Source: container.AbsFHSSys.Append("devices"), Optional: true}},
+						{FilesystemConfig: &hst.FSBind{Source: fhs.AbsEtc.Append("resolv.conf"), Optional: true}},
+						{FilesystemConfig: &hst.FSBind{Source: fhs.AbsSys.Append("block"), Optional: true}},
+						{FilesystemConfig: &hst.FSBind{Source: fhs.AbsSys.Append("bus"), Optional: true}},
+						{FilesystemConfig: &hst.FSBind{Source: fhs.AbsSys.Append("class"), Optional: true}},
+						{FilesystemConfig: &hst.FSBind{Source: fhs.AbsSys.Append("dev"), Optional: true}},
+						{FilesystemConfig: &hst.FSBind{Source: fhs.AbsSys.Append("devices"), Optional: true}},
 					}...)
 					appendGPUFilesystem(config)
 					return config
@@ -308,7 +309,7 @@ func main() {

 			if a.GPU {
 				config.Container.Filesystem = append(config.Container.Filesystem,
-					hst.FilesystemConfigJSON{FilesystemConfig: &hst.FSBind{Source: pathSet.nixPath.Append(".nixGL"), Target: hst.AbsTmp.Append("nixGL")}})
+					hst.FilesystemConfigJSON{FilesystemConfig: &hst.FSBind{Source: pathSet.nixPath.Append(".nixGL"), Target: hst.AbsPrivateTmp.Append("nixGL")}})
 				appendGPUFilesystem(config)
 			}

@@ -316,7 +317,7 @@ func main() {
 				Spawn app.
 			*/

-			mustRunApp(ctx, config, func() {})
+			mustRunApp(ctx, msg, config, func() {})
 			return errSuccess
 		}).
 			Flag(&flagDropShellNixGL, "s", command.BoolFlag(false), "Drop to a shell on nixGL build").
@@ -324,9 +325,9 @@ func main() {
 	}

 	c.MustParse(os.Args[1:], func(err error) {
-		hlog.Verbosef("command returned %v", err)
+		msg.Verbosef("command returned %v", err)
 		if errors.Is(err, errSuccess) {
-			hlog.BeforeExit()
+			msg.BeforeExit()
 			os.Exit(0)
 		}
 	})
--- a/cmd/hpkg/paths.go
+++ b/cmd/hpkg/paths.go
@@ -7,36 +7,37 @@ import (
 	"strconv"
 	"sync/atomic"

-	"hakurei.app/container"
+	"hakurei.app/container/check"
+	"hakurei.app/container/fhs"
 	"hakurei.app/hst"
-	"hakurei.app/internal/hlog"
+	"hakurei.app/message"
 )

 const bash = "bash"

 var (
-	dataHome *container.Absolute
+	dataHome *check.Absolute
 )

 func init() {
 	// dataHome
-	if a, err := container.NewAbs(os.Getenv("HAKUREI_DATA_HOME")); err == nil {
+	if a, err := check.NewAbs(os.Getenv("HAKUREI_DATA_HOME")); err == nil {
 		dataHome = a
 	} else {
-		dataHome = container.AbsFHSVarLib.Append("hakurei/" + strconv.Itoa(os.Getuid()))
+		dataHome = fhs.AbsVarLib.Append("hakurei/" + strconv.Itoa(os.Getuid()))
 	}
 }

 var (
-	pathBin = container.AbsFHSRoot.Append("bin")
+	pathBin = fhs.AbsRoot.Append("bin")

-	pathNix           = container.MustAbs("/nix/")
+	pathNix           = check.MustAbs("/nix/")
 	pathNixStore      = pathNix.Append("store/")
-	pathCurrentSystem = container.AbsFHSRun.Append("current-system")
+	pathCurrentSystem = fhs.AbsRun.Append("current-system")
 	pathSwBin         = pathCurrentSystem.Append("sw/bin/")
 	pathShell         = pathSwBin.Append(bash)

-	pathData     = container.MustAbs("/data")
+	pathData     = check.MustAbs("/data")
 	pathDataData = pathData.Append("data")
 )

@@ -51,8 +52,8 @@ func lookPath(file string) string {

 var beforeRunFail = new(atomic.Pointer[func()])

-func mustRun(name string, arg ...string) {
-	hlog.Verbosef("spawning process: %q %q", name, arg)
+func mustRun(msg message.Msg, name string, arg ...string) {
+	msg.Verbosef("spawning process: %q %q", name, arg)
 	cmd := exec.Command(name, arg...)
 	cmd.Stdin, cmd.Stdout, cmd.Stderr = os.Stdin, os.Stdout, os.Stderr
 	if err := cmd.Run(); err != nil {
@@ -65,15 +66,15 @@ func mustRun(name string, arg ...string) {

 type appPathSet struct {
 	// ${dataHome}/${id}
-	baseDir *container.Absolute
+	baseDir *check.Absolute
 	// ${baseDir}/app
-	metaPath *container.Absolute
+	metaPath *check.Absolute
 	// ${baseDir}/files
-	homeDir *container.Absolute
+	homeDir *check.Absolute
 	// ${baseDir}/cache
-	cacheDir *container.Absolute
+	cacheDir *check.Absolute
 	// ${baseDir}/cache/nix
-	nixPath *container.Absolute
+	nixPath *check.Absolute
 }

 func pathSetByApp(id string) *appPathSet {
@@ -89,28 +90,28 @@ func pathSetByApp(id string) *appPathSet {
 func appendGPUFilesystem(config *hst.Config) {
 	config.Container.Filesystem = append(config.Container.Filesystem, []hst.FilesystemConfigJSON{
 		// flatpak commit 763a686d874dd668f0236f911de00b80766ffe79
-		{FilesystemConfig: &hst.FSBind{Source: container.AbsFHSDev.Append("dri"), Device: true, Optional: true}},
+		{FilesystemConfig: &hst.FSBind{Source: fhs.AbsDev.Append("dri"), Device: true, Optional: true}},
 		// mali
-		{FilesystemConfig: &hst.FSBind{Source: container.AbsFHSDev.Append("mali"), Device: true, Optional: true}},
-		{FilesystemConfig: &hst.FSBind{Source: container.AbsFHSDev.Append("mali0"), Device: true, Optional: true}},
-		{FilesystemConfig: &hst.FSBind{Source: container.AbsFHSDev.Append("umplock"), Device: true, Optional: true}},
+		{FilesystemConfig: &hst.FSBind{Source: fhs.AbsDev.Append("mali"), Device: true, Optional: true}},
+		{FilesystemConfig: &hst.FSBind{Source: fhs.AbsDev.Append("mali0"), Device: true, Optional: true}},
+		{FilesystemConfig: &hst.FSBind{Source: fhs.AbsDev.Append("umplock"), Device: true, Optional: true}},
 		// nvidia
-		{FilesystemConfig: &hst.FSBind{Source: container.AbsFHSDev.Append("nvidiactl"), Device: true, Optional: true}},
-		{FilesystemConfig: &hst.FSBind{Source: container.AbsFHSDev.Append("nvidia-modeset"), Device: true, Optional: true}},
+		{FilesystemConfig: &hst.FSBind{Source: fhs.AbsDev.Append("nvidiactl"), Device: true, Optional: true}},
+		{FilesystemConfig: &hst.FSBind{Source: fhs.AbsDev.Append("nvidia-modeset"), Device: true, Optional: true}},
 		// nvidia OpenCL/CUDA
-		{FilesystemConfig: &hst.FSBind{Source: container.AbsFHSDev.Append("nvidia-uvm"), Device: true, Optional: true}},
-		{FilesystemConfig: &hst.FSBind{Source: container.AbsFHSDev.Append("nvidia-uvm-tools"), Device: true, Optional: true}},
+		{FilesystemConfig: &hst.FSBind{Source: fhs.AbsDev.Append("nvidia-uvm"), Device: true, Optional: true}},
+		{FilesystemConfig: &hst.FSBind{Source: fhs.AbsDev.Append("nvidia-uvm-tools"), Device: true, Optional: true}},

 		// flatpak commit d2dff2875bb3b7e2cd92d8204088d743fd07f3ff
-		{FilesystemConfig: &hst.FSBind{Source: container.AbsFHSDev.Append("nvidia0"), Device: true, Optional: true}}, {FilesystemConfig: &hst.FSBind{Source: container.AbsFHSDev.Append("nvidia1"), Device: true, Optional: true}},
-		{FilesystemConfig: &hst.FSBind{Source: container.AbsFHSDev.Append("nvidia2"), Device: true, Optional: true}}, {FilesystemConfig: &hst.FSBind{Source: container.AbsFHSDev.Append("nvidia3"), Device: true, Optional: true}},
-		{FilesystemConfig: &hst.FSBind{Source: container.AbsFHSDev.Append("nvidia4"), Device: true, Optional: true}}, {FilesystemConfig: &hst.FSBind{Source: container.AbsFHSDev.Append("nvidia5"), Device: true, Optional: true}},
-		{FilesystemConfig: &hst.FSBind{Source: container.AbsFHSDev.Append("nvidia6"), Device: true, Optional: true}}, {FilesystemConfig: &hst.FSBind{Source: container.AbsFHSDev.Append("nvidia7"), Device: true, Optional: true}},
-		{FilesystemConfig: &hst.FSBind{Source: container.AbsFHSDev.Append("nvidia8"), Device: true, Optional: true}}, {FilesystemConfig: &hst.FSBind{Source: container.AbsFHSDev.Append("nvidia9"), Device: true, Optional: true}},
-		{FilesystemConfig: &hst.FSBind{Source: container.AbsFHSDev.Append("nvidia10"), Device: true, Optional: true}}, {FilesystemConfig: &hst.FSBind{Source: container.AbsFHSDev.Append("nvidia11"), Device: true, Optional: true}},
-		{FilesystemConfig: &hst.FSBind{Source: container.AbsFHSDev.Append("nvidia12"), Device: true, Optional: true}}, {FilesystemConfig: &hst.FSBind{Source: container.AbsFHSDev.Append("nvidia13"), Device: true, Optional: true}},
-		{FilesystemConfig: &hst.FSBind{Source: container.AbsFHSDev.Append("nvidia14"), Device: true, Optional: true}}, {FilesystemConfig: &hst.FSBind{Source: container.AbsFHSDev.Append("nvidia15"), Device: true, Optional: true}},
-		{FilesystemConfig: &hst.FSBind{Source: container.AbsFHSDev.Append("nvidia16"), Device: true, Optional: true}}, {FilesystemConfig: &hst.FSBind{Source: container.AbsFHSDev.Append("nvidia17"), Device: true, Optional: true}},
-		{FilesystemConfig: &hst.FSBind{Source: container.AbsFHSDev.Append("nvidia18"), Device: true, Optional: true}}, {FilesystemConfig: &hst.FSBind{Source: container.AbsFHSDev.Append("nvidia19"), Device: true, Optional: true}},
+		{FilesystemConfig: &hst.FSBind{Source: fhs.AbsDev.Append("nvidia0"), Device: true, Optional: true}}, {FilesystemConfig: &hst.FSBind{Source: fhs.AbsDev.Append("nvidia1"), Device: true, Optional: true}},
+		{FilesystemConfig: &hst.FSBind{Source: fhs.AbsDev.Append("nvidia2"), Device: true, Optional: true}}, {FilesystemConfig: &hst.FSBind{Source: fhs.AbsDev.Append("nvidia3"), Device: true, Optional: true}},
+		{FilesystemConfig: &hst.FSBind{Source: fhs.AbsDev.Append("nvidia4"), Device: true, Optional: true}}, {FilesystemConfig: &hst.FSBind{Source: fhs.AbsDev.Append("nvidia5"), Device: true, Optional: true}},
+		{FilesystemConfig: &hst.FSBind{Source: fhs.AbsDev.Append("nvidia6"), Device: true, Optional: true}}, {FilesystemConfig: &hst.FSBind{Source: fhs.AbsDev.Append("nvidia7"), Device: true, Optional: true}},
+		{FilesystemConfig: &hst.FSBind{Source: fhs.AbsDev.Append("nvidia8"), Device: true, Optional: true}}, {FilesystemConfig: &hst.FSBind{Source: fhs.AbsDev.Append("nvidia9"), Device: true, Optional: true}},
+		{FilesystemConfig: &hst.FSBind{Source: fhs.AbsDev.Append("nvidia10"), Device: true, Optional: true}}, {FilesystemConfig: &hst.FSBind{Source: fhs.AbsDev.Append("nvidia11"), Device: true, Optional: true}},
+		{FilesystemConfig: &hst.FSBind{Source: fhs.AbsDev.Append("nvidia12"), Device: true, Optional: true}}, {FilesystemConfig: &hst.FSBind{Source: fhs.AbsDev.Append("nvidia13"), Device: true, Optional: true}},
+		{FilesystemConfig: &hst.FSBind{Source: fhs.AbsDev.Append("nvidia14"), Device: true, Optional: true}}, {FilesystemConfig: &hst.FSBind{Source: fhs.AbsDev.Append("nvidia15"), Device: true, Optional: true}},
+		{FilesystemConfig: &hst.FSBind{Source: fhs.AbsDev.Append("nvidia16"), Device: true, Optional: true}}, {FilesystemConfig: &hst.FSBind{Source: fhs.AbsDev.Append("nvidia17"), Device: true, Optional: true}},
+		{FilesystemConfig: &hst.FSBind{Source: fhs.AbsDev.Append("nvidia18"), Device: true, Optional: true}}, {FilesystemConfig: &hst.FSBind{Source: fhs.AbsDev.Append("nvidia19"), Device: true, Optional: true}},
 	}...)
 }
--- a/cmd/hpkg/proc.go
+++ b/cmd/hpkg/proc.go
@@ -11,12 +11,12 @@ import (

 	"hakurei.app/hst"
 	"hakurei.app/internal"
-	"hakurei.app/internal/hlog"
+	"hakurei.app/message"
 )

 var hakureiPath = internal.MustHakureiPath()

-func mustRunApp(ctx context.Context, config *hst.Config, beforeFail func()) {
+func mustRunApp(ctx context.Context, msg message.Msg, config *hst.Config, beforeFail func()) {
 	var (
 		cmd *exec.Cmd
 		st  io.WriteCloser
@@ -26,10 +26,10 @@ func mustRunApp(ctx context.Context, config *hst.Config, beforeFail func()) {
 		beforeFail()
 		log.Fatalf("cannot pipe: %v", err)
 	} else {
-		if hlog.Load() {
-			cmd = exec.CommandContext(ctx, hakureiPath, "-v", "app", "3")
+		if msg.IsVerbose() {
+			cmd = exec.CommandContext(ctx, hakureiPath.String(), "-v", "app", "3")
 		} else {
-			cmd = exec.CommandContext(ctx, hakureiPath, "app", "3")
+			cmd = exec.CommandContext(ctx, hakureiPath.String(), "app", "3")
 		}
 		cmd.Stdin, cmd.Stdout, cmd.Stderr = os.Stdin, os.Stdout, os.Stderr
 		cmd.ExtraFiles = []*os.File{r}
@@ -51,7 +51,8 @@ func mustRunApp(ctx context.Context, config *hst.Config, beforeFail func()) {
 		var exitError *exec.ExitError
 		if errors.As(err, &exitError) {
 			beforeFail()
-			internal.Exit(exitError.ExitCode())
+			msg.BeforeExit()
+			os.Exit(exitError.ExitCode())
 		} else {
 			beforeFail()
 			log.Fatalf("cannot wait: %v", err)
--- a/cmd/hpkg/test/test.py
+++ b/cmd/hpkg/test/test.py
@@ -62,11 +62,11 @@ def check_state(name, enablements):

    config = instance['config']

-    if len(config['args']) != 1 or not (config['args'][0].startswith("/nix/store/")) or f"hakurei-{name}-" not in (config['args'][0]):
-        raise Exception(f"unexpected args {instance['config']['args']}")
+    if len(config['container']['args']) != 1 or not (config['container']['args'][0].startswith("/nix/store/")) or f"hakurei-{name}-" not in (config['container']['args'][0]):
+        raise Exception(f"unexpected args {config['container']['args']}")

    if config['enablements'] != enablements:
-        raise Exception(f"unexpected enablements {instance['config']['enablements']}")
+        raise Exception(f"unexpected enablements {config['enablements']}")


 start_all()
--- a/cmd/hpkg/with.go
+++ b/cmd/hpkg/with.go
@@ -2,38 +2,24 @@ package main

 import (
 	"context"
+	"os"
 	"strings"

-	"hakurei.app/container"
-	"hakurei.app/container/seccomp"
+	"hakurei.app/container/check"
+	"hakurei.app/container/fhs"
 	"hakurei.app/hst"
-	"hakurei.app/internal"
+	"hakurei.app/message"
 )

 func withNixDaemon(
 	ctx context.Context,
+	msg message.Msg,
 	action string, command []string, net bool, updateConfig func(config *hst.Config) *hst.Config,
 	app *appInfo, pathSet *appPathSet, dropShell bool, beforeFail func(),
 ) {
-	mustRunAppDropShell(ctx, updateConfig(&hst.Config{
+	mustRunAppDropShell(ctx, msg, updateConfig(&hst.Config{
 		ID: app.ID,

-		Path: pathShell,
-		Args: []string{bash, "-lc", "rm -f /nix/var/nix/daemon-socket/socket && " +
-			// start nix-daemon
-			"nix-daemon --store / & " +
-			// wait for socket to appear
-			"(while [ ! -S /nix/var/nix/daemon-socket/socket ]; do sleep 0.01; done) && " +
-			// create directory so nix stops complaining
-			"mkdir -p /nix/var/nix/profiles/per-user/root/channels && " +
-			strings.Join(command, " && ") +
-			// terminate nix-daemon
-			" && pkill nix-daemon",
-		},
-
-		Username: "hakurei",
-		Shell:    pathShell,
-		Home:     pathDataData.Append(app.ID),
 		ExtraPerms: []*hst.ExtraPermConfig{
 			{Path: dataHome, Execute: true},
 			{Ensure: true, Path: pathSet.baseDir, Read: true, Write: true, Execute: true},
@@ -42,36 +28,48 @@ func withNixDaemon(
 		Identity: app.Identity,

 		Container: &hst.ContainerConfig{
-			Hostname:     formatHostname(app.Name) + "-" + action,
-			Userns:       true, // nix sandbox requires userns
-			HostNet:      net,
-			SeccompFlags: seccomp.AllowMultiarch,
-			Tty:          dropShell,
+			Hostname:  formatHostname(app.Name) + "-" + action,
+			Userns:    true, // nix sandbox requires userns
+			HostNet:   net,
+			Multiarch: true,
+			Tty:       dropShell,
 			Filesystem: []hst.FilesystemConfigJSON{
-				{FilesystemConfig: &hst.FSBind{Target: container.AbsFHSEtc, Source: pathSet.cacheDir.Append("etc"), Special: true}},
+				{FilesystemConfig: &hst.FSBind{Target: fhs.AbsEtc, Source: pathSet.cacheDir.Append("etc"), Special: true}},
 				{FilesystemConfig: &hst.FSBind{Source: pathSet.nixPath, Target: pathNix, Write: true}},
 				{FilesystemConfig: &hst.FSLink{Target: pathCurrentSystem, Linkname: app.CurrentSystem.String()}},
 				{FilesystemConfig: &hst.FSLink{Target: pathBin, Linkname: pathSwBin.String()}},
-				{FilesystemConfig: &hst.FSLink{Target: container.AbsFHSUsrBin, Linkname: pathSwBin.String()}},
+				{FilesystemConfig: &hst.FSLink{Target: fhs.AbsUsrBin, Linkname: pathSwBin.String()}},
 				{FilesystemConfig: &hst.FSBind{Target: pathDataData.Append(app.ID), Source: pathSet.homeDir, Write: true, Ensure: true}},
 			},
+
+			Username: "hakurei",
+			Shell:    pathShell,
+			Home:     pathDataData.Append(app.ID),
+
+			Path: pathShell,
+			Args: []string{bash, "-lc", "rm -f /nix/var/nix/daemon-socket/socket && " +
+				// start nix-daemon
+				"nix-daemon --store / & " +
+				// wait for socket to appear
+				"(while [ ! -S /nix/var/nix/daemon-socket/socket ]; do sleep 0.01; done) && " +
+				// create directory so nix stops complaining
+				"mkdir -p /nix/var/nix/profiles/per-user/root/channels && " +
+				strings.Join(command, " && ") +
+				// terminate nix-daemon
+				" && pkill nix-daemon",
+			},
 		},
 	}), dropShell, beforeFail)
 }

 func withCacheDir(
 	ctx context.Context,
-	action string, command []string, workDir *container.Absolute,
+	msg message.Msg,
+	action string, command []string, workDir *check.Absolute,
 	app *appInfo, pathSet *appPathSet, dropShell bool, beforeFail func()) {
-	mustRunAppDropShell(ctx, &hst.Config{
+	mustRunAppDropShell(ctx, msg, &hst.Config{
 		ID: app.ID,

-		Path: pathShell,
-		Args: []string{bash, "-lc", strings.Join(command, " && ")},
-
-		Username: "nixos",
-		Shell:    pathShell,
-		Home:     pathDataData.Append(app.ID, "cache"),
 		ExtraPerms: []*hst.ExtraPermConfig{
 			{Path: dataHome, Execute: true},
 			{Ensure: true, Path: pathSet.baseDir, Read: true, Write: true, Execute: true},
@@ -81,28 +79,38 @@ func withCacheDir(
 		Identity: app.Identity,

 		Container: &hst.ContainerConfig{
-			Hostname:     formatHostname(app.Name) + "-" + action,
-			SeccompFlags: seccomp.AllowMultiarch,
-			Tty:          dropShell,
+			Hostname:  formatHostname(app.Name) + "-" + action,
+			Multiarch: true,
+			Tty:       dropShell,
 			Filesystem: []hst.FilesystemConfigJSON{
-				{FilesystemConfig: &hst.FSBind{Target: container.AbsFHSEtc, Source: workDir.Append(container.FHSEtc), Special: true}},
+				{FilesystemConfig: &hst.FSBind{Target: fhs.AbsEtc, Source: workDir.Append(fhs.Etc), Special: true}},
 				{FilesystemConfig: &hst.FSBind{Source: workDir.Append("nix"), Target: pathNix}},
 				{FilesystemConfig: &hst.FSLink{Target: pathCurrentSystem, Linkname: app.CurrentSystem.String()}},
 				{FilesystemConfig: &hst.FSLink{Target: pathBin, Linkname: pathSwBin.String()}},
-				{FilesystemConfig: &hst.FSLink{Target: container.AbsFHSUsrBin, Linkname: pathSwBin.String()}},
-				{FilesystemConfig: &hst.FSBind{Source: workDir, Target: hst.AbsTmp.Append("bundle")}},
+				{FilesystemConfig: &hst.FSLink{Target: fhs.AbsUsrBin, Linkname: pathSwBin.String()}},
+				{FilesystemConfig: &hst.FSBind{Source: workDir, Target: hst.AbsPrivateTmp.Append("bundle")}},
 				{FilesystemConfig: &hst.FSBind{Target: pathDataData.Append(app.ID, "cache"), Source: pathSet.cacheDir, Write: true, Ensure: true}},
 			},
+
+			Username: "nixos",
+			Shell:    pathShell,
+			Home:     pathDataData.Append(app.ID, "cache"),
+
+			Path: pathShell,
+			Args: []string{bash, "-lc", strings.Join(command, " && ")},
 		},
 	}, dropShell, beforeFail)
 }

-func mustRunAppDropShell(ctx context.Context, config *hst.Config, dropShell bool, beforeFail func()) {
+func mustRunAppDropShell(ctx context.Context, msg message.Msg, config *hst.Config, dropShell bool, beforeFail func()) {
 	if dropShell {
-		config.Args = []string{bash, "-l"}
-		mustRunApp(ctx, config, beforeFail)
+		if config.Container != nil {
+			config.Container.Args = []string{bash, "-l"}
+		}
+		mustRunApp(ctx, msg, config, beforeFail)
 		beforeFail()
-		internal.Exit(0)
+		msg.BeforeExit()
+		os.Exit(0)
 	}
-	mustRunApp(ctx, config, beforeFail)
+	mustRunApp(ctx, msg, config, beforeFail)
 }
--- a/cmd/hsu/main.go
+++ b/cmd/hsu/main.go
@@ -1,5 +1,7 @@
 package main

+// minimise imports to avoid inadvertently calling init or global variable functions
+
 import (
 	"bytes"
 	"fmt"
@@ -15,10 +17,13 @@ import (
 const (
 	hsuConfFile = "/etc/hsurc"
 	envShim     = "HAKUREI_SHIM"
-	envAID      = "HAKUREI_APP_ID"
+	envIdentity = "HAKUREI_IDENTITY"
 	envGroups   = "HAKUREI_GROUPS"

 	PR_SET_NO_NEW_PRIVS = 0x26
+
+	identityMin = 0
+	identityMax = 9999
 )

 func main() {
@@ -29,6 +34,9 @@ func main() {
 	if os.Geteuid() != 0 {
 		log.Fatal("this program must be owned by uid 0 and have the setuid bit set")
 	}
+	if os.Getegid() != os.Getgid() {
+		log.Fatal("this program must not have the setgid bit set")
+	}

 	puid := os.Getuid()
 	if puid == 0 {
@@ -48,8 +56,8 @@ func main() {
 	}

 	// uid = 1000000 +
-	//   fid * 10000 +
-	//           aid
+	//    id * 10000 +
+	//      identity
 	uid := 1000000

 	// refuse to run if hsurc is not protected correctly
@@ -62,29 +70,25 @@ func main() {
 	}

 	// authenticate before accepting user input
+	var id int
 	if f, err := os.Open(hsuConfFile); err != nil {
 		log.Fatal(err)
-	} else if fid, ok := mustParseConfig(f, puid); !ok {
+	} else if v, ok := mustParseConfig(f, puid); !ok {
 		log.Fatalf("uid %d is not in the hsurc file", puid)
 	} else {
-		uid += fid * 10000
-	}
+		id = v
+		if err = f.Close(); err != nil {
+			log.Fatal(err)
+		}

-	// allowed aid range 0 to 9999
-	if as, ok := os.LookupEnv(envAID); !ok {
-		log.Fatal("HAKUREI_APP_ID not set")
-	} else if aid, err := parseUint32Fast(as); err != nil || aid < 0 || aid > 9999 {
-		log.Fatal("invalid aid")
-	} else {
-		uid += aid
+		uid += id * 10000
 	}

 	// pass through setup fd to shim
 	var shimSetupFd string
 	if s, ok := os.LookupEnv(envShim); !ok {
-		// hakurei requests target uid
-		// print resolved uid and exit
-		fmt.Print(uid)
+		// hakurei requests hsurc user id
+		fmt.Print(id)
 		os.Exit(0)
 	} else if len(s) != 1 || s[0] > '9' || s[0] < '3' {
 		log.Fatal("HAKUREI_SHIM holds an invalid value")
@@ -92,6 +96,15 @@ func main() {
 		shimSetupFd = s
 	}

+	// allowed identity range 0 to 9999
+	if as, ok := os.LookupEnv(envIdentity); !ok {
+		log.Fatal("HAKUREI_IDENTITY not set")
+	} else if identity, err := parseUint32Fast(as); err != nil || identity < identityMin || identity > identityMax {
+		log.Fatal("invalid identity")
+	} else {
+		uid += identity
+	}
+
 	// supplementary groups
 	var suppGroups, suppCurrent []int

--- a/cmd/hsu/parse_test.go
+++ b/cmd/hsu/parse_test.go
@@ -6,32 +6,46 @@ import (
 	"testing"
 )

-func Test_parseUint32Fast(t *testing.T) {
+func TestParseUint32Fast(t *testing.T) {
+	t.Parallel()
+
 	t.Run("zero-length", func(t *testing.T) {
+		t.Parallel()
+
 		if _, err := parseUint32Fast(""); err == nil || err.Error() != "zero length string" {
 			t.Errorf(`parseUint32Fast(""): error = %v`, err)
 			return
 		}
 	})
+
 	t.Run("overflow", func(t *testing.T) {
+		t.Parallel()
+
 		if _, err := parseUint32Fast("10000000000"); err == nil || err.Error() != "string too long" {
 			t.Errorf("parseUint32Fast: error = %v", err)
 			return
 		}
 	})
+
 	t.Run("invalid byte", func(t *testing.T) {
+		t.Parallel()
+
 		if _, err := parseUint32Fast("meow"); err == nil || err.Error() != "invalid character 'm' at index 0" {
 			t.Errorf(`parseUint32Fast("meow"): error = %v`, err)
 			return
 		}
 	})
+
 	t.Run("full range", func(t *testing.T) {
+		t.Parallel()
+
 		testRange := func(i, end int) {
 			for ; i < end; i++ {
 				s := strconv.Itoa(i)
 				w := i
 				t.Run("parse "+s, func(t *testing.T) {
 					t.Parallel()
+
 					v, err := parseUint32Fast(s)
 					if err != nil {
 						t.Errorf("parseUint32Fast(%q): error = %v",
@@ -55,7 +69,9 @@ func Test_parseUint32Fast(t *testing.T) {
 	})
 }

-func Test_parseConfig(t *testing.T) {
+func TestParseConfig(t *testing.T) {
+	t.Parallel()
+
 	testCases := []struct {
 		name       string
 		puid, want int
@@ -71,6 +87,8 @@ func Test_parseConfig(t *testing.T) {

 	for _, tc := range testCases {
 		t.Run(tc.name, func(t *testing.T) {
+			t.Parallel()
+
 			fid, ok, err := parseConfig(bytes.NewBufferString(tc.rc), tc.puid)
 			if err == nil && tc.wantErr != "" {
 				t.Errorf("parseConfig: error = %v; wantErr %q",
--- a/command/builder_test.go
+++ b/command/builder_test.go
@@ -7,6 +7,7 @@ import (
 )

 func TestBuild(t *testing.T) {
+	t.Parallel()
 	c := command.New(nil, nil, "test", nil)
 	stubHandler := func([]string) error { panic("unreachable") }

--- a/command/parse_test.go
+++ b/command/parse_test.go
@@ -14,6 +14,8 @@ import (
 )

 func TestParse(t *testing.T) {
+	t.Parallel()
+
 	testCases := []struct {
 		name      string
 		buildTree func(wout, wlog io.Writer) command.Command
@@ -251,6 +253,7 @@ Commands:
 	}
 	for _, tc := range testCases {
 		t.Run(tc.name, func(t *testing.T) {
+			t.Parallel()
 			wout, wlog := new(bytes.Buffer), new(bytes.Buffer)
 			c := tc.buildTree(wout, wlog)

--- a/command/unreachable_test.go
+++ b/command/unreachable_test.go
@@ -6,15 +6,19 @@ import (
 )

 func TestParseUnreachable(t *testing.T) {
+	t.Parallel()
+
 	// top level bypasses name matching and recursive calls to Parse
 	// returns when encountering zero-length args
 	t.Run("zero-length args", func(t *testing.T) {
+		t.Parallel()
 		defer checkRecover(t, "Parse", "attempted to parse with zero length args")
 		_ = newNode(panicWriter{}, nil, " ", " ").Parse(nil)
 	})

 	// top level must not have siblings
 	t.Run("toplevel siblings", func(t *testing.T) {
+		t.Parallel()
 		defer checkRecover(t, "Parse", "invalid toplevel state")
 		n := newNode(panicWriter{}, nil, " ", "")
 		n.append(newNode(panicWriter{}, nil, "  ", " "))
@@ -23,6 +27,7 @@ func TestParseUnreachable(t *testing.T) {

 	// a node with descendents must not have a direct handler
 	t.Run("sub handle conflict", func(t *testing.T) {
+		t.Parallel()
 		defer checkRecover(t, "Parse", "invalid subcommand tree state")
 		n := newNode(panicWriter{}, nil, " ", " ")
 		n.adopt(newNode(panicWriter{}, nil, " ", " "))
@@ -32,6 +37,7 @@ func TestParseUnreachable(t *testing.T) {

 	// this would only happen if a node was matched twice
 	t.Run("parsed flag set", func(t *testing.T) {
+		t.Parallel()
 		defer checkRecover(t, "Parse", "invalid set state")
 		n := newNode(panicWriter{}, nil, " ", "")
 		set := flag.NewFlagSet("parsed", flag.ContinueOnError)
--- a/container/autoetc.go
+++ b/container/autoetc.go
@@ -3,16 +3,18 @@ package container
 import (
 	"encoding/gob"
 	"fmt"
-	"io/fs"
+
+	"hakurei.app/container/check"
+	"hakurei.app/container/fhs"
 )

 func init() { gob.Register(new(AutoEtcOp)) }

 // Etc appends an [Op] that expands host /etc into a toplevel symlink mirror with /etc semantics.
 // This is not a generic setup op. It is implemented here to reduce ipc overhead.
-func (f *Ops) Etc(host *Absolute, prefix string) *Ops {
+func (f *Ops) Etc(host *check.Absolute, prefix string) *Ops {
 	e := &AutoEtcOp{prefix}
-	f.Mkdir(AbsFHSEtc, 0755)
+	f.Mkdir(fhs.AbsEtc, 0755)
 	f.Bind(host, e.hostPath(), 0)
 	*f = append(*f, e)
 	return f
@@ -24,18 +26,18 @@ func (e *AutoEtcOp) Valid() bool                                { return e != ni
 func (e *AutoEtcOp) early(*setupState, syscallDispatcher) error { return nil }
 func (e *AutoEtcOp) apply(state *setupState, k syscallDispatcher) error {
 	if state.nonrepeatable&nrAutoEtc != 0 {
-		return msg.WrapErr(fs.ErrInvalid, "autoetc is not repeatable")
+		return OpRepeatError("autoetc")
 	}
 	state.nonrepeatable |= nrAutoEtc

-	const target = sysrootPath + FHSEtc
+	const target = sysrootPath + fhs.Etc
 	rel := e.hostRel() + "/"

 	if err := k.mkdirAll(target, 0755); err != nil {
-		return wrapErrSelf(err)
+		return err
 	}
 	if d, err := k.readdir(toSysroot(e.hostPath().String())); err != nil {
-		return wrapErrSelf(err)
+		return err
 	} else {
 		for _, ent := range d {
 			n := ent.Name()
@@ -43,13 +45,13 @@ func (e *AutoEtcOp) apply(state *setupState, k syscallDispatcher) error {
 			case ".host", "passwd", "group":

 			case "mtab":
-				if err = k.symlink(FHSProc+"mounts", target+n); err != nil {
-					return wrapErrSelf(err)
+				if err = k.symlink(fhs.Proc+"mounts", target+n); err != nil {
+					return err
 				}

 			default:
 				if err = k.symlink(rel+n, target+n); err != nil {
-					return wrapErrSelf(err)
+					return err
 				}
 			}
 		}
@@ -58,12 +60,12 @@ func (e *AutoEtcOp) apply(state *setupState, k syscallDispatcher) error {
 	return nil
 }

-func (e *AutoEtcOp) hostPath() *Absolute { return AbsFHSEtc.Append(e.hostRel()) }
-func (e *AutoEtcOp) hostRel() string     { return ".host/" + e.Prefix }
+func (e *AutoEtcOp) hostPath() *check.Absolute { return fhs.AbsEtc.Append(e.hostRel()) }
+func (e *AutoEtcOp) hostRel() string           { return ".host/" + e.Prefix }

 func (e *AutoEtcOp) Is(op Op) bool {
 	ve, ok := op.(*AutoEtcOp)
 	return ok && e.Valid() && ve.Valid() && *e == *ve
 }
-func (*AutoEtcOp) prefix() string   { return "setting up" }
-func (e *AutoEtcOp) String() string { return fmt.Sprintf("auto etc %s", e.Prefix) }
+func (*AutoEtcOp) prefix() (string, bool) { return "setting up", true }
+func (e *AutoEtcOp) String() string       { return fmt.Sprintf("auto etc %s", e.Prefix) }
--- a/container/autoetc_test.go
+++ b/container/autoetc_test.go
@@ -2,14 +2,19 @@ package container

 import (
 	"errors"
-	"io/fs"
 	"os"
 	"testing"
+
+	"hakurei.app/container/check"
+	"hakurei.app/container/stub"
 )

 func TestAutoEtcOp(t *testing.T) {
+	t.Parallel()
+
 	t.Run("nonrepeatable", func(t *testing.T) {
-		wantErr := msg.WrapErr(fs.ErrInvalid, "autoetc is not repeatable")
+		t.Parallel()
+		wantErr := OpRepeatError("autoetc")
 		if err := (&AutoEtcOp{Prefix: "81ceabb30d37bbdb3868004629cb84e9"}).apply(&setupState{nonrepeatable: nrAutoEtc}, nil); !errors.Is(err, wantErr) {
 			t.Errorf("apply: error = %v, want %v", err, wantErr)
 		}
@@ -18,22 +23,22 @@ func TestAutoEtcOp(t *testing.T) {
 	checkOpBehaviour(t, []opBehaviourTestCase{
 		{"mkdirAll", new(Params), &AutoEtcOp{
 			Prefix: "81ceabb30d37bbdb3868004629cb84e9",
-		}, nil, nil, []kexpect{
-			{"mkdirAll", expectArgs{"/sysroot/etc/", os.FileMode(0755)}, nil, errUnique},
-		}, wrapErrSelf(errUnique)},
+		}, nil, nil, []stub.Call{
+			call("mkdirAll", stub.ExpectArgs{"/sysroot/etc/", os.FileMode(0755)}, nil, stub.UniqueError(3)),
+		}, stub.UniqueError(3)},

 		{"readdir", new(Params), &AutoEtcOp{
 			Prefix: "81ceabb30d37bbdb3868004629cb84e9",
-		}, nil, nil, []kexpect{
-			{"mkdirAll", expectArgs{"/sysroot/etc/", os.FileMode(0755)}, nil, nil},
-			{"readdir", expectArgs{"/sysroot/etc/.host/81ceabb30d37bbdb3868004629cb84e9"}, stubDir(), errUnique},
-		}, wrapErrSelf(errUnique)},
+		}, nil, nil, []stub.Call{
+			call("mkdirAll", stub.ExpectArgs{"/sysroot/etc/", os.FileMode(0755)}, nil, nil),
+			call("readdir", stub.ExpectArgs{"/sysroot/etc/.host/81ceabb30d37bbdb3868004629cb84e9"}, stubDir(), stub.UniqueError(2)),
+		}, stub.UniqueError(2)},

 		{"symlink", new(Params), &AutoEtcOp{
 			Prefix: "81ceabb30d37bbdb3868004629cb84e9",
-		}, nil, nil, []kexpect{
-			{"mkdirAll", expectArgs{"/sysroot/etc/", os.FileMode(0755)}, nil, nil},
-			{"readdir", expectArgs{"/sysroot/etc/.host/81ceabb30d37bbdb3868004629cb84e9"}, stubDir(".host",
+		}, nil, nil, []stub.Call{
+			call("mkdirAll", stub.ExpectArgs{"/sysroot/etc/", os.FileMode(0755)}, nil, nil),
+			call("readdir", stub.ExpectArgs{"/sysroot/etc/.host/81ceabb30d37bbdb3868004629cb84e9"}, stubDir(".host",
 				"alsa", "bash_logout", "bashrc", "binfmt.d", "dbus-1", "default", "dhcpcd.exit-hook", "fonts",
 				"fstab", "fuse.conf", "group", "host.conf", "hostname", "hosts", "hsurc", "inputrc", "issue", "kbd",
 				"locale.conf", "login.defs", "lsb-release", "lvm", "machine-id", "man_db.conf", "mdadm.conf",
@@ -41,15 +46,15 @@ func TestAutoEtcOp(t *testing.T) {
 				"nsswitch.conf", "os-release", "pam", "pam.d", "passwd", "pipewire", "pki", "polkit-1", "profile",
 				"protocols", "resolv.conf", "resolvconf.conf", "rpc", "services", "set-environment", "shadow", "shells",
 				"ssh", "ssl", "static", "subgid", "subuid", "sudoers", "sway", "sysctl.d", "systemd", "terminfo",
-				"tmpfiles.d", "udev", "vconsole.conf", "X11", "xdg", "zoneinfo"), nil},
-			{"symlink", expectArgs{".host/81ceabb30d37bbdb3868004629cb84e9/alsa", "/sysroot/etc/alsa"}, nil, errUnique},
-		}, wrapErrSelf(errUnique)},
+				"tmpfiles.d", "udev", "vconsole.conf", "X11", "xdg", "zoneinfo"), nil),
+			call("symlink", stub.ExpectArgs{".host/81ceabb30d37bbdb3868004629cb84e9/alsa", "/sysroot/etc/alsa"}, nil, stub.UniqueError(1)),
+		}, stub.UniqueError(1)},

 		{"symlink mtab", new(Params), &AutoEtcOp{
 			Prefix: "81ceabb30d37bbdb3868004629cb84e9",
-		}, nil, nil, []kexpect{
-			{"mkdirAll", expectArgs{"/sysroot/etc/", os.FileMode(0755)}, nil, nil},
-			{"readdir", expectArgs{"/sysroot/etc/.host/81ceabb30d37bbdb3868004629cb84e9"}, stubDir(".host",
+		}, nil, nil, []stub.Call{
+			call("mkdirAll", stub.ExpectArgs{"/sysroot/etc/", os.FileMode(0755)}, nil, nil),
+			call("readdir", stub.ExpectArgs{"/sysroot/etc/.host/81ceabb30d37bbdb3868004629cb84e9"}, stubDir(".host",
 				"alsa", "bash_logout", "bashrc", "binfmt.d", "dbus-1", "default", "dhcpcd.exit-hook", "fonts",
 				"fstab", "fuse.conf", "group", "host.conf", "hostname", "hosts", "hsurc", "inputrc", "issue", "kbd",
 				"locale.conf", "login.defs", "lsb-release", "lvm", "machine-id", "man_db.conf", "mdadm.conf",
@@ -57,41 +62,41 @@ func TestAutoEtcOp(t *testing.T) {
 				"nsswitch.conf", "os-release", "pam", "pam.d", "passwd", "pipewire", "pki", "polkit-1", "profile",
 				"protocols", "resolv.conf", "resolvconf.conf", "rpc", "services", "set-environment", "shadow", "shells",
 				"ssh", "ssl", "static", "subgid", "subuid", "sudoers", "sway", "sysctl.d", "systemd", "terminfo",
-				"tmpfiles.d", "udev", "vconsole.conf", "X11", "xdg", "zoneinfo"), nil},
-			{"symlink", expectArgs{".host/81ceabb30d37bbdb3868004629cb84e9/alsa", "/sysroot/etc/alsa"}, nil, nil},
-			{"symlink", expectArgs{".host/81ceabb30d37bbdb3868004629cb84e9/bash_logout", "/sysroot/etc/bash_logout"}, nil, nil},
-			{"symlink", expectArgs{".host/81ceabb30d37bbdb3868004629cb84e9/bashrc", "/sysroot/etc/bashrc"}, nil, nil},
-			{"symlink", expectArgs{".host/81ceabb30d37bbdb3868004629cb84e9/binfmt.d", "/sysroot/etc/binfmt.d"}, nil, nil},
-			{"symlink", expectArgs{".host/81ceabb30d37bbdb3868004629cb84e9/dbus-1", "/sysroot/etc/dbus-1"}, nil, nil},
-			{"symlink", expectArgs{".host/81ceabb30d37bbdb3868004629cb84e9/default", "/sysroot/etc/default"}, nil, nil},
-			{"symlink", expectArgs{".host/81ceabb30d37bbdb3868004629cb84e9/dhcpcd.exit-hook", "/sysroot/etc/dhcpcd.exit-hook"}, nil, nil},
-			{"symlink", expectArgs{".host/81ceabb30d37bbdb3868004629cb84e9/fonts", "/sysroot/etc/fonts"}, nil, nil},
-			{"symlink", expectArgs{".host/81ceabb30d37bbdb3868004629cb84e9/fstab", "/sysroot/etc/fstab"}, nil, nil},
-			{"symlink", expectArgs{".host/81ceabb30d37bbdb3868004629cb84e9/fuse.conf", "/sysroot/etc/fuse.conf"}, nil, nil},
-			{"symlink", expectArgs{".host/81ceabb30d37bbdb3868004629cb84e9/host.conf", "/sysroot/etc/host.conf"}, nil, nil},
-			{"symlink", expectArgs{".host/81ceabb30d37bbdb3868004629cb84e9/hostname", "/sysroot/etc/hostname"}, nil, nil},
-			{"symlink", expectArgs{".host/81ceabb30d37bbdb3868004629cb84e9/hosts", "/sysroot/etc/hosts"}, nil, nil},
-			{"symlink", expectArgs{".host/81ceabb30d37bbdb3868004629cb84e9/hsurc", "/sysroot/etc/hsurc"}, nil, nil},
-			{"symlink", expectArgs{".host/81ceabb30d37bbdb3868004629cb84e9/inputrc", "/sysroot/etc/inputrc"}, nil, nil},
-			{"symlink", expectArgs{".host/81ceabb30d37bbdb3868004629cb84e9/issue", "/sysroot/etc/issue"}, nil, nil},
-			{"symlink", expectArgs{".host/81ceabb30d37bbdb3868004629cb84e9/kbd", "/sysroot/etc/kbd"}, nil, nil},
-			{"symlink", expectArgs{".host/81ceabb30d37bbdb3868004629cb84e9/locale.conf", "/sysroot/etc/locale.conf"}, nil, nil},
-			{"symlink", expectArgs{".host/81ceabb30d37bbdb3868004629cb84e9/login.defs", "/sysroot/etc/login.defs"}, nil, nil},
-			{"symlink", expectArgs{".host/81ceabb30d37bbdb3868004629cb84e9/lsb-release", "/sysroot/etc/lsb-release"}, nil, nil},
-			{"symlink", expectArgs{".host/81ceabb30d37bbdb3868004629cb84e9/lvm", "/sysroot/etc/lvm"}, nil, nil},
-			{"symlink", expectArgs{".host/81ceabb30d37bbdb3868004629cb84e9/machine-id", "/sysroot/etc/machine-id"}, nil, nil},
-			{"symlink", expectArgs{".host/81ceabb30d37bbdb3868004629cb84e9/man_db.conf", "/sysroot/etc/man_db.conf"}, nil, nil},
-			{"symlink", expectArgs{".host/81ceabb30d37bbdb3868004629cb84e9/mdadm.conf", "/sysroot/etc/mdadm.conf"}, nil, nil},
-			{"symlink", expectArgs{".host/81ceabb30d37bbdb3868004629cb84e9/modprobe.d", "/sysroot/etc/modprobe.d"}, nil, nil},
-			{"symlink", expectArgs{".host/81ceabb30d37bbdb3868004629cb84e9/modules-load.d", "/sysroot/etc/modules-load.d"}, nil, nil},
-			{"symlink", expectArgs{"/proc/mounts", "/sysroot/etc/mtab"}, nil, errUnique},
-		}, wrapErrSelf(errUnique)},
+				"tmpfiles.d", "udev", "vconsole.conf", "X11", "xdg", "zoneinfo"), nil),
+			call("symlink", stub.ExpectArgs{".host/81ceabb30d37bbdb3868004629cb84e9/alsa", "/sysroot/etc/alsa"}, nil, nil),
+			call("symlink", stub.ExpectArgs{".host/81ceabb30d37bbdb3868004629cb84e9/bash_logout", "/sysroot/etc/bash_logout"}, nil, nil),
+			call("symlink", stub.ExpectArgs{".host/81ceabb30d37bbdb3868004629cb84e9/bashrc", "/sysroot/etc/bashrc"}, nil, nil),
+			call("symlink", stub.ExpectArgs{".host/81ceabb30d37bbdb3868004629cb84e9/binfmt.d", "/sysroot/etc/binfmt.d"}, nil, nil),
+			call("symlink", stub.ExpectArgs{".host/81ceabb30d37bbdb3868004629cb84e9/dbus-1", "/sysroot/etc/dbus-1"}, nil, nil),
+			call("symlink", stub.ExpectArgs{".host/81ceabb30d37bbdb3868004629cb84e9/default", "/sysroot/etc/default"}, nil, nil),
+			call("symlink", stub.ExpectArgs{".host/81ceabb30d37bbdb3868004629cb84e9/dhcpcd.exit-hook", "/sysroot/etc/dhcpcd.exit-hook"}, nil, nil),
+			call("symlink", stub.ExpectArgs{".host/81ceabb30d37bbdb3868004629cb84e9/fonts", "/sysroot/etc/fonts"}, nil, nil),
+			call("symlink", stub.ExpectArgs{".host/81ceabb30d37bbdb3868004629cb84e9/fstab", "/sysroot/etc/fstab"}, nil, nil),
+			call("symlink", stub.ExpectArgs{".host/81ceabb30d37bbdb3868004629cb84e9/fuse.conf", "/sysroot/etc/fuse.conf"}, nil, nil),
+			call("symlink", stub.ExpectArgs{".host/81ceabb30d37bbdb3868004629cb84e9/host.conf", "/sysroot/etc/host.conf"}, nil, nil),
+			call("symlink", stub.ExpectArgs{".host/81ceabb30d37bbdb3868004629cb84e9/hostname", "/sysroot/etc/hostname"}, nil, nil),
+			call("symlink", stub.ExpectArgs{".host/81ceabb30d37bbdb3868004629cb84e9/hosts", "/sysroot/etc/hosts"}, nil, nil),
+			call("symlink", stub.ExpectArgs{".host/81ceabb30d37bbdb3868004629cb84e9/hsurc", "/sysroot/etc/hsurc"}, nil, nil),
+			call("symlink", stub.ExpectArgs{".host/81ceabb30d37bbdb3868004629cb84e9/inputrc", "/sysroot/etc/inputrc"}, nil, nil),
+			call("symlink", stub.ExpectArgs{".host/81ceabb30d37bbdb3868004629cb84e9/issue", "/sysroot/etc/issue"}, nil, nil),
+			call("symlink", stub.ExpectArgs{".host/81ceabb30d37bbdb3868004629cb84e9/kbd", "/sysroot/etc/kbd"}, nil, nil),
+			call("symlink", stub.ExpectArgs{".host/81ceabb30d37bbdb3868004629cb84e9/locale.conf", "/sysroot/etc/locale.conf"}, nil, nil),
+			call("symlink", stub.ExpectArgs{".host/81ceabb30d37bbdb3868004629cb84e9/login.defs", "/sysroot/etc/login.defs"}, nil, nil),
+			call("symlink", stub.ExpectArgs{".host/81ceabb30d37bbdb3868004629cb84e9/lsb-release", "/sysroot/etc/lsb-release"}, nil, nil),
+			call("symlink", stub.ExpectArgs{".host/81ceabb30d37bbdb3868004629cb84e9/lvm", "/sysroot/etc/lvm"}, nil, nil),
+			call("symlink", stub.ExpectArgs{".host/81ceabb30d37bbdb3868004629cb84e9/machine-id", "/sysroot/etc/machine-id"}, nil, nil),
+			call("symlink", stub.ExpectArgs{".host/81ceabb30d37bbdb3868004629cb84e9/man_db.conf", "/sysroot/etc/man_db.conf"}, nil, nil),
+			call("symlink", stub.ExpectArgs{".host/81ceabb30d37bbdb3868004629cb84e9/mdadm.conf", "/sysroot/etc/mdadm.conf"}, nil, nil),
+			call("symlink", stub.ExpectArgs{".host/81ceabb30d37bbdb3868004629cb84e9/modprobe.d", "/sysroot/etc/modprobe.d"}, nil, nil),
+			call("symlink", stub.ExpectArgs{".host/81ceabb30d37bbdb3868004629cb84e9/modules-load.d", "/sysroot/etc/modules-load.d"}, nil, nil),
+			call("symlink", stub.ExpectArgs{"/proc/mounts", "/sysroot/etc/mtab"}, nil, stub.UniqueError(0)),
+		}, stub.UniqueError(0)},

 		{"success nested", new(Params), &AutoEtcOp{
 			Prefix: "81ceabb30d37bbdb3868004629cb84e9",
-		}, nil, nil, []kexpect{
-			{"mkdirAll", expectArgs{"/sysroot/etc/", os.FileMode(0755)}, nil, nil},
-			{"readdir", expectArgs{"/sysroot/etc/.host/81ceabb30d37bbdb3868004629cb84e9"}, stubDir(".host",
+		}, nil, nil, []stub.Call{
+			call("mkdirAll", stub.ExpectArgs{"/sysroot/etc/", os.FileMode(0755)}, nil, nil),
+			call("readdir", stub.ExpectArgs{"/sysroot/etc/.host/81ceabb30d37bbdb3868004629cb84e9"}, stubDir(".host",
 				"alsa", "bash_logout", "bashrc", "binfmt.d", "dbus-1", "default", "dhcpcd.exit-hook", "fonts",
 				"fstab", "fuse.conf", "group", "host.conf", "hostname", "hosts", "hsurc", "inputrc", "issue", "kbd",
 				"locale.conf", "login.defs", "lsb-release", "lvm", "machine-id", "man_db.conf", "mdadm.conf",
@@ -99,79 +104,79 @@ func TestAutoEtcOp(t *testing.T) {
 				"nsswitch.conf", "os-release", "pam", "pam.d", "passwd", "pipewire", "pki", "polkit-1", "profile",
 				"protocols", "resolv.conf", "resolvconf.conf", "rpc", "services", "set-environment", "shadow", "shells",
 				"ssh", "ssl", "static", "subgid", "subuid", "sudoers", "sway", "sysctl.d", "systemd", "terminfo",
-				"tmpfiles.d", "udev", "vconsole.conf", "X11", "xdg", "zoneinfo"), nil},
-			{"symlink", expectArgs{".host/81ceabb30d37bbdb3868004629cb84e9/alsa", "/sysroot/etc/alsa"}, nil, nil},
-			{"symlink", expectArgs{".host/81ceabb30d37bbdb3868004629cb84e9/bash_logout", "/sysroot/etc/bash_logout"}, nil, nil},
-			{"symlink", expectArgs{".host/81ceabb30d37bbdb3868004629cb84e9/bashrc", "/sysroot/etc/bashrc"}, nil, nil},
-			{"symlink", expectArgs{".host/81ceabb30d37bbdb3868004629cb84e9/binfmt.d", "/sysroot/etc/binfmt.d"}, nil, nil},
-			{"symlink", expectArgs{".host/81ceabb30d37bbdb3868004629cb84e9/dbus-1", "/sysroot/etc/dbus-1"}, nil, nil},
-			{"symlink", expectArgs{".host/81ceabb30d37bbdb3868004629cb84e9/default", "/sysroot/etc/default"}, nil, nil},
-			{"symlink", expectArgs{".host/81ceabb30d37bbdb3868004629cb84e9/dhcpcd.exit-hook", "/sysroot/etc/dhcpcd.exit-hook"}, nil, nil},
-			{"symlink", expectArgs{".host/81ceabb30d37bbdb3868004629cb84e9/fonts", "/sysroot/etc/fonts"}, nil, nil},
-			{"symlink", expectArgs{".host/81ceabb30d37bbdb3868004629cb84e9/fstab", "/sysroot/etc/fstab"}, nil, nil},
-			{"symlink", expectArgs{".host/81ceabb30d37bbdb3868004629cb84e9/fuse.conf", "/sysroot/etc/fuse.conf"}, nil, nil},
-			{"symlink", expectArgs{".host/81ceabb30d37bbdb3868004629cb84e9/host.conf", "/sysroot/etc/host.conf"}, nil, nil},
-			{"symlink", expectArgs{".host/81ceabb30d37bbdb3868004629cb84e9/hostname", "/sysroot/etc/hostname"}, nil, nil},
-			{"symlink", expectArgs{".host/81ceabb30d37bbdb3868004629cb84e9/hosts", "/sysroot/etc/hosts"}, nil, nil},
-			{"symlink", expectArgs{".host/81ceabb30d37bbdb3868004629cb84e9/hsurc", "/sysroot/etc/hsurc"}, nil, nil},
-			{"symlink", expectArgs{".host/81ceabb30d37bbdb3868004629cb84e9/inputrc", "/sysroot/etc/inputrc"}, nil, nil},
-			{"symlink", expectArgs{".host/81ceabb30d37bbdb3868004629cb84e9/issue", "/sysroot/etc/issue"}, nil, nil},
-			{"symlink", expectArgs{".host/81ceabb30d37bbdb3868004629cb84e9/kbd", "/sysroot/etc/kbd"}, nil, nil},
-			{"symlink", expectArgs{".host/81ceabb30d37bbdb3868004629cb84e9/locale.conf", "/sysroot/etc/locale.conf"}, nil, nil},
-			{"symlink", expectArgs{".host/81ceabb30d37bbdb3868004629cb84e9/login.defs", "/sysroot/etc/login.defs"}, nil, nil},
-			{"symlink", expectArgs{".host/81ceabb30d37bbdb3868004629cb84e9/lsb-release", "/sysroot/etc/lsb-release"}, nil, nil},
-			{"symlink", expectArgs{".host/81ceabb30d37bbdb3868004629cb84e9/lvm", "/sysroot/etc/lvm"}, nil, nil},
-			{"symlink", expectArgs{".host/81ceabb30d37bbdb3868004629cb84e9/machine-id", "/sysroot/etc/machine-id"}, nil, nil},
-			{"symlink", expectArgs{".host/81ceabb30d37bbdb3868004629cb84e9/man_db.conf", "/sysroot/etc/man_db.conf"}, nil, nil},
-			{"symlink", expectArgs{".host/81ceabb30d37bbdb3868004629cb84e9/mdadm.conf", "/sysroot/etc/mdadm.conf"}, nil, nil},
-			{"symlink", expectArgs{".host/81ceabb30d37bbdb3868004629cb84e9/modprobe.d", "/sysroot/etc/modprobe.d"}, nil, nil},
-			{"symlink", expectArgs{".host/81ceabb30d37bbdb3868004629cb84e9/modules-load.d", "/sysroot/etc/modules-load.d"}, nil, nil},
-			{"symlink", expectArgs{"/proc/mounts", "/sysroot/etc/mtab"}, nil, nil},
-			{"symlink", expectArgs{".host/81ceabb30d37bbdb3868004629cb84e9/nanorc", "/sysroot/etc/nanorc"}, nil, nil},
-			{"symlink", expectArgs{".host/81ceabb30d37bbdb3868004629cb84e9/netgroup", "/sysroot/etc/netgroup"}, nil, nil},
-			{"symlink", expectArgs{".host/81ceabb30d37bbdb3868004629cb84e9/nix", "/sysroot/etc/nix"}, nil, nil},
-			{"symlink", expectArgs{".host/81ceabb30d37bbdb3868004629cb84e9/nixos", "/sysroot/etc/nixos"}, nil, nil},
-			{"symlink", expectArgs{".host/81ceabb30d37bbdb3868004629cb84e9/NIXOS", "/sysroot/etc/NIXOS"}, nil, nil},
-			{"symlink", expectArgs{".host/81ceabb30d37bbdb3868004629cb84e9/nscd.conf", "/sysroot/etc/nscd.conf"}, nil, nil},
-			{"symlink", expectArgs{".host/81ceabb30d37bbdb3868004629cb84e9/nsswitch.conf", "/sysroot/etc/nsswitch.conf"}, nil, nil},
-			{"symlink", expectArgs{".host/81ceabb30d37bbdb3868004629cb84e9/os-release", "/sysroot/etc/os-release"}, nil, nil},
-			{"symlink", expectArgs{".host/81ceabb30d37bbdb3868004629cb84e9/pam", "/sysroot/etc/pam"}, nil, nil},
-			{"symlink", expectArgs{".host/81ceabb30d37bbdb3868004629cb84e9/pam.d", "/sysroot/etc/pam.d"}, nil, nil},
-			{"symlink", expectArgs{".host/81ceabb30d37bbdb3868004629cb84e9/pipewire", "/sysroot/etc/pipewire"}, nil, nil},
-			{"symlink", expectArgs{".host/81ceabb30d37bbdb3868004629cb84e9/pki", "/sysroot/etc/pki"}, nil, nil},
-			{"symlink", expectArgs{".host/81ceabb30d37bbdb3868004629cb84e9/polkit-1", "/sysroot/etc/polkit-1"}, nil, nil},
-			{"symlink", expectArgs{".host/81ceabb30d37bbdb3868004629cb84e9/profile", "/sysroot/etc/profile"}, nil, nil},
-			{"symlink", expectArgs{".host/81ceabb30d37bbdb3868004629cb84e9/protocols", "/sysroot/etc/protocols"}, nil, nil},
-			{"symlink", expectArgs{".host/81ceabb30d37bbdb3868004629cb84e9/resolv.conf", "/sysroot/etc/resolv.conf"}, nil, nil},
-			{"symlink", expectArgs{".host/81ceabb30d37bbdb3868004629cb84e9/resolvconf.conf", "/sysroot/etc/resolvconf.conf"}, nil, nil},
-			{"symlink", expectArgs{".host/81ceabb30d37bbdb3868004629cb84e9/rpc", "/sysroot/etc/rpc"}, nil, nil},
-			{"symlink", expectArgs{".host/81ceabb30d37bbdb3868004629cb84e9/services", "/sysroot/etc/services"}, nil, nil},
-			{"symlink", expectArgs{".host/81ceabb30d37bbdb3868004629cb84e9/set-environment", "/sysroot/etc/set-environment"}, nil, nil},
-			{"symlink", expectArgs{".host/81ceabb30d37bbdb3868004629cb84e9/shadow", "/sysroot/etc/shadow"}, nil, nil},
-			{"symlink", expectArgs{".host/81ceabb30d37bbdb3868004629cb84e9/shells", "/sysroot/etc/shells"}, nil, nil},
-			{"symlink", expectArgs{".host/81ceabb30d37bbdb3868004629cb84e9/ssh", "/sysroot/etc/ssh"}, nil, nil},
-			{"symlink", expectArgs{".host/81ceabb30d37bbdb3868004629cb84e9/ssl", "/sysroot/etc/ssl"}, nil, nil},
-			{"symlink", expectArgs{".host/81ceabb30d37bbdb3868004629cb84e9/static", "/sysroot/etc/static"}, nil, nil},
-			{"symlink", expectArgs{".host/81ceabb30d37bbdb3868004629cb84e9/subgid", "/sysroot/etc/subgid"}, nil, nil},
-			{"symlink", expectArgs{".host/81ceabb30d37bbdb3868004629cb84e9/subuid", "/sysroot/etc/subuid"}, nil, nil},
-			{"symlink", expectArgs{".host/81ceabb30d37bbdb3868004629cb84e9/sudoers", "/sysroot/etc/sudoers"}, nil, nil},
-			{"symlink", expectArgs{".host/81ceabb30d37bbdb3868004629cb84e9/sway", "/sysroot/etc/sway"}, nil, nil},
-			{"symlink", expectArgs{".host/81ceabb30d37bbdb3868004629cb84e9/sysctl.d", "/sysroot/etc/sysctl.d"}, nil, nil},
-			{"symlink", expectArgs{".host/81ceabb30d37bbdb3868004629cb84e9/systemd", "/sysroot/etc/systemd"}, nil, nil},
-			{"symlink", expectArgs{".host/81ceabb30d37bbdb3868004629cb84e9/terminfo", "/sysroot/etc/terminfo"}, nil, nil},
-			{"symlink", expectArgs{".host/81ceabb30d37bbdb3868004629cb84e9/tmpfiles.d", "/sysroot/etc/tmpfiles.d"}, nil, nil},
-			{"symlink", expectArgs{".host/81ceabb30d37bbdb3868004629cb84e9/udev", "/sysroot/etc/udev"}, nil, nil},
-			{"symlink", expectArgs{".host/81ceabb30d37bbdb3868004629cb84e9/vconsole.conf", "/sysroot/etc/vconsole.conf"}, nil, nil},
-			{"symlink", expectArgs{".host/81ceabb30d37bbdb3868004629cb84e9/X11", "/sysroot/etc/X11"}, nil, nil},
-			{"symlink", expectArgs{".host/81ceabb30d37bbdb3868004629cb84e9/xdg", "/sysroot/etc/xdg"}, nil, nil},
-			{"symlink", expectArgs{".host/81ceabb30d37bbdb3868004629cb84e9/zoneinfo", "/sysroot/etc/zoneinfo"}, nil, nil},
+				"tmpfiles.d", "udev", "vconsole.conf", "X11", "xdg", "zoneinfo"), nil),
+			call("symlink", stub.ExpectArgs{".host/81ceabb30d37bbdb3868004629cb84e9/alsa", "/sysroot/etc/alsa"}, nil, nil),
+			call("symlink", stub.ExpectArgs{".host/81ceabb30d37bbdb3868004629cb84e9/bash_logout", "/sysroot/etc/bash_logout"}, nil, nil),
+			call("symlink", stub.ExpectArgs{".host/81ceabb30d37bbdb3868004629cb84e9/bashrc", "/sysroot/etc/bashrc"}, nil, nil),
+			call("symlink", stub.ExpectArgs{".host/81ceabb30d37bbdb3868004629cb84e9/binfmt.d", "/sysroot/etc/binfmt.d"}, nil, nil),
+			call("symlink", stub.ExpectArgs{".host/81ceabb30d37bbdb3868004629cb84e9/dbus-1", "/sysroot/etc/dbus-1"}, nil, nil),
+			call("symlink", stub.ExpectArgs{".host/81ceabb30d37bbdb3868004629cb84e9/default", "/sysroot/etc/default"}, nil, nil),
+			call("symlink", stub.ExpectArgs{".host/81ceabb30d37bbdb3868004629cb84e9/dhcpcd.exit-hook", "/sysroot/etc/dhcpcd.exit-hook"}, nil, nil),
+			call("symlink", stub.ExpectArgs{".host/81ceabb30d37bbdb3868004629cb84e9/fonts", "/sysroot/etc/fonts"}, nil, nil),
+			call("symlink", stub.ExpectArgs{".host/81ceabb30d37bbdb3868004629cb84e9/fstab", "/sysroot/etc/fstab"}, nil, nil),
+			call("symlink", stub.ExpectArgs{".host/81ceabb30d37bbdb3868004629cb84e9/fuse.conf", "/sysroot/etc/fuse.conf"}, nil, nil),
+			call("symlink", stub.ExpectArgs{".host/81ceabb30d37bbdb3868004629cb84e9/host.conf", "/sysroot/etc/host.conf"}, nil, nil),
+			call("symlink", stub.ExpectArgs{".host/81ceabb30d37bbdb3868004629cb84e9/hostname", "/sysroot/etc/hostname"}, nil, nil),
+			call("symlink", stub.ExpectArgs{".host/81ceabb30d37bbdb3868004629cb84e9/hosts", "/sysroot/etc/hosts"}, nil, nil),
+			call("symlink", stub.ExpectArgs{".host/81ceabb30d37bbdb3868004629cb84e9/hsurc", "/sysroot/etc/hsurc"}, nil, nil),
+			call("symlink", stub.ExpectArgs{".host/81ceabb30d37bbdb3868004629cb84e9/inputrc", "/sysroot/etc/inputrc"}, nil, nil),
+			call("symlink", stub.ExpectArgs{".host/81ceabb30d37bbdb3868004629cb84e9/issue", "/sysroot/etc/issue"}, nil, nil),
+			call("symlink", stub.ExpectArgs{".host/81ceabb30d37bbdb3868004629cb84e9/kbd", "/sysroot/etc/kbd"}, nil, nil),
+			call("symlink", stub.ExpectArgs{".host/81ceabb30d37bbdb3868004629cb84e9/locale.conf", "/sysroot/etc/locale.conf"}, nil, nil),
+			call("symlink", stub.ExpectArgs{".host/81ceabb30d37bbdb3868004629cb84e9/login.defs", "/sysroot/etc/login.defs"}, nil, nil),
+			call("symlink", stub.ExpectArgs{".host/81ceabb30d37bbdb3868004629cb84e9/lsb-release", "/sysroot/etc/lsb-release"}, nil, nil),
+			call("symlink", stub.ExpectArgs{".host/81ceabb30d37bbdb3868004629cb84e9/lvm", "/sysroot/etc/lvm"}, nil, nil),
+			call("symlink", stub.ExpectArgs{".host/81ceabb30d37bbdb3868004629cb84e9/machine-id", "/sysroot/etc/machine-id"}, nil, nil),
+			call("symlink", stub.ExpectArgs{".host/81ceabb30d37bbdb3868004629cb84e9/man_db.conf", "/sysroot/etc/man_db.conf"}, nil, nil),
+			call("symlink", stub.ExpectArgs{".host/81ceabb30d37bbdb3868004629cb84e9/mdadm.conf", "/sysroot/etc/mdadm.conf"}, nil, nil),
+			call("symlink", stub.ExpectArgs{".host/81ceabb30d37bbdb3868004629cb84e9/modprobe.d", "/sysroot/etc/modprobe.d"}, nil, nil),
+			call("symlink", stub.ExpectArgs{".host/81ceabb30d37bbdb3868004629cb84e9/modules-load.d", "/sysroot/etc/modules-load.d"}, nil, nil),
+			call("symlink", stub.ExpectArgs{"/proc/mounts", "/sysroot/etc/mtab"}, nil, nil),
+			call("symlink", stub.ExpectArgs{".host/81ceabb30d37bbdb3868004629cb84e9/nanorc", "/sysroot/etc/nanorc"}, nil, nil),
+			call("symlink", stub.ExpectArgs{".host/81ceabb30d37bbdb3868004629cb84e9/netgroup", "/sysroot/etc/netgroup"}, nil, nil),
+			call("symlink", stub.ExpectArgs{".host/81ceabb30d37bbdb3868004629cb84e9/nix", "/sysroot/etc/nix"}, nil, nil),
+			call("symlink", stub.ExpectArgs{".host/81ceabb30d37bbdb3868004629cb84e9/nixos", "/sysroot/etc/nixos"}, nil, nil),
+			call("symlink", stub.ExpectArgs{".host/81ceabb30d37bbdb3868004629cb84e9/NIXOS", "/sysroot/etc/NIXOS"}, nil, nil),
+			call("symlink", stub.ExpectArgs{".host/81ceabb30d37bbdb3868004629cb84e9/nscd.conf", "/sysroot/etc/nscd.conf"}, nil, nil),
+			call("symlink", stub.ExpectArgs{".host/81ceabb30d37bbdb3868004629cb84e9/nsswitch.conf", "/sysroot/etc/nsswitch.conf"}, nil, nil),
+			call("symlink", stub.ExpectArgs{".host/81ceabb30d37bbdb3868004629cb84e9/os-release", "/sysroot/etc/os-release"}, nil, nil),
+			call("symlink", stub.ExpectArgs{".host/81ceabb30d37bbdb3868004629cb84e9/pam", "/sysroot/etc/pam"}, nil, nil),
+			call("symlink", stub.ExpectArgs{".host/81ceabb30d37bbdb3868004629cb84e9/pam.d", "/sysroot/etc/pam.d"}, nil, nil),
+			call("symlink", stub.ExpectArgs{".host/81ceabb30d37bbdb3868004629cb84e9/pipewire", "/sysroot/etc/pipewire"}, nil, nil),
+			call("symlink", stub.ExpectArgs{".host/81ceabb30d37bbdb3868004629cb84e9/pki", "/sysroot/etc/pki"}, nil, nil),
+			call("symlink", stub.ExpectArgs{".host/81ceabb30d37bbdb3868004629cb84e9/polkit-1", "/sysroot/etc/polkit-1"}, nil, nil),
+			call("symlink", stub.ExpectArgs{".host/81ceabb30d37bbdb3868004629cb84e9/profile", "/sysroot/etc/profile"}, nil, nil),
+			call("symlink", stub.ExpectArgs{".host/81ceabb30d37bbdb3868004629cb84e9/protocols", "/sysroot/etc/protocols"}, nil, nil),
+			call("symlink", stub.ExpectArgs{".host/81ceabb30d37bbdb3868004629cb84e9/resolv.conf", "/sysroot/etc/resolv.conf"}, nil, nil),
+			call("symlink", stub.ExpectArgs{".host/81ceabb30d37bbdb3868004629cb84e9/resolvconf.conf", "/sysroot/etc/resolvconf.conf"}, nil, nil),
+			call("symlink", stub.ExpectArgs{".host/81ceabb30d37bbdb3868004629cb84e9/rpc", "/sysroot/etc/rpc"}, nil, nil),
+			call("symlink", stub.ExpectArgs{".host/81ceabb30d37bbdb3868004629cb84e9/services", "/sysroot/etc/services"}, nil, nil),
+			call("symlink", stub.ExpectArgs{".host/81ceabb30d37bbdb3868004629cb84e9/set-environment", "/sysroot/etc/set-environment"}, nil, nil),
+			call("symlink", stub.ExpectArgs{".host/81ceabb30d37bbdb3868004629cb84e9/shadow", "/sysroot/etc/shadow"}, nil, nil),
+			call("symlink", stub.ExpectArgs{".host/81ceabb30d37bbdb3868004629cb84e9/shells", "/sysroot/etc/shells"}, nil, nil),
+			call("symlink", stub.ExpectArgs{".host/81ceabb30d37bbdb3868004629cb84e9/ssh", "/sysroot/etc/ssh"}, nil, nil),
+			call("symlink", stub.ExpectArgs{".host/81ceabb30d37bbdb3868004629cb84e9/ssl", "/sysroot/etc/ssl"}, nil, nil),
+			call("symlink", stub.ExpectArgs{".host/81ceabb30d37bbdb3868004629cb84e9/static", "/sysroot/etc/static"}, nil, nil),
+			call("symlink", stub.ExpectArgs{".host/81ceabb30d37bbdb3868004629cb84e9/subgid", "/sysroot/etc/subgid"}, nil, nil),
+			call("symlink", stub.ExpectArgs{".host/81ceabb30d37bbdb3868004629cb84e9/subuid", "/sysroot/etc/subuid"}, nil, nil),
+			call("symlink", stub.ExpectArgs{".host/81ceabb30d37bbdb3868004629cb84e9/sudoers", "/sysroot/etc/sudoers"}, nil, nil),
+			call("symlink", stub.ExpectArgs{".host/81ceabb30d37bbdb3868004629cb84e9/sway", "/sysroot/etc/sway"}, nil, nil),
+			call("symlink", stub.ExpectArgs{".host/81ceabb30d37bbdb3868004629cb84e9/sysctl.d", "/sysroot/etc/sysctl.d"}, nil, nil),
+			call("symlink", stub.ExpectArgs{".host/81ceabb30d37bbdb3868004629cb84e9/systemd", "/sysroot/etc/systemd"}, nil, nil),
+			call("symlink", stub.ExpectArgs{".host/81ceabb30d37bbdb3868004629cb84e9/terminfo", "/sysroot/etc/terminfo"}, nil, nil),
+			call("symlink", stub.ExpectArgs{".host/81ceabb30d37bbdb3868004629cb84e9/tmpfiles.d", "/sysroot/etc/tmpfiles.d"}, nil, nil),
+			call("symlink", stub.ExpectArgs{".host/81ceabb30d37bbdb3868004629cb84e9/udev", "/sysroot/etc/udev"}, nil, nil),
+			call("symlink", stub.ExpectArgs{".host/81ceabb30d37bbdb3868004629cb84e9/vconsole.conf", "/sysroot/etc/vconsole.conf"}, nil, nil),
+			call("symlink", stub.ExpectArgs{".host/81ceabb30d37bbdb3868004629cb84e9/X11", "/sysroot/etc/X11"}, nil, nil),
+			call("symlink", stub.ExpectArgs{".host/81ceabb30d37bbdb3868004629cb84e9/xdg", "/sysroot/etc/xdg"}, nil, nil),
+			call("symlink", stub.ExpectArgs{".host/81ceabb30d37bbdb3868004629cb84e9/zoneinfo", "/sysroot/etc/zoneinfo"}, nil, nil),
 		}, nil},

 		{"success", new(Params), &AutoEtcOp{
 			Prefix: "81ceabb30d37bbdb3868004629cb84e9",
-		}, nil, nil, []kexpect{
-			{"mkdirAll", expectArgs{"/sysroot/etc/", os.FileMode(0755)}, nil, nil},
-			{"readdir", expectArgs{"/sysroot/etc/.host/81ceabb30d37bbdb3868004629cb84e9"}, stubDir(
+		}, nil, nil, []stub.Call{
+			call("mkdirAll", stub.ExpectArgs{"/sysroot/etc/", os.FileMode(0755)}, nil, nil),
+			call("readdir", stub.ExpectArgs{"/sysroot/etc/.host/81ceabb30d37bbdb3868004629cb84e9"}, stubDir(
 				"alsa", "bash_logout", "bashrc", "binfmt.d", "dbus-1", "default", "dhcpcd.exit-hook", "fonts",
 				"fstab", "fuse.conf", "group", "host.conf", "hostname", "hosts", "hsurc", "inputrc", "issue", "kbd",
 				"locale.conf", "login.defs", "lsb-release", "lvm", "machine-id", "man_db.conf", "mdadm.conf",
@@ -179,72 +184,72 @@ func TestAutoEtcOp(t *testing.T) {
 				"nsswitch.conf", "os-release", "pam", "pam.d", "passwd", "pipewire", "pki", "polkit-1", "profile",
 				"protocols", "resolv.conf", "resolvconf.conf", "rpc", "services", "set-environment", "shadow", "shells",
 				"ssh", "ssl", "static", "subgid", "subuid", "sudoers", "sway", "sysctl.d", "systemd", "terminfo",
-				"tmpfiles.d", "udev", "vconsole.conf", "X11", "xdg", "zoneinfo"), nil},
-			{"symlink", expectArgs{".host/81ceabb30d37bbdb3868004629cb84e9/alsa", "/sysroot/etc/alsa"}, nil, nil},
-			{"symlink", expectArgs{".host/81ceabb30d37bbdb3868004629cb84e9/bash_logout", "/sysroot/etc/bash_logout"}, nil, nil},
-			{"symlink", expectArgs{".host/81ceabb30d37bbdb3868004629cb84e9/bashrc", "/sysroot/etc/bashrc"}, nil, nil},
-			{"symlink", expectArgs{".host/81ceabb30d37bbdb3868004629cb84e9/binfmt.d", "/sysroot/etc/binfmt.d"}, nil, nil},
-			{"symlink", expectArgs{".host/81ceabb30d37bbdb3868004629cb84e9/dbus-1", "/sysroot/etc/dbus-1"}, nil, nil},
-			{"symlink", expectArgs{".host/81ceabb30d37bbdb3868004629cb84e9/default", "/sysroot/etc/default"}, nil, nil},
-			{"symlink", expectArgs{".host/81ceabb30d37bbdb3868004629cb84e9/dhcpcd.exit-hook", "/sysroot/etc/dhcpcd.exit-hook"}, nil, nil},
-			{"symlink", expectArgs{".host/81ceabb30d37bbdb3868004629cb84e9/fonts", "/sysroot/etc/fonts"}, nil, nil},
-			{"symlink", expectArgs{".host/81ceabb30d37bbdb3868004629cb84e9/fstab", "/sysroot/etc/fstab"}, nil, nil},
-			{"symlink", expectArgs{".host/81ceabb30d37bbdb3868004629cb84e9/fuse.conf", "/sysroot/etc/fuse.conf"}, nil, nil},
-			{"symlink", expectArgs{".host/81ceabb30d37bbdb3868004629cb84e9/host.conf", "/sysroot/etc/host.conf"}, nil, nil},
-			{"symlink", expectArgs{".host/81ceabb30d37bbdb3868004629cb84e9/hostname", "/sysroot/etc/hostname"}, nil, nil},
-			{"symlink", expectArgs{".host/81ceabb30d37bbdb3868004629cb84e9/hosts", "/sysroot/etc/hosts"}, nil, nil},
-			{"symlink", expectArgs{".host/81ceabb30d37bbdb3868004629cb84e9/hsurc", "/sysroot/etc/hsurc"}, nil, nil},
-			{"symlink", expectArgs{".host/81ceabb30d37bbdb3868004629cb84e9/inputrc", "/sysroot/etc/inputrc"}, nil, nil},
-			{"symlink", expectArgs{".host/81ceabb30d37bbdb3868004629cb84e9/issue", "/sysroot/etc/issue"}, nil, nil},
-			{"symlink", expectArgs{".host/81ceabb30d37bbdb3868004629cb84e9/kbd", "/sysroot/etc/kbd"}, nil, nil},
-			{"symlink", expectArgs{".host/81ceabb30d37bbdb3868004629cb84e9/locale.conf", "/sysroot/etc/locale.conf"}, nil, nil},
-			{"symlink", expectArgs{".host/81ceabb30d37bbdb3868004629cb84e9/login.defs", "/sysroot/etc/login.defs"}, nil, nil},
-			{"symlink", expectArgs{".host/81ceabb30d37bbdb3868004629cb84e9/lsb-release", "/sysroot/etc/lsb-release"}, nil, nil},
-			{"symlink", expectArgs{".host/81ceabb30d37bbdb3868004629cb84e9/lvm", "/sysroot/etc/lvm"}, nil, nil},
-			{"symlink", expectArgs{".host/81ceabb30d37bbdb3868004629cb84e9/machine-id", "/sysroot/etc/machine-id"}, nil, nil},
-			{"symlink", expectArgs{".host/81ceabb30d37bbdb3868004629cb84e9/man_db.conf", "/sysroot/etc/man_db.conf"}, nil, nil},
-			{"symlink", expectArgs{".host/81ceabb30d37bbdb3868004629cb84e9/mdadm.conf", "/sysroot/etc/mdadm.conf"}, nil, nil},
-			{"symlink", expectArgs{".host/81ceabb30d37bbdb3868004629cb84e9/modprobe.d", "/sysroot/etc/modprobe.d"}, nil, nil},
-			{"symlink", expectArgs{".host/81ceabb30d37bbdb3868004629cb84e9/modules-load.d", "/sysroot/etc/modules-load.d"}, nil, nil},
-			{"symlink", expectArgs{"/proc/mounts", "/sysroot/etc/mtab"}, nil, nil},
-			{"symlink", expectArgs{".host/81ceabb30d37bbdb3868004629cb84e9/nanorc", "/sysroot/etc/nanorc"}, nil, nil},
-			{"symlink", expectArgs{".host/81ceabb30d37bbdb3868004629cb84e9/netgroup", "/sysroot/etc/netgroup"}, nil, nil},
-			{"symlink", expectArgs{".host/81ceabb30d37bbdb3868004629cb84e9/nix", "/sysroot/etc/nix"}, nil, nil},
-			{"symlink", expectArgs{".host/81ceabb30d37bbdb3868004629cb84e9/nixos", "/sysroot/etc/nixos"}, nil, nil},
-			{"symlink", expectArgs{".host/81ceabb30d37bbdb3868004629cb84e9/NIXOS", "/sysroot/etc/NIXOS"}, nil, nil},
-			{"symlink", expectArgs{".host/81ceabb30d37bbdb3868004629cb84e9/nscd.conf", "/sysroot/etc/nscd.conf"}, nil, nil},
-			{"symlink", expectArgs{".host/81ceabb30d37bbdb3868004629cb84e9/nsswitch.conf", "/sysroot/etc/nsswitch.conf"}, nil, nil},
-			{"symlink", expectArgs{".host/81ceabb30d37bbdb3868004629cb84e9/os-release", "/sysroot/etc/os-release"}, nil, nil},
-			{"symlink", expectArgs{".host/81ceabb30d37bbdb3868004629cb84e9/pam", "/sysroot/etc/pam"}, nil, nil},
-			{"symlink", expectArgs{".host/81ceabb30d37bbdb3868004629cb84e9/pam.d", "/sysroot/etc/pam.d"}, nil, nil},
-			{"symlink", expectArgs{".host/81ceabb30d37bbdb3868004629cb84e9/pipewire", "/sysroot/etc/pipewire"}, nil, nil},
-			{"symlink", expectArgs{".host/81ceabb30d37bbdb3868004629cb84e9/pki", "/sysroot/etc/pki"}, nil, nil},
-			{"symlink", expectArgs{".host/81ceabb30d37bbdb3868004629cb84e9/polkit-1", "/sysroot/etc/polkit-1"}, nil, nil},
-			{"symlink", expectArgs{".host/81ceabb30d37bbdb3868004629cb84e9/profile", "/sysroot/etc/profile"}, nil, nil},
-			{"symlink", expectArgs{".host/81ceabb30d37bbdb3868004629cb84e9/protocols", "/sysroot/etc/protocols"}, nil, nil},
-			{"symlink", expectArgs{".host/81ceabb30d37bbdb3868004629cb84e9/resolv.conf", "/sysroot/etc/resolv.conf"}, nil, nil},
-			{"symlink", expectArgs{".host/81ceabb30d37bbdb3868004629cb84e9/resolvconf.conf", "/sysroot/etc/resolvconf.conf"}, nil, nil},
-			{"symlink", expectArgs{".host/81ceabb30d37bbdb3868004629cb84e9/rpc", "/sysroot/etc/rpc"}, nil, nil},
-			{"symlink", expectArgs{".host/81ceabb30d37bbdb3868004629cb84e9/services", "/sysroot/etc/services"}, nil, nil},
-			{"symlink", expectArgs{".host/81ceabb30d37bbdb3868004629cb84e9/set-environment", "/sysroot/etc/set-environment"}, nil, nil},
-			{"symlink", expectArgs{".host/81ceabb30d37bbdb3868004629cb84e9/shadow", "/sysroot/etc/shadow"}, nil, nil},
-			{"symlink", expectArgs{".host/81ceabb30d37bbdb3868004629cb84e9/shells", "/sysroot/etc/shells"}, nil, nil},
-			{"symlink", expectArgs{".host/81ceabb30d37bbdb3868004629cb84e9/ssh", "/sysroot/etc/ssh"}, nil, nil},
-			{"symlink", expectArgs{".host/81ceabb30d37bbdb3868004629cb84e9/ssl", "/sysroot/etc/ssl"}, nil, nil},
-			{"symlink", expectArgs{".host/81ceabb30d37bbdb3868004629cb84e9/static", "/sysroot/etc/static"}, nil, nil},
-			{"symlink", expectArgs{".host/81ceabb30d37bbdb3868004629cb84e9/subgid", "/sysroot/etc/subgid"}, nil, nil},
-			{"symlink", expectArgs{".host/81ceabb30d37bbdb3868004629cb84e9/subuid", "/sysroot/etc/subuid"}, nil, nil},
-			{"symlink", expectArgs{".host/81ceabb30d37bbdb3868004629cb84e9/sudoers", "/sysroot/etc/sudoers"}, nil, nil},
-			{"symlink", expectArgs{".host/81ceabb30d37bbdb3868004629cb84e9/sway", "/sysroot/etc/sway"}, nil, nil},
-			{"symlink", expectArgs{".host/81ceabb30d37bbdb3868004629cb84e9/sysctl.d", "/sysroot/etc/sysctl.d"}, nil, nil},
-			{"symlink", expectArgs{".host/81ceabb30d37bbdb3868004629cb84e9/systemd", "/sysroot/etc/systemd"}, nil, nil},
-			{"symlink", expectArgs{".host/81ceabb30d37bbdb3868004629cb84e9/terminfo", "/sysroot/etc/terminfo"}, nil, nil},
-			{"symlink", expectArgs{".host/81ceabb30d37bbdb3868004629cb84e9/tmpfiles.d", "/sysroot/etc/tmpfiles.d"}, nil, nil},
-			{"symlink", expectArgs{".host/81ceabb30d37bbdb3868004629cb84e9/udev", "/sysroot/etc/udev"}, nil, nil},
-			{"symlink", expectArgs{".host/81ceabb30d37bbdb3868004629cb84e9/vconsole.conf", "/sysroot/etc/vconsole.conf"}, nil, nil},
-			{"symlink", expectArgs{".host/81ceabb30d37bbdb3868004629cb84e9/X11", "/sysroot/etc/X11"}, nil, nil},
-			{"symlink", expectArgs{".host/81ceabb30d37bbdb3868004629cb84e9/xdg", "/sysroot/etc/xdg"}, nil, nil},
-			{"symlink", expectArgs{".host/81ceabb30d37bbdb3868004629cb84e9/zoneinfo", "/sysroot/etc/zoneinfo"}, nil, nil},
+				"tmpfiles.d", "udev", "vconsole.conf", "X11", "xdg", "zoneinfo"), nil),
+			call("symlink", stub.ExpectArgs{".host/81ceabb30d37bbdb3868004629cb84e9/alsa", "/sysroot/etc/alsa"}, nil, nil),
+			call("symlink", stub.ExpectArgs{".host/81ceabb30d37bbdb3868004629cb84e9/bash_logout", "/sysroot/etc/bash_logout"}, nil, nil),
+			call("symlink", stub.ExpectArgs{".host/81ceabb30d37bbdb3868004629cb84e9/bashrc", "/sysroot/etc/bashrc"}, nil, nil),
+			call("symlink", stub.ExpectArgs{".host/81ceabb30d37bbdb3868004629cb84e9/binfmt.d", "/sysroot/etc/binfmt.d"}, nil, nil),
+			call("symlink", stub.ExpectArgs{".host/81ceabb30d37bbdb3868004629cb84e9/dbus-1", "/sysroot/etc/dbus-1"}, nil, nil),
+			call("symlink", stub.ExpectArgs{".host/81ceabb30d37bbdb3868004629cb84e9/default", "/sysroot/etc/default"}, nil, nil),
+			call("symlink", stub.ExpectArgs{".host/81ceabb30d37bbdb3868004629cb84e9/dhcpcd.exit-hook", "/sysroot/etc/dhcpcd.exit-hook"}, nil, nil),
+			call("symlink", stub.ExpectArgs{".host/81ceabb30d37bbdb3868004629cb84e9/fonts", "/sysroot/etc/fonts"}, nil, nil),
+			call("symlink", stub.ExpectArgs{".host/81ceabb30d37bbdb3868004629cb84e9/fstab", "/sysroot/etc/fstab"}, nil, nil),
+			call("symlink", stub.ExpectArgs{".host/81ceabb30d37bbdb3868004629cb84e9/fuse.conf", "/sysroot/etc/fuse.conf"}, nil, nil),
+			call("symlink", stub.ExpectArgs{".host/81ceabb30d37bbdb3868004629cb84e9/host.conf", "/sysroot/etc/host.conf"}, nil, nil),
+			call("symlink", stub.ExpectArgs{".host/81ceabb30d37bbdb3868004629cb84e9/hostname", "/sysroot/etc/hostname"}, nil, nil),
+			call("symlink", stub.ExpectArgs{".host/81ceabb30d37bbdb3868004629cb84e9/hosts", "/sysroot/etc/hosts"}, nil, nil),
+			call("symlink", stub.ExpectArgs{".host/81ceabb30d37bbdb3868004629cb84e9/hsurc", "/sysroot/etc/hsurc"}, nil, nil),
+			call("symlink", stub.ExpectArgs{".host/81ceabb30d37bbdb3868004629cb84e9/inputrc", "/sysroot/etc/inputrc"}, nil, nil),
+			call("symlink", stub.ExpectArgs{".host/81ceabb30d37bbdb3868004629cb84e9/issue", "/sysroot/etc/issue"}, nil, nil),
+			call("symlink", stub.ExpectArgs{".host/81ceabb30d37bbdb3868004629cb84e9/kbd", "/sysroot/etc/kbd"}, nil, nil),
+			call("symlink", stub.ExpectArgs{".host/81ceabb30d37bbdb3868004629cb84e9/locale.conf", "/sysroot/etc/locale.conf"}, nil, nil),
+			call("symlink", stub.ExpectArgs{".host/81ceabb30d37bbdb3868004629cb84e9/login.defs", "/sysroot/etc/login.defs"}, nil, nil),
+			call("symlink", stub.ExpectArgs{".host/81ceabb30d37bbdb3868004629cb84e9/lsb-release", "/sysroot/etc/lsb-release"}, nil, nil),
+			call("symlink", stub.ExpectArgs{".host/81ceabb30d37bbdb3868004629cb84e9/lvm", "/sysroot/etc/lvm"}, nil, nil),
+			call("symlink", stub.ExpectArgs{".host/81ceabb30d37bbdb3868004629cb84e9/machine-id", "/sysroot/etc/machine-id"}, nil, nil),
+			call("symlink", stub.ExpectArgs{".host/81ceabb30d37bbdb3868004629cb84e9/man_db.conf", "/sysroot/etc/man_db.conf"}, nil, nil),
+			call("symlink", stub.ExpectArgs{".host/81ceabb30d37bbdb3868004629cb84e9/mdadm.conf", "/sysroot/etc/mdadm.conf"}, nil, nil),
+			call("symlink", stub.ExpectArgs{".host/81ceabb30d37bbdb3868004629cb84e9/modprobe.d", "/sysroot/etc/modprobe.d"}, nil, nil),
+			call("symlink", stub.ExpectArgs{".host/81ceabb30d37bbdb3868004629cb84e9/modules-load.d", "/sysroot/etc/modules-load.d"}, nil, nil),
+			call("symlink", stub.ExpectArgs{"/proc/mounts", "/sysroot/etc/mtab"}, nil, nil),
+			call("symlink", stub.ExpectArgs{".host/81ceabb30d37bbdb3868004629cb84e9/nanorc", "/sysroot/etc/nanorc"}, nil, nil),
+			call("symlink", stub.ExpectArgs{".host/81ceabb30d37bbdb3868004629cb84e9/netgroup", "/sysroot/etc/netgroup"}, nil, nil),
+			call("symlink", stub.ExpectArgs{".host/81ceabb30d37bbdb3868004629cb84e9/nix", "/sysroot/etc/nix"}, nil, nil),
+			call("symlink", stub.ExpectArgs{".host/81ceabb30d37bbdb3868004629cb84e9/nixos", "/sysroot/etc/nixos"}, nil, nil),
+			call("symlink", stub.ExpectArgs{".host/81ceabb30d37bbdb3868004629cb84e9/NIXOS", "/sysroot/etc/NIXOS"}, nil, nil),
+			call("symlink", stub.ExpectArgs{".host/81ceabb30d37bbdb3868004629cb84e9/nscd.conf", "/sysroot/etc/nscd.conf"}, nil, nil),
+			call("symlink", stub.ExpectArgs{".host/81ceabb30d37bbdb3868004629cb84e9/nsswitch.conf", "/sysroot/etc/nsswitch.conf"}, nil, nil),
+			call("symlink", stub.ExpectArgs{".host/81ceabb30d37bbdb3868004629cb84e9/os-release", "/sysroot/etc/os-release"}, nil, nil),
+			call("symlink", stub.ExpectArgs{".host/81ceabb30d37bbdb3868004629cb84e9/pam", "/sysroot/etc/pam"}, nil, nil),
+			call("symlink", stub.ExpectArgs{".host/81ceabb30d37bbdb3868004629cb84e9/pam.d", "/sysroot/etc/pam.d"}, nil, nil),
+			call("symlink", stub.ExpectArgs{".host/81ceabb30d37bbdb3868004629cb84e9/pipewire", "/sysroot/etc/pipewire"}, nil, nil),
+			call("symlink", stub.ExpectArgs{".host/81ceabb30d37bbdb3868004629cb84e9/pki", "/sysroot/etc/pki"}, nil, nil),
+			call("symlink", stub.ExpectArgs{".host/81ceabb30d37bbdb3868004629cb84e9/polkit-1", "/sysroot/etc/polkit-1"}, nil, nil),
+			call("symlink", stub.ExpectArgs{".host/81ceabb30d37bbdb3868004629cb84e9/profile", "/sysroot/etc/profile"}, nil, nil),
+			call("symlink", stub.ExpectArgs{".host/81ceabb30d37bbdb3868004629cb84e9/protocols", "/sysroot/etc/protocols"}, nil, nil),
+			call("symlink", stub.ExpectArgs{".host/81ceabb30d37bbdb3868004629cb84e9/resolv.conf", "/sysroot/etc/resolv.conf"}, nil, nil),
+			call("symlink", stub.ExpectArgs{".host/81ceabb30d37bbdb3868004629cb84e9/resolvconf.conf", "/sysroot/etc/resolvconf.conf"}, nil, nil),
+			call("symlink", stub.ExpectArgs{".host/81ceabb30d37bbdb3868004629cb84e9/rpc", "/sysroot/etc/rpc"}, nil, nil),
+			call("symlink", stub.ExpectArgs{".host/81ceabb30d37bbdb3868004629cb84e9/services", "/sysroot/etc/services"}, nil, nil),
+			call("symlink", stub.ExpectArgs{".host/81ceabb30d37bbdb3868004629cb84e9/set-environment", "/sysroot/etc/set-environment"}, nil, nil),
+			call("symlink", stub.ExpectArgs{".host/81ceabb30d37bbdb3868004629cb84e9/shadow", "/sysroot/etc/shadow"}, nil, nil),
+			call("symlink", stub.ExpectArgs{".host/81ceabb30d37bbdb3868004629cb84e9/shells", "/sysroot/etc/shells"}, nil, nil),
+			call("symlink", stub.ExpectArgs{".host/81ceabb30d37bbdb3868004629cb84e9/ssh", "/sysroot/etc/ssh"}, nil, nil),
+			call("symlink", stub.ExpectArgs{".host/81ceabb30d37bbdb3868004629cb84e9/ssl", "/sysroot/etc/ssl"}, nil, nil),
+			call("symlink", stub.ExpectArgs{".host/81ceabb30d37bbdb3868004629cb84e9/static", "/sysroot/etc/static"}, nil, nil),
+			call("symlink", stub.ExpectArgs{".host/81ceabb30d37bbdb3868004629cb84e9/subgid", "/sysroot/etc/subgid"}, nil, nil),
+			call("symlink", stub.ExpectArgs{".host/81ceabb30d37bbdb3868004629cb84e9/subuid", "/sysroot/etc/subuid"}, nil, nil),
+			call("symlink", stub.ExpectArgs{".host/81ceabb30d37bbdb3868004629cb84e9/sudoers", "/sysroot/etc/sudoers"}, nil, nil),
+			call("symlink", stub.ExpectArgs{".host/81ceabb30d37bbdb3868004629cb84e9/sway", "/sysroot/etc/sway"}, nil, nil),
+			call("symlink", stub.ExpectArgs{".host/81ceabb30d37bbdb3868004629cb84e9/sysctl.d", "/sysroot/etc/sysctl.d"}, nil, nil),
+			call("symlink", stub.ExpectArgs{".host/81ceabb30d37bbdb3868004629cb84e9/systemd", "/sysroot/etc/systemd"}, nil, nil),
+			call("symlink", stub.ExpectArgs{".host/81ceabb30d37bbdb3868004629cb84e9/terminfo", "/sysroot/etc/terminfo"}, nil, nil),
+			call("symlink", stub.ExpectArgs{".host/81ceabb30d37bbdb3868004629cb84e9/tmpfiles.d", "/sysroot/etc/tmpfiles.d"}, nil, nil),
+			call("symlink", stub.ExpectArgs{".host/81ceabb30d37bbdb3868004629cb84e9/udev", "/sysroot/etc/udev"}, nil, nil),
+			call("symlink", stub.ExpectArgs{".host/81ceabb30d37bbdb3868004629cb84e9/vconsole.conf", "/sysroot/etc/vconsole.conf"}, nil, nil),
+			call("symlink", stub.ExpectArgs{".host/81ceabb30d37bbdb3868004629cb84e9/X11", "/sysroot/etc/X11"}, nil, nil),
+			call("symlink", stub.ExpectArgs{".host/81ceabb30d37bbdb3868004629cb84e9/xdg", "/sysroot/etc/xdg"}, nil, nil),
+			call("symlink", stub.ExpectArgs{".host/81ceabb30d37bbdb3868004629cb84e9/zoneinfo", "/sysroot/etc/zoneinfo"}, nil, nil),
 		}, nil},
 	})

@@ -255,11 +260,11 @@ func TestAutoEtcOp(t *testing.T) {
 	})

 	checkOpsBuilder(t, []opsBuilderTestCase{
-		{"pd", new(Ops).Etc(MustAbs("/etc/"), "048090b6ed8f9ebb10e275ff5d8c0659"), Ops{
-			&MkdirOp{Path: MustAbs("/etc/"), Perm: 0755},
+		{"pd", new(Ops).Etc(check.MustAbs("/etc/"), "048090b6ed8f9ebb10e275ff5d8c0659"), Ops{
+			&MkdirOp{Path: check.MustAbs("/etc/"), Perm: 0755},
 			&BindMountOp{
-				Source: MustAbs("/etc/"),
-				Target: MustAbs("/etc/.host/048090b6ed8f9ebb10e275ff5d8c0659"),
+				Source: check.MustAbs("/etc/"),
+				Target: check.MustAbs("/etc/.host/048090b6ed8f9ebb10e275ff5d8c0659"),
 			},
 			&AutoEtcOp{Prefix: "048090b6ed8f9ebb10e275ff5d8c0659"},
 		}},
@@ -278,6 +283,7 @@ func TestAutoEtcOp(t *testing.T) {
 	})

 	t.Run("host path rel", func(t *testing.T) {
+		t.Parallel()
 		op := &AutoEtcOp{Prefix: "048090b6ed8f9ebb10e275ff5d8c0659"}
 		wantHostPath := "/etc/.host/048090b6ed8f9ebb10e275ff5d8c0659"
 		wantHostRel := ".host/048090b6ed8f9ebb10e275ff5d8c0659"
--- a/container/autoroot.go
+++ b/container/autoroot.go
@@ -3,42 +3,46 @@ package container
 import (
 	"encoding/gob"
 	"fmt"
-	"io/fs"
+
+	"hakurei.app/container/check"
+	"hakurei.app/container/fhs"
+	"hakurei.app/message"
 )

 func init() { gob.Register(new(AutoRootOp)) }

 // Root appends an [Op] that expands a directory into a toplevel bind mount mirror on container root.
 // This is not a generic setup op. It is implemented here to reduce ipc overhead.
-func (f *Ops) Root(host *Absolute, flags int) *Ops {
+func (f *Ops) Root(host *check.Absolute, flags int) *Ops {
 	*f = append(*f, &AutoRootOp{host, flags, nil})
 	return f
 }

 type AutoRootOp struct {
-	Host *Absolute
+	Host *check.Absolute
 	// passed through to bindMount
 	Flags int

 	// obtained during early;
 	// these wrap the underlying Op because BindMountOp is relatively complex,
 	// so duplicating that code would be unwise
-	resolved []Op
+	resolved []*BindMountOp
 }

 func (r *AutoRootOp) Valid() bool { return r != nil && r.Host != nil }

 func (r *AutoRootOp) early(state *setupState, k syscallDispatcher) error {
 	if d, err := k.readdir(r.Host.String()); err != nil {
-		return wrapErrSelf(err)
+		return err
 	} else {
-		r.resolved = make([]Op, 0, len(d))
+		r.resolved = make([]*BindMountOp, 0, len(d))
 		for _, ent := range d {
 			name := ent.Name()
-			if IsAutoRootBindable(name) {
+			if IsAutoRootBindable(state, name) {
+				// careful: the Valid method is skipped, make sure this is always valid
 				op := &BindMountOp{
 					Source: r.Host.Append(name),
-					Target: AbsFHSRoot.Append(name),
+					Target: fhs.AbsRoot.Append(name),
 					Flags:  r.Flags,
 				}
 				if err = op.early(state, k); err != nil {
@@ -53,12 +57,12 @@ func (r *AutoRootOp) early(state *setupState, k syscallDispatcher) error {

 func (r *AutoRootOp) apply(state *setupState, k syscallDispatcher) error {
 	if state.nonrepeatable&nrAutoRoot != 0 {
-		return msg.WrapErr(fs.ErrInvalid, "autoroot is not repeatable")
+		return OpRepeatError("autoroot")
 	}
 	state.nonrepeatable |= nrAutoRoot

 	for _, op := range r.resolved {
-		k.verbosef("%s %s", op.prefix(), op)
+		// these are exclusively BindMountOp, do not attempt to print identifying message
 		if err := op.apply(state, k); err != nil {
 			return err
 		}
@@ -72,13 +76,13 @@ func (r *AutoRootOp) Is(op Op) bool {
 		r.Host.Is(vr.Host) &&
 		r.Flags == vr.Flags
 }
-func (*AutoRootOp) prefix() string { return "setting up" }
+func (*AutoRootOp) prefix() (string, bool) { return "setting up", true }
 func (r *AutoRootOp) String() string {
 	return fmt.Sprintf("auto root %q flags %#x", r.Host, r.Flags)
 }

 // IsAutoRootBindable returns whether a dir entry name is selected for AutoRoot.
-func IsAutoRootBindable(name string) bool {
+func IsAutoRootBindable(msg message.Msg, name string) bool {
 	switch name {
 	case "proc", "dev", "tmp", "mnt", "etc":

--- a/container/autoroot_test.go
+++ b/container/autoroot_test.go
@@ -2,14 +2,19 @@ package container

 import (
 	"errors"
-	"io/fs"
 	"os"
 	"testing"
+
+	"hakurei.app/container/bits"
+	"hakurei.app/container/check"
+	"hakurei.app/container/stub"
+	"hakurei.app/message"
 )

 func TestAutoRootOp(t *testing.T) {
 	t.Run("nonrepeatable", func(t *testing.T) {
-		wantErr := msg.WrapErr(fs.ErrInvalid, "autoroot is not repeatable")
+		t.Parallel()
+		wantErr := OpRepeatError("autoroot")
 		if err := new(AutoRootOp).apply(&setupState{nonrepeatable: nrAutoRoot}, nil); !errors.Is(err, wantErr) {
 			t.Errorf("apply: error = %v, want %v", err, wantErr)
 		}
@@ -17,116 +22,115 @@ func TestAutoRootOp(t *testing.T) {

 	checkOpBehaviour(t, []opBehaviourTestCase{
 		{"readdir", &Params{ParentPerm: 0750}, &AutoRootOp{
-			Host:  MustAbs("/"),
-			Flags: BindWritable,
-		}, []kexpect{
-			{"readdir", expectArgs{"/"}, stubDir(), errUnique},
-		}, wrapErrSelf(errUnique), nil, nil},
+			Host:  check.MustAbs("/"),
+			Flags: bits.BindWritable,
+		}, []stub.Call{
+			call("readdir", stub.ExpectArgs{"/"}, stubDir(), stub.UniqueError(2)),
+		}, stub.UniqueError(2), nil, nil},

 		{"early", &Params{ParentPerm: 0750}, &AutoRootOp{
-			Host:  MustAbs("/"),
-			Flags: BindWritable,
-		}, []kexpect{
-			{"readdir", expectArgs{"/"}, stubDir("bin", "dev", "etc", "home", "lib64",
-				"lost+found", "mnt", "nix", "proc", "root", "run", "srv", "sys", "tmp", "usr", "var"), nil},
-			{"evalSymlinks", expectArgs{"/bin"}, "", errUnique},
-		}, wrapErrSelf(errUnique), nil, nil},
+			Host:  check.MustAbs("/"),
+			Flags: bits.BindWritable,
+		}, []stub.Call{
+			call("readdir", stub.ExpectArgs{"/"}, stubDir("bin", "dev", "etc", "home", "lib64",
+				"lost+found", "mnt", "nix", "proc", "root", "run", "srv", "sys", "tmp", "usr", "var"), nil),
+			call("evalSymlinks", stub.ExpectArgs{"/bin"}, "", stub.UniqueError(1)),
+		}, stub.UniqueError(1), nil, nil},

 		{"apply", &Params{ParentPerm: 0750}, &AutoRootOp{
-			Host:  MustAbs("/"),
-			Flags: BindWritable,
-		}, []kexpect{
-			{"readdir", expectArgs{"/"}, stubDir("bin", "dev", "etc", "home", "lib64",
-				"lost+found", "mnt", "nix", "proc", "root", "run", "srv", "sys", "tmp", "usr", "var"), nil},
-			{"evalSymlinks", expectArgs{"/bin"}, "/usr/bin", nil},
-			{"evalSymlinks", expectArgs{"/home"}, "/home", nil},
-			{"evalSymlinks", expectArgs{"/lib64"}, "/lib64", nil},
-			{"evalSymlinks", expectArgs{"/lost+found"}, "/lost+found", nil},
-			{"evalSymlinks", expectArgs{"/nix"}, "/nix", nil},
-			{"evalSymlinks", expectArgs{"/root"}, "/root", nil},
-			{"evalSymlinks", expectArgs{"/run"}, "/run", nil},
-			{"evalSymlinks", expectArgs{"/srv"}, "/srv", nil},
-			{"evalSymlinks", expectArgs{"/sys"}, "/sys", nil},
-			{"evalSymlinks", expectArgs{"/usr"}, "/usr", nil},
-			{"evalSymlinks", expectArgs{"/var"}, "/var", nil},
-		}, nil, []kexpect{
-			{"verbosef", expectArgs{"%s %s", []any{"mounting", &BindMountOp{MustAbs("/usr/bin"), MustAbs("/bin"), MustAbs("/bin"), BindWritable}}}, nil, nil},
-			{"stat", expectArgs{"/host/usr/bin"}, isDirFi(false), errUnique},
-		}, wrapErrSelf(errUnique)},
+			Host:  check.MustAbs("/"),
+			Flags: bits.BindWritable,
+		}, []stub.Call{
+			call("readdir", stub.ExpectArgs{"/"}, stubDir("bin", "dev", "etc", "home", "lib64",
+				"lost+found", "mnt", "nix", "proc", "root", "run", "srv", "sys", "tmp", "usr", "var"), nil),
+			call("evalSymlinks", stub.ExpectArgs{"/bin"}, "/usr/bin", nil),
+			call("evalSymlinks", stub.ExpectArgs{"/home"}, "/home", nil),
+			call("evalSymlinks", stub.ExpectArgs{"/lib64"}, "/lib64", nil),
+			call("evalSymlinks", stub.ExpectArgs{"/lost+found"}, "/lost+found", nil),
+			call("evalSymlinks", stub.ExpectArgs{"/nix"}, "/nix", nil),
+			call("evalSymlinks", stub.ExpectArgs{"/root"}, "/root", nil),
+			call("evalSymlinks", stub.ExpectArgs{"/run"}, "/run", nil),
+			call("evalSymlinks", stub.ExpectArgs{"/srv"}, "/srv", nil),
+			call("evalSymlinks", stub.ExpectArgs{"/sys"}, "/sys", nil),
+			call("evalSymlinks", stub.ExpectArgs{"/usr"}, "/usr", nil),
+			call("evalSymlinks", stub.ExpectArgs{"/var"}, "/var", nil),
+		}, nil, []stub.Call{
+			call("stat", stub.ExpectArgs{"/host/usr/bin"}, isDirFi(false), stub.UniqueError(0)),
+		}, stub.UniqueError(0)},

 		{"success pd", &Params{ParentPerm: 0750}, &AutoRootOp{
-			Host:  MustAbs("/"),
-			Flags: BindWritable,
-		}, []kexpect{
-			{"readdir", expectArgs{"/"}, stubDir("bin", "dev", "etc", "home", "lib64",
-				"lost+found", "mnt", "nix", "proc", "root", "run", "srv", "sys", "tmp", "usr", "var"), nil},
-			{"evalSymlinks", expectArgs{"/bin"}, "/usr/bin", nil},
-			{"evalSymlinks", expectArgs{"/home"}, "/home", nil},
-			{"evalSymlinks", expectArgs{"/lib64"}, "/lib64", nil},
-			{"evalSymlinks", expectArgs{"/lost+found"}, "/lost+found", nil},
-			{"evalSymlinks", expectArgs{"/nix"}, "/nix", nil},
-			{"evalSymlinks", expectArgs{"/root"}, "/root", nil},
-			{"evalSymlinks", expectArgs{"/run"}, "/run", nil},
-			{"evalSymlinks", expectArgs{"/srv"}, "/srv", nil},
-			{"evalSymlinks", expectArgs{"/sys"}, "/sys", nil},
-			{"evalSymlinks", expectArgs{"/usr"}, "/usr", nil},
-			{"evalSymlinks", expectArgs{"/var"}, "/var", nil},
-		}, nil, []kexpect{
-			{"verbosef", expectArgs{"%s %s", []any{"mounting", &BindMountOp{MustAbs("/usr/bin"), MustAbs("/bin"), MustAbs("/bin"), BindWritable}}}, nil, nil}, {"stat", expectArgs{"/host/usr/bin"}, isDirFi(true), nil}, {"mkdirAll", expectArgs{"/sysroot/bin", os.FileMode(0700)}, nil, nil}, {"bindMount", expectArgs{"/host/usr/bin", "/sysroot/bin", uintptr(0x4004), false}, nil, nil},
-			{"verbosef", expectArgs{"%s %s", []any{"mounting", &BindMountOp{MustAbs("/home"), MustAbs("/home"), MustAbs("/home"), BindWritable}}}, nil, nil}, {"stat", expectArgs{"/host/home"}, isDirFi(true), nil}, {"mkdirAll", expectArgs{"/sysroot/home", os.FileMode(0700)}, nil, nil}, {"bindMount", expectArgs{"/host/home", "/sysroot/home", uintptr(0x4004), false}, nil, nil},
-			{"verbosef", expectArgs{"%s %s", []any{"mounting", &BindMountOp{MustAbs("/lib64"), MustAbs("/lib64"), MustAbs("/lib64"), BindWritable}}}, nil, nil}, {"stat", expectArgs{"/host/lib64"}, isDirFi(true), nil}, {"mkdirAll", expectArgs{"/sysroot/lib64", os.FileMode(0700)}, nil, nil}, {"bindMount", expectArgs{"/host/lib64", "/sysroot/lib64", uintptr(0x4004), false}, nil, nil},
-			{"verbosef", expectArgs{"%s %s", []any{"mounting", &BindMountOp{MustAbs("/lost+found"), MustAbs("/lost+found"), MustAbs("/lost+found"), BindWritable}}}, nil, nil}, {"stat", expectArgs{"/host/lost+found"}, isDirFi(true), nil}, {"mkdirAll", expectArgs{"/sysroot/lost+found", os.FileMode(0700)}, nil, nil}, {"bindMount", expectArgs{"/host/lost+found", "/sysroot/lost+found", uintptr(0x4004), false}, nil, nil},
-			{"verbosef", expectArgs{"%s %s", []any{"mounting", &BindMountOp{MustAbs("/nix"), MustAbs("/nix"), MustAbs("/nix"), BindWritable}}}, nil, nil}, {"stat", expectArgs{"/host/nix"}, isDirFi(true), nil}, {"mkdirAll", expectArgs{"/sysroot/nix", os.FileMode(0700)}, nil, nil}, {"bindMount", expectArgs{"/host/nix", "/sysroot/nix", uintptr(0x4004), false}, nil, nil},
-			{"verbosef", expectArgs{"%s %s", []any{"mounting", &BindMountOp{MustAbs("/root"), MustAbs("/root"), MustAbs("/root"), BindWritable}}}, nil, nil}, {"stat", expectArgs{"/host/root"}, isDirFi(true), nil}, {"mkdirAll", expectArgs{"/sysroot/root", os.FileMode(0700)}, nil, nil}, {"bindMount", expectArgs{"/host/root", "/sysroot/root", uintptr(0x4004), false}, nil, nil},
-			{"verbosef", expectArgs{"%s %s", []any{"mounting", &BindMountOp{MustAbs("/run"), MustAbs("/run"), MustAbs("/run"), BindWritable}}}, nil, nil}, {"stat", expectArgs{"/host/run"}, isDirFi(true), nil}, {"mkdirAll", expectArgs{"/sysroot/run", os.FileMode(0700)}, nil, nil}, {"bindMount", expectArgs{"/host/run", "/sysroot/run", uintptr(0x4004), false}, nil, nil},
-			{"verbosef", expectArgs{"%s %s", []any{"mounting", &BindMountOp{MustAbs("/srv"), MustAbs("/srv"), MustAbs("/srv"), BindWritable}}}, nil, nil}, {"stat", expectArgs{"/host/srv"}, isDirFi(true), nil}, {"mkdirAll", expectArgs{"/sysroot/srv", os.FileMode(0700)}, nil, nil}, {"bindMount", expectArgs{"/host/srv", "/sysroot/srv", uintptr(0x4004), false}, nil, nil},
-			{"verbosef", expectArgs{"%s %s", []any{"mounting", &BindMountOp{MustAbs("/sys"), MustAbs("/sys"), MustAbs("/sys"), BindWritable}}}, nil, nil}, {"stat", expectArgs{"/host/sys"}, isDirFi(true), nil}, {"mkdirAll", expectArgs{"/sysroot/sys", os.FileMode(0700)}, nil, nil}, {"bindMount", expectArgs{"/host/sys", "/sysroot/sys", uintptr(0x4004), false}, nil, nil},
-			{"verbosef", expectArgs{"%s %s", []any{"mounting", &BindMountOp{MustAbs("/usr"), MustAbs("/usr"), MustAbs("/usr"), BindWritable}}}, nil, nil}, {"stat", expectArgs{"/host/usr"}, isDirFi(true), nil}, {"mkdirAll", expectArgs{"/sysroot/usr", os.FileMode(0700)}, nil, nil}, {"bindMount", expectArgs{"/host/usr", "/sysroot/usr", uintptr(0x4004), false}, nil, nil},
-			{"verbosef", expectArgs{"%s %s", []any{"mounting", &BindMountOp{MustAbs("/var"), MustAbs("/var"), MustAbs("/var"), BindWritable}}}, nil, nil}, {"stat", expectArgs{"/host/var"}, isDirFi(true), nil}, {"mkdirAll", expectArgs{"/sysroot/var", os.FileMode(0700)}, nil, nil}, {"bindMount", expectArgs{"/host/var", "/sysroot/var", uintptr(0x4004), false}, nil, nil},
+			Host:  check.MustAbs("/"),
+			Flags: bits.BindWritable,
+		}, []stub.Call{
+			call("readdir", stub.ExpectArgs{"/"}, stubDir("bin", "dev", "etc", "home", "lib64",
+				"lost+found", "mnt", "nix", "proc", "root", "run", "srv", "sys", "tmp", "usr", "var"), nil),
+			call("evalSymlinks", stub.ExpectArgs{"/bin"}, "/usr/bin", nil),
+			call("evalSymlinks", stub.ExpectArgs{"/home"}, "/home", nil),
+			call("evalSymlinks", stub.ExpectArgs{"/lib64"}, "/lib64", nil),
+			call("evalSymlinks", stub.ExpectArgs{"/lost+found"}, "/lost+found", nil),
+			call("evalSymlinks", stub.ExpectArgs{"/nix"}, "/nix", nil),
+			call("evalSymlinks", stub.ExpectArgs{"/root"}, "/root", nil),
+			call("evalSymlinks", stub.ExpectArgs{"/run"}, "/run", nil),
+			call("evalSymlinks", stub.ExpectArgs{"/srv"}, "/srv", nil),
+			call("evalSymlinks", stub.ExpectArgs{"/sys"}, "/sys", nil),
+			call("evalSymlinks", stub.ExpectArgs{"/usr"}, "/usr", nil),
+			call("evalSymlinks", stub.ExpectArgs{"/var"}, "/var", nil),
+		}, nil, []stub.Call{
+			call("stat", stub.ExpectArgs{"/host/usr/bin"}, isDirFi(true), nil), call("mkdirAll", stub.ExpectArgs{"/sysroot/bin", os.FileMode(0700)}, nil, nil), call("verbosef", stub.ExpectArgs{"mounting %q on %q flags %#x", []any{"/host/usr/bin", "/sysroot/bin", uintptr(0x4004)}}, nil, nil), call("bindMount", stub.ExpectArgs{"/host/usr/bin", "/sysroot/bin", uintptr(0x4004), false}, nil, nil),
+			call("stat", stub.ExpectArgs{"/host/home"}, isDirFi(true), nil), call("mkdirAll", stub.ExpectArgs{"/sysroot/home", os.FileMode(0700)}, nil, nil), call("verbosef", stub.ExpectArgs{"mounting %q flags %#x", []any{"/sysroot/home", uintptr(0x4004)}}, nil, nil), call("bindMount", stub.ExpectArgs{"/host/home", "/sysroot/home", uintptr(0x4004), false}, nil, nil),
+			call("stat", stub.ExpectArgs{"/host/lib64"}, isDirFi(true), nil), call("mkdirAll", stub.ExpectArgs{"/sysroot/lib64", os.FileMode(0700)}, nil, nil), call("verbosef", stub.ExpectArgs{"mounting %q flags %#x", []any{"/sysroot/lib64", uintptr(0x4004)}}, nil, nil), call("bindMount", stub.ExpectArgs{"/host/lib64", "/sysroot/lib64", uintptr(0x4004), false}, nil, nil),
+			call("stat", stub.ExpectArgs{"/host/lost+found"}, isDirFi(true), nil), call("mkdirAll", stub.ExpectArgs{"/sysroot/lost+found", os.FileMode(0700)}, nil, nil), call("verbosef", stub.ExpectArgs{"mounting %q flags %#x", []any{"/sysroot/lost+found", uintptr(0x4004)}}, nil, nil), call("bindMount", stub.ExpectArgs{"/host/lost+found", "/sysroot/lost+found", uintptr(0x4004), false}, nil, nil),
+			call("stat", stub.ExpectArgs{"/host/nix"}, isDirFi(true), nil), call("mkdirAll", stub.ExpectArgs{"/sysroot/nix", os.FileMode(0700)}, nil, nil), call("verbosef", stub.ExpectArgs{"mounting %q flags %#x", []any{"/sysroot/nix", uintptr(0x4004)}}, nil, nil), call("bindMount", stub.ExpectArgs{"/host/nix", "/sysroot/nix", uintptr(0x4004), false}, nil, nil),
+			call("stat", stub.ExpectArgs{"/host/root"}, isDirFi(true), nil), call("mkdirAll", stub.ExpectArgs{"/sysroot/root", os.FileMode(0700)}, nil, nil), call("verbosef", stub.ExpectArgs{"mounting %q flags %#x", []any{"/sysroot/root", uintptr(0x4004)}}, nil, nil), call("bindMount", stub.ExpectArgs{"/host/root", "/sysroot/root", uintptr(0x4004), false}, nil, nil),
+			call("stat", stub.ExpectArgs{"/host/run"}, isDirFi(true), nil), call("mkdirAll", stub.ExpectArgs{"/sysroot/run", os.FileMode(0700)}, nil, nil), call("verbosef", stub.ExpectArgs{"mounting %q flags %#x", []any{"/sysroot/run", uintptr(0x4004)}}, nil, nil), call("bindMount", stub.ExpectArgs{"/host/run", "/sysroot/run", uintptr(0x4004), false}, nil, nil),
+			call("stat", stub.ExpectArgs{"/host/srv"}, isDirFi(true), nil), call("mkdirAll", stub.ExpectArgs{"/sysroot/srv", os.FileMode(0700)}, nil, nil), call("verbosef", stub.ExpectArgs{"mounting %q flags %#x", []any{"/sysroot/srv", uintptr(0x4004)}}, nil, nil), call("bindMount", stub.ExpectArgs{"/host/srv", "/sysroot/srv", uintptr(0x4004), false}, nil, nil),
+			call("stat", stub.ExpectArgs{"/host/sys"}, isDirFi(true), nil), call("mkdirAll", stub.ExpectArgs{"/sysroot/sys", os.FileMode(0700)}, nil, nil), call("verbosef", stub.ExpectArgs{"mounting %q flags %#x", []any{"/sysroot/sys", uintptr(0x4004)}}, nil, nil), call("bindMount", stub.ExpectArgs{"/host/sys", "/sysroot/sys", uintptr(0x4004), false}, nil, nil),
+			call("stat", stub.ExpectArgs{"/host/usr"}, isDirFi(true), nil), call("mkdirAll", stub.ExpectArgs{"/sysroot/usr", os.FileMode(0700)}, nil, nil), call("verbosef", stub.ExpectArgs{"mounting %q flags %#x", []any{"/sysroot/usr", uintptr(0x4004)}}, nil, nil), call("bindMount", stub.ExpectArgs{"/host/usr", "/sysroot/usr", uintptr(0x4004), false}, nil, nil),
+			call("stat", stub.ExpectArgs{"/host/var"}, isDirFi(true), nil), call("mkdirAll", stub.ExpectArgs{"/sysroot/var", os.FileMode(0700)}, nil, nil), call("verbosef", stub.ExpectArgs{"mounting %q flags %#x", []any{"/sysroot/var", uintptr(0x4004)}}, nil, nil), call("bindMount", stub.ExpectArgs{"/host/var", "/sysroot/var", uintptr(0x4004), false}, nil, nil),
 		}, nil},

 		{"success", &Params{ParentPerm: 0750}, &AutoRootOp{
-			Host: MustAbs("/var/lib/planterette/base/debian:f92c9052"),
-		}, []kexpect{
-			{"readdir", expectArgs{"/var/lib/planterette/base/debian:f92c9052"}, stubDir("bin", "dev", "etc", "home", "lib64",
-				"lost+found", "mnt", "nix", "proc", "root", "run", "srv", "sys", "tmp", "usr", "var"), nil},
-			{"evalSymlinks", expectArgs{"/var/lib/planterette/base/debian:f92c9052/bin"}, "/var/lib/planterette/base/debian:f92c9052/usr/bin", nil},
-			{"evalSymlinks", expectArgs{"/var/lib/planterette/base/debian:f92c9052/home"}, "/var/lib/planterette/base/debian:f92c9052/home", nil},
-			{"evalSymlinks", expectArgs{"/var/lib/planterette/base/debian:f92c9052/lib64"}, "/var/lib/planterette/base/debian:f92c9052/lib64", nil},
-			{"evalSymlinks", expectArgs{"/var/lib/planterette/base/debian:f92c9052/lost+found"}, "/var/lib/planterette/base/debian:f92c9052/lost+found", nil},
-			{"evalSymlinks", expectArgs{"/var/lib/planterette/base/debian:f92c9052/nix"}, "/var/lib/planterette/base/debian:f92c9052/nix", nil},
-			{"evalSymlinks", expectArgs{"/var/lib/planterette/base/debian:f92c9052/root"}, "/var/lib/planterette/base/debian:f92c9052/root", nil},
-			{"evalSymlinks", expectArgs{"/var/lib/planterette/base/debian:f92c9052/run"}, "/var/lib/planterette/base/debian:f92c9052/run", nil},
-			{"evalSymlinks", expectArgs{"/var/lib/planterette/base/debian:f92c9052/srv"}, "/var/lib/planterette/base/debian:f92c9052/srv", nil},
-			{"evalSymlinks", expectArgs{"/var/lib/planterette/base/debian:f92c9052/sys"}, "/var/lib/planterette/base/debian:f92c9052/sys", nil},
-			{"evalSymlinks", expectArgs{"/var/lib/planterette/base/debian:f92c9052/usr"}, "/var/lib/planterette/base/debian:f92c9052/usr", nil},
-			{"evalSymlinks", expectArgs{"/var/lib/planterette/base/debian:f92c9052/var"}, "/var/lib/planterette/base/debian:f92c9052/var", nil},
-		}, nil, []kexpect{
-			{"verbosef", expectArgs{"%s %s", []any{"mounting", &BindMountOp{MustAbs("/var/lib/planterette/base/debian:f92c9052/usr/bin"), MustAbs("/var/lib/planterette/base/debian:f92c9052/bin"), MustAbs("/bin"), 0}}}, nil, nil}, {"stat", expectArgs{"/host/var/lib/planterette/base/debian:f92c9052/usr/bin"}, isDirFi(true), nil}, {"mkdirAll", expectArgs{"/sysroot/bin", os.FileMode(0700)}, nil, nil}, {"bindMount", expectArgs{"/host/var/lib/planterette/base/debian:f92c9052/usr/bin", "/sysroot/bin", uintptr(0x4005), false}, nil, nil},
-			{"verbosef", expectArgs{"%s %s", []any{"mounting", &BindMountOp{MustAbs("/var/lib/planterette/base/debian:f92c9052/home"), MustAbs("/var/lib/planterette/base/debian:f92c9052/home"), MustAbs("/home"), 0}}}, nil, nil}, {"stat", expectArgs{"/host/var/lib/planterette/base/debian:f92c9052/home"}, isDirFi(true), nil}, {"mkdirAll", expectArgs{"/sysroot/home", os.FileMode(0700)}, nil, nil}, {"bindMount", expectArgs{"/host/var/lib/planterette/base/debian:f92c9052/home", "/sysroot/home", uintptr(0x4005), false}, nil, nil},
-			{"verbosef", expectArgs{"%s %s", []any{"mounting", &BindMountOp{MustAbs("/var/lib/planterette/base/debian:f92c9052/lib64"), MustAbs("/var/lib/planterette/base/debian:f92c9052/lib64"), MustAbs("/lib64"), 0}}}, nil, nil}, {"stat", expectArgs{"/host/var/lib/planterette/base/debian:f92c9052/lib64"}, isDirFi(true), nil}, {"mkdirAll", expectArgs{"/sysroot/lib64", os.FileMode(0700)}, nil, nil}, {"bindMount", expectArgs{"/host/var/lib/planterette/base/debian:f92c9052/lib64", "/sysroot/lib64", uintptr(0x4005), false}, nil, nil},
-			{"verbosef", expectArgs{"%s %s", []any{"mounting", &BindMountOp{MustAbs("/var/lib/planterette/base/debian:f92c9052/lost+found"), MustAbs("/var/lib/planterette/base/debian:f92c9052/lost+found"), MustAbs("/lost+found"), 0}}}, nil, nil}, {"stat", expectArgs{"/host/var/lib/planterette/base/debian:f92c9052/lost+found"}, isDirFi(true), nil}, {"mkdirAll", expectArgs{"/sysroot/lost+found", os.FileMode(0700)}, nil, nil}, {"bindMount", expectArgs{"/host/var/lib/planterette/base/debian:f92c9052/lost+found", "/sysroot/lost+found", uintptr(0x4005), false}, nil, nil},
-			{"verbosef", expectArgs{"%s %s", []any{"mounting", &BindMountOp{MustAbs("/var/lib/planterette/base/debian:f92c9052/nix"), MustAbs("/var/lib/planterette/base/debian:f92c9052/nix"), MustAbs("/nix"), 0}}}, nil, nil}, {"stat", expectArgs{"/host/var/lib/planterette/base/debian:f92c9052/nix"}, isDirFi(true), nil}, {"mkdirAll", expectArgs{"/sysroot/nix", os.FileMode(0700)}, nil, nil}, {"bindMount", expectArgs{"/host/var/lib/planterette/base/debian:f92c9052/nix", "/sysroot/nix", uintptr(0x4005), false}, nil, nil},
-			{"verbosef", expectArgs{"%s %s", []any{"mounting", &BindMountOp{MustAbs("/var/lib/planterette/base/debian:f92c9052/root"), MustAbs("/var/lib/planterette/base/debian:f92c9052/root"), MustAbs("/root"), 0}}}, nil, nil}, {"stat", expectArgs{"/host/var/lib/planterette/base/debian:f92c9052/root"}, isDirFi(true), nil}, {"mkdirAll", expectArgs{"/sysroot/root", os.FileMode(0700)}, nil, nil}, {"bindMount", expectArgs{"/host/var/lib/planterette/base/debian:f92c9052/root", "/sysroot/root", uintptr(0x4005), false}, nil, nil},
-			{"verbosef", expectArgs{"%s %s", []any{"mounting", &BindMountOp{MustAbs("/var/lib/planterette/base/debian:f92c9052/run"), MustAbs("/var/lib/planterette/base/debian:f92c9052/run"), MustAbs("/run"), 0}}}, nil, nil}, {"stat", expectArgs{"/host/var/lib/planterette/base/debian:f92c9052/run"}, isDirFi(true), nil}, {"mkdirAll", expectArgs{"/sysroot/run", os.FileMode(0700)}, nil, nil}, {"bindMount", expectArgs{"/host/var/lib/planterette/base/debian:f92c9052/run", "/sysroot/run", uintptr(0x4005), false}, nil, nil},
-			{"verbosef", expectArgs{"%s %s", []any{"mounting", &BindMountOp{MustAbs("/var/lib/planterette/base/debian:f92c9052/srv"), MustAbs("/var/lib/planterette/base/debian:f92c9052/srv"), MustAbs("/srv"), 0}}}, nil, nil}, {"stat", expectArgs{"/host/var/lib/planterette/base/debian:f92c9052/srv"}, isDirFi(true), nil}, {"mkdirAll", expectArgs{"/sysroot/srv", os.FileMode(0700)}, nil, nil}, {"bindMount", expectArgs{"/host/var/lib/planterette/base/debian:f92c9052/srv", "/sysroot/srv", uintptr(0x4005), false}, nil, nil},
-			{"verbosef", expectArgs{"%s %s", []any{"mounting", &BindMountOp{MustAbs("/var/lib/planterette/base/debian:f92c9052/sys"), MustAbs("/var/lib/planterette/base/debian:f92c9052/sys"), MustAbs("/sys"), 0}}}, nil, nil}, {"stat", expectArgs{"/host/var/lib/planterette/base/debian:f92c9052/sys"}, isDirFi(true), nil}, {"mkdirAll", expectArgs{"/sysroot/sys", os.FileMode(0700)}, nil, nil}, {"bindMount", expectArgs{"/host/var/lib/planterette/base/debian:f92c9052/sys", "/sysroot/sys", uintptr(0x4005), false}, nil, nil},
-			{"verbosef", expectArgs{"%s %s", []any{"mounting", &BindMountOp{MustAbs("/var/lib/planterette/base/debian:f92c9052/usr"), MustAbs("/var/lib/planterette/base/debian:f92c9052/usr"), MustAbs("/usr"), 0}}}, nil, nil}, {"stat", expectArgs{"/host/var/lib/planterette/base/debian:f92c9052/usr"}, isDirFi(true), nil}, {"mkdirAll", expectArgs{"/sysroot/usr", os.FileMode(0700)}, nil, nil}, {"bindMount", expectArgs{"/host/var/lib/planterette/base/debian:f92c9052/usr", "/sysroot/usr", uintptr(0x4005), false}, nil, nil},
-			{"verbosef", expectArgs{"%s %s", []any{"mounting", &BindMountOp{MustAbs("/var/lib/planterette/base/debian:f92c9052/var"), MustAbs("/var/lib/planterette/base/debian:f92c9052/var"), MustAbs("/var"), 0}}}, nil, nil}, {"stat", expectArgs{"/host/var/lib/planterette/base/debian:f92c9052/var"}, isDirFi(true), nil}, {"mkdirAll", expectArgs{"/sysroot/var", os.FileMode(0700)}, nil, nil}, {"bindMount", expectArgs{"/host/var/lib/planterette/base/debian:f92c9052/var", "/sysroot/var", uintptr(0x4005), false}, nil, nil},
+			Host: check.MustAbs("/var/lib/planterette/base/debian:f92c9052"),
+		}, []stub.Call{
+			call("readdir", stub.ExpectArgs{"/var/lib/planterette/base/debian:f92c9052"}, stubDir("bin", "dev", "etc", "home", "lib64",
+				"lost+found", "mnt", "nix", "proc", "root", "run", "srv", "sys", "tmp", "usr", "var"), nil),
+			call("evalSymlinks", stub.ExpectArgs{"/var/lib/planterette/base/debian:f92c9052/bin"}, "/var/lib/planterette/base/debian:f92c9052/usr/bin", nil),
+			call("evalSymlinks", stub.ExpectArgs{"/var/lib/planterette/base/debian:f92c9052/home"}, "/var/lib/planterette/base/debian:f92c9052/home", nil),
+			call("evalSymlinks", stub.ExpectArgs{"/var/lib/planterette/base/debian:f92c9052/lib64"}, "/var/lib/planterette/base/debian:f92c9052/lib64", nil),
+			call("evalSymlinks", stub.ExpectArgs{"/var/lib/planterette/base/debian:f92c9052/lost+found"}, "/var/lib/planterette/base/debian:f92c9052/lost+found", nil),
+			call("evalSymlinks", stub.ExpectArgs{"/var/lib/planterette/base/debian:f92c9052/nix"}, "/var/lib/planterette/base/debian:f92c9052/nix", nil),
+			call("evalSymlinks", stub.ExpectArgs{"/var/lib/planterette/base/debian:f92c9052/root"}, "/var/lib/planterette/base/debian:f92c9052/root", nil),
+			call("evalSymlinks", stub.ExpectArgs{"/var/lib/planterette/base/debian:f92c9052/run"}, "/var/lib/planterette/base/debian:f92c9052/run", nil),
+			call("evalSymlinks", stub.ExpectArgs{"/var/lib/planterette/base/debian:f92c9052/srv"}, "/var/lib/planterette/base/debian:f92c9052/srv", nil),
+			call("evalSymlinks", stub.ExpectArgs{"/var/lib/planterette/base/debian:f92c9052/sys"}, "/var/lib/planterette/base/debian:f92c9052/sys", nil),
+			call("evalSymlinks", stub.ExpectArgs{"/var/lib/planterette/base/debian:f92c9052/usr"}, "/var/lib/planterette/base/debian:f92c9052/usr", nil),
+			call("evalSymlinks", stub.ExpectArgs{"/var/lib/planterette/base/debian:f92c9052/var"}, "/var/lib/planterette/base/debian:f92c9052/var", nil),
+		}, nil, []stub.Call{
+			call("stat", stub.ExpectArgs{"/host/var/lib/planterette/base/debian:f92c9052/usr/bin"}, isDirFi(true), nil), call("mkdirAll", stub.ExpectArgs{"/sysroot/bin", os.FileMode(0700)}, nil, nil), call("verbosef", stub.ExpectArgs{"mounting %q on %q flags %#x", []any{"/host/var/lib/planterette/base/debian:f92c9052/usr/bin", "/sysroot/bin", uintptr(0x4005)}}, nil, nil), call("bindMount", stub.ExpectArgs{"/host/var/lib/planterette/base/debian:f92c9052/usr/bin", "/sysroot/bin", uintptr(0x4005), false}, nil, nil),
+			call("stat", stub.ExpectArgs{"/host/var/lib/planterette/base/debian:f92c9052/home"}, isDirFi(true), nil), call("mkdirAll", stub.ExpectArgs{"/sysroot/home", os.FileMode(0700)}, nil, nil), call("verbosef", stub.ExpectArgs{"mounting %q on %q flags %#x", []any{"/host/var/lib/planterette/base/debian:f92c9052/home", "/sysroot/home", uintptr(0x4005)}}, nil, nil), call("bindMount", stub.ExpectArgs{"/host/var/lib/planterette/base/debian:f92c9052/home", "/sysroot/home", uintptr(0x4005), false}, nil, nil),
+			call("stat", stub.ExpectArgs{"/host/var/lib/planterette/base/debian:f92c9052/lib64"}, isDirFi(true), nil), call("mkdirAll", stub.ExpectArgs{"/sysroot/lib64", os.FileMode(0700)}, nil, nil), call("verbosef", stub.ExpectArgs{"mounting %q on %q flags %#x", []any{"/host/var/lib/planterette/base/debian:f92c9052/lib64", "/sysroot/lib64", uintptr(0x4005)}}, nil, nil), call("bindMount", stub.ExpectArgs{"/host/var/lib/planterette/base/debian:f92c9052/lib64", "/sysroot/lib64", uintptr(0x4005), false}, nil, nil),
+			call("stat", stub.ExpectArgs{"/host/var/lib/planterette/base/debian:f92c9052/lost+found"}, isDirFi(true), nil), call("mkdirAll", stub.ExpectArgs{"/sysroot/lost+found", os.FileMode(0700)}, nil, nil), call("verbosef", stub.ExpectArgs{"mounting %q on %q flags %#x", []any{"/host/var/lib/planterette/base/debian:f92c9052/lost+found", "/sysroot/lost+found", uintptr(0x4005)}}, nil, nil), call("bindMount", stub.ExpectArgs{"/host/var/lib/planterette/base/debian:f92c9052/lost+found", "/sysroot/lost+found", uintptr(0x4005), false}, nil, nil),
+			call("stat", stub.ExpectArgs{"/host/var/lib/planterette/base/debian:f92c9052/nix"}, isDirFi(true), nil), call("mkdirAll", stub.ExpectArgs{"/sysroot/nix", os.FileMode(0700)}, nil, nil), call("verbosef", stub.ExpectArgs{"mounting %q on %q flags %#x", []any{"/host/var/lib/planterette/base/debian:f92c9052/nix", "/sysroot/nix", uintptr(0x4005)}}, nil, nil), call("bindMount", stub.ExpectArgs{"/host/var/lib/planterette/base/debian:f92c9052/nix", "/sysroot/nix", uintptr(0x4005), false}, nil, nil),
+			call("stat", stub.ExpectArgs{"/host/var/lib/planterette/base/debian:f92c9052/root"}, isDirFi(true), nil), call("mkdirAll", stub.ExpectArgs{"/sysroot/root", os.FileMode(0700)}, nil, nil), call("verbosef", stub.ExpectArgs{"mounting %q on %q flags %#x", []any{"/host/var/lib/planterette/base/debian:f92c9052/root", "/sysroot/root", uintptr(0x4005)}}, nil, nil), call("bindMount", stub.ExpectArgs{"/host/var/lib/planterette/base/debian:f92c9052/root", "/sysroot/root", uintptr(0x4005), false}, nil, nil),
+			call("stat", stub.ExpectArgs{"/host/var/lib/planterette/base/debian:f92c9052/run"}, isDirFi(true), nil), call("mkdirAll", stub.ExpectArgs{"/sysroot/run", os.FileMode(0700)}, nil, nil), call("verbosef", stub.ExpectArgs{"mounting %q on %q flags %#x", []any{"/host/var/lib/planterette/base/debian:f92c9052/run", "/sysroot/run", uintptr(0x4005)}}, nil, nil), call("bindMount", stub.ExpectArgs{"/host/var/lib/planterette/base/debian:f92c9052/run", "/sysroot/run", uintptr(0x4005), false}, nil, nil),
+			call("stat", stub.ExpectArgs{"/host/var/lib/planterette/base/debian:f92c9052/srv"}, isDirFi(true), nil), call("mkdirAll", stub.ExpectArgs{"/sysroot/srv", os.FileMode(0700)}, nil, nil), call("verbosef", stub.ExpectArgs{"mounting %q on %q flags %#x", []any{"/host/var/lib/planterette/base/debian:f92c9052/srv", "/sysroot/srv", uintptr(0x4005)}}, nil, nil), call("bindMount", stub.ExpectArgs{"/host/var/lib/planterette/base/debian:f92c9052/srv", "/sysroot/srv", uintptr(0x4005), false}, nil, nil),
+			call("stat", stub.ExpectArgs{"/host/var/lib/planterette/base/debian:f92c9052/sys"}, isDirFi(true), nil), call("mkdirAll", stub.ExpectArgs{"/sysroot/sys", os.FileMode(0700)}, nil, nil), call("verbosef", stub.ExpectArgs{"mounting %q on %q flags %#x", []any{"/host/var/lib/planterette/base/debian:f92c9052/sys", "/sysroot/sys", uintptr(0x4005)}}, nil, nil), call("bindMount", stub.ExpectArgs{"/host/var/lib/planterette/base/debian:f92c9052/sys", "/sysroot/sys", uintptr(0x4005), false}, nil, nil),
+			call("stat", stub.ExpectArgs{"/host/var/lib/planterette/base/debian:f92c9052/usr"}, isDirFi(true), nil), call("mkdirAll", stub.ExpectArgs{"/sysroot/usr", os.FileMode(0700)}, nil, nil), call("verbosef", stub.ExpectArgs{"mounting %q on %q flags %#x", []any{"/host/var/lib/planterette/base/debian:f92c9052/usr", "/sysroot/usr", uintptr(0x4005)}}, nil, nil), call("bindMount", stub.ExpectArgs{"/host/var/lib/planterette/base/debian:f92c9052/usr", "/sysroot/usr", uintptr(0x4005), false}, nil, nil),
+			call("stat", stub.ExpectArgs{"/host/var/lib/planterette/base/debian:f92c9052/var"}, isDirFi(true), nil), call("mkdirAll", stub.ExpectArgs{"/sysroot/var", os.FileMode(0700)}, nil, nil), call("verbosef", stub.ExpectArgs{"mounting %q on %q flags %#x", []any{"/host/var/lib/planterette/base/debian:f92c9052/var", "/sysroot/var", uintptr(0x4005)}}, nil, nil), call("bindMount", stub.ExpectArgs{"/host/var/lib/planterette/base/debian:f92c9052/var", "/sysroot/var", uintptr(0x4005), false}, nil, nil),
 		}, nil},
 	})

 	checkOpsValid(t, []opValidTestCase{
 		{"nil", (*AutoRootOp)(nil), false},
 		{"zero", new(AutoRootOp), false},
-		{"valid", &AutoRootOp{Host: MustAbs("/")}, true},
+		{"valid", &AutoRootOp{Host: check.MustAbs("/")}, true},
 	})

 	checkOpsBuilder(t, []opsBuilderTestCase{
-		{"pd", new(Ops).Root(MustAbs("/"), BindWritable), Ops{
+		{"pd", new(Ops).Root(check.MustAbs("/"), bits.BindWritable), Ops{
 			&AutoRootOp{
-				Host:  MustAbs("/"),
-				Flags: BindWritable,
+				Host:  check.MustAbs("/"),
+				Flags: bits.BindWritable,
 			},
 		}},
 	})
@@ -135,64 +139,74 @@ func TestAutoRootOp(t *testing.T) {
 		{"zero", new(AutoRootOp), new(AutoRootOp), false},

 		{"internal ne", &AutoRootOp{
-			Host:  MustAbs("/"),
-			Flags: BindWritable,
+			Host:  check.MustAbs("/"),
+			Flags: bits.BindWritable,
 		}, &AutoRootOp{
-			Host:     MustAbs("/"),
-			Flags:    BindWritable,
-			resolved: []Op{new(BindMountOp)},
+			Host:     check.MustAbs("/"),
+			Flags:    bits.BindWritable,
+			resolved: []*BindMountOp{new(BindMountOp)},
 		}, true},

 		{"flags differs", &AutoRootOp{
-			Host:  MustAbs("/"),
-			Flags: BindWritable | BindDevice,
+			Host:  check.MustAbs("/"),
+			Flags: bits.BindWritable | bits.BindDevice,
 		}, &AutoRootOp{
-			Host:  MustAbs("/"),
-			Flags: BindWritable,
+			Host:  check.MustAbs("/"),
+			Flags: bits.BindWritable,
 		}, false},

 		{"host differs", &AutoRootOp{
-			Host:  MustAbs("/tmp/"),
-			Flags: BindWritable,
+			Host:  check.MustAbs("/tmp/"),
+			Flags: bits.BindWritable,
 		}, &AutoRootOp{
-			Host:  MustAbs("/"),
-			Flags: BindWritable,
+			Host:  check.MustAbs("/"),
+			Flags: bits.BindWritable,
 		}, false},

 		{"equals", &AutoRootOp{
-			Host:  MustAbs("/"),
-			Flags: BindWritable,
+			Host:  check.MustAbs("/"),
+			Flags: bits.BindWritable,
 		}, &AutoRootOp{
-			Host:  MustAbs("/"),
-			Flags: BindWritable,
+			Host:  check.MustAbs("/"),
+			Flags: bits.BindWritable,
 		}, true},
 	})

 	checkOpMeta(t, []opMetaTestCase{
 		{"root", &AutoRootOp{
-			Host:  MustAbs("/"),
-			Flags: BindWritable,
+			Host:  check.MustAbs("/"),
+			Flags: bits.BindWritable,
 		}, "setting up", `auto root "/" flags 0x2`},
 	})
 }

 func TestIsAutoRootBindable(t *testing.T) {
+	t.Parallel()
+
 	testCases := []struct {
 		name string
 		want bool
+		log  bool
 	}{
-		{"proc", false},
-		{"dev", false},
-		{"tmp", false},
-		{"mnt", false},
-		{"etc", false},
-		{"", false},
+		{"proc", false, false},
+		{"dev", false, false},
+		{"tmp", false, false},
+		{"mnt", false, false},
+		{"etc", false, false},
+		{"", false, true},

-		{"var", true},
+		{"var", true, false},
 	}
 	for _, tc := range testCases {
 		t.Run(tc.name, func(t *testing.T) {
-			if got := IsAutoRootBindable(tc.name); got != tc.want {
+			t.Parallel()
+			var msg message.Msg
+			if tc.log {
+				msg = &kstub{nil, stub.New(t, func(s *stub.Stub[syscallDispatcher]) syscallDispatcher { panic("unreachable") }, stub.Expect{Calls: []stub.Call{
+					call("verbose", stub.ExpectArgs{[]any{"got unexpected root entry"}}, nil, nil),
+				}})}
+			}
+			if got := IsAutoRootBindable(msg, tc.name); got != tc.want {
 				t.Errorf("IsAutoRootBindable: %v, want %v", got, tc.want)
 			}
 		})
--- a/container/bits/bits.go
+++ b/container/bits/bits.go
@@ -0,0 +1,13 @@
+// Package bits contains constants for configuring the container.
+package bits
+
+const (
+	// BindOptional skips nonexistent host paths.
+	BindOptional = 1 << iota
+	// BindWritable mounts filesystem read-write.
+	BindWritable
+	// BindDevice allows access to devices (special files) on this filesystem.
+	BindDevice
+	// BindEnsure attempts to create the host path if it does not exist.
+	BindEnsure
+)
--- a/container/bits/seccomp.go
+++ b/container/bits/seccomp.go
@@ -0,0 +1,20 @@
+package bits
+
+// FilterPreset specifies parts of the syscall filter preset to enable.
+type FilterPreset int
+
+const (
+	// PresetExt are project-specific extensions.
+	PresetExt FilterPreset = 1 << iota
+	// PresetDenyNS denies namespace setup syscalls.
+	PresetDenyNS
+	// PresetDenyTTY denies faking input.
+	PresetDenyTTY
+	// PresetDenyDevel denies development-related syscalls.
+	PresetDenyDevel
+	// PresetLinux32 sets PER_LINUX32.
+	PresetLinux32
+
+	// PresetStrict is a strict preset useful as a default value.
+	PresetStrict = PresetExt | PresetDenyNS | PresetDenyTTY | PresetDenyDevel
+)
--- a/container/capability_test.go
+++ b/container/capability_test.go
@@ -3,6 +3,8 @@ package container
 import "testing"

 func TestCapToIndex(t *testing.T) {
+	t.Parallel()
+
 	testCases := []struct {
 		name string
 		cap  uintptr
@@ -14,6 +16,7 @@ func TestCapToIndex(t *testing.T) {
 	}
 	for _, tc := range testCases {
 		t.Run(tc.name, func(t *testing.T) {
+			t.Parallel()
 			if got := capToIndex(tc.cap); got != tc.want {
 				t.Errorf("capToIndex: %#x, want %#x", got, tc.want)
 			}
@@ -22,6 +25,8 @@ func TestCapToIndex(t *testing.T) {
 }

 func TestCapToMask(t *testing.T) {
+	t.Parallel()
+
 	testCases := []struct {
 		name string
 		cap  uintptr
@@ -33,6 +38,7 @@ func TestCapToMask(t *testing.T) {
 	}
 	for _, tc := range testCases {
 		t.Run(tc.name, func(t *testing.T) {
+			t.Parallel()
 			if got := capToMask(tc.cap); got != tc.want {
 				t.Errorf("capToMask: %#x, want %#x", got, tc.want)
 			}
--- a/container/check/absolute.go
+++ b/container/check/absolute.go
@@ -1,4 +1,5 @@
-package container
+// Package check provides types yielding values checked to meet a condition.
+package check

 import (
 	"encoding/json"
@@ -11,9 +12,7 @@ import (
 )

 // AbsoluteError is returned by [NewAbs] and holds the invalid pathname.
-type AbsoluteError struct {
-	Pathname string
-}
+type AbsoluteError struct{ Pathname string }

 func (e *AbsoluteError) Error() string { return fmt.Sprintf("path %q is not absolute", e.Pathname) }
 func (e *AbsoluteError) Is(target error) bool {
@@ -25,15 +24,13 @@ func (e *AbsoluteError) Is(target error) bool {
 }

 // Absolute holds a pathname checked to be absolute.
-type Absolute struct {
-	pathname string
-}
+type Absolute struct{ pathname string }

-// isAbs wraps [path.IsAbs] in case additional checks are added in the future.
-func isAbs(pathname string) bool { return path.IsAbs(pathname) }
+// unsafeAbs returns [check.Absolute] on any string value.
+func unsafeAbs(pathname string) *Absolute { return &Absolute{pathname} }

 func (a *Absolute) String() string {
-	if a.pathname == zeroString {
+	if a.pathname == "" {
 		panic("attempted use of zero Absolute")
 	}
 	return a.pathname
@@ -44,16 +41,16 @@ func (a *Absolute) Is(v *Absolute) bool {
 		return true
 	}
 	return a != nil && v != nil &&
-		a.pathname != zeroString && v.pathname != zeroString &&
+		a.pathname != "" && v.pathname != "" &&
 		a.pathname == v.pathname
 }

 // NewAbs checks pathname and returns a new [Absolute] if pathname is absolute.
 func NewAbs(pathname string) (*Absolute, error) {
-	if !isAbs(pathname) {
+	if !path.IsAbs(pathname) {
 		return nil, &AbsoluteError{pathname}
 	}
-	return &Absolute{pathname}, nil
+	return unsafeAbs(pathname), nil
 }

 // MustAbs calls [NewAbs] and panics on error.
@@ -67,16 +64,16 @@ func MustAbs(pathname string) *Absolute {

 // Append calls [path.Join] with [Absolute] as the first element.
 func (a *Absolute) Append(elem ...string) *Absolute {
-	return &Absolute{path.Join(append([]string{a.String()}, elem...)...)}
+	return unsafeAbs(path.Join(append([]string{a.String()}, elem...)...))
 }

 // Dir calls [path.Dir] with [Absolute] as its argument.
-func (a *Absolute) Dir() *Absolute { return &Absolute{path.Dir(a.String())} }
+func (a *Absolute) Dir() *Absolute { return unsafeAbs(path.Dir(a.String())) }

 func (a *Absolute) GobEncode() ([]byte, error) { return []byte(a.String()), nil }
 func (a *Absolute) GobDecode(data []byte) error {
 	pathname := string(data)
-	if !isAbs(pathname) {
+	if !path.IsAbs(pathname) {
 		return &AbsoluteError{pathname}
 	}
 	a.pathname = pathname
@@ -89,7 +86,7 @@ func (a *Absolute) UnmarshalJSON(data []byte) error {
 	if err := json.Unmarshal(data, &pathname); err != nil {
 		return err
 	}
-	if !isAbs(pathname) {
+	if !path.IsAbs(pathname) {
 		return &AbsoluteError{pathname}
 	}
 	a.pathname = pathname
--- a/container/check/absolute_test.go
+++ b/container/check/absolute_test.go
@@ -1,4 +1,4 @@
-package container
+package check_test

 import (
 	"bytes"
@@ -9,9 +9,17 @@ import (
 	"strings"
 	"syscall"
 	"testing"
+	_ "unsafe"
+
+	. "hakurei.app/container/check"
 )

+//go:linkname unsafeAbs hakurei.app/container/check.unsafeAbs
+func unsafeAbs(_ string) *Absolute
+
 func TestAbsoluteError(t *testing.T) {
+	t.Parallel()
+
 	testCases := []struct {
 		name string

@@ -21,8 +29,8 @@ func TestAbsoluteError(t *testing.T) {
 	}{
 		{"EINVAL", new(AbsoluteError), syscall.EINVAL, true},
 		{"not EINVAL", new(AbsoluteError), syscall.EBADE, false},
-		{"ne val", new(AbsoluteError), &AbsoluteError{"etc"}, false},
-		{"equals", &AbsoluteError{"etc"}, &AbsoluteError{"etc"}, true},
+		{"ne val", new(AbsoluteError), &AbsoluteError{Pathname: "etc"}, false},
+		{"equals", &AbsoluteError{Pathname: "etc"}, &AbsoluteError{Pathname: "etc"}, true},
 	}

 	for _, tc := range testCases {
@@ -32,14 +40,18 @@ func TestAbsoluteError(t *testing.T) {
 	}

 	t.Run("string", func(t *testing.T) {
+		t.Parallel()
+
 		want := `path "etc" is not absolute`
-		if got := (&AbsoluteError{"etc"}).Error(); got != want {
+		if got := (&AbsoluteError{Pathname: "etc"}).Error(); got != want {
 			t.Errorf("Error: %q, want %q", got, want)
 		}
 	})
 }

 func TestNewAbs(t *testing.T) {
+	t.Parallel()
+
 	testCases := []struct {
 		name string

@@ -48,12 +60,14 @@ func TestNewAbs(t *testing.T) {
 		wantErr  error
 	}{
 		{"good", "/etc", MustAbs("/etc"), nil},
-		{"not absolute", "etc", nil, &AbsoluteError{"etc"}},
-		{"zero", "", nil, &AbsoluteError{""}},
+		{"not absolute", "etc", nil, &AbsoluteError{Pathname: "etc"}},
+		{"zero", "", nil, &AbsoluteError{Pathname: ""}},
 	}

 	for _, tc := range testCases {
 		t.Run(tc.name, func(t *testing.T) {
+			t.Parallel()
+
 			got, err := NewAbs(tc.pathname)
 			if !reflect.DeepEqual(got, tc.want) {
 				t.Errorf("NewAbs: %#v, want %#v", got, tc.want)
@@ -65,6 +79,8 @@ func TestNewAbs(t *testing.T) {
 	}

 	t.Run("must", func(t *testing.T) {
+		t.Parallel()
+
 		defer func() {
 			wantPanic := `path "etc" is not absolute`

@@ -79,13 +95,17 @@ func TestNewAbs(t *testing.T) {

 func TestAbsoluteString(t *testing.T) {
 	t.Run("passthrough", func(t *testing.T) {
+		t.Parallel()
+
 		pathname := "/etc"
-		if got := (&Absolute{pathname}).String(); got != pathname {
+		if got := unsafeAbs(pathname).String(); got != pathname {
 			t.Errorf("String: %q, want %q", got, pathname)
 		}
 	})

 	t.Run("zero", func(t *testing.T) {
+		t.Parallel()
+
 		defer func() {
 			wantPanic := "attempted use of zero Absolute"

@@ -99,6 +119,8 @@ func TestAbsoluteString(t *testing.T) {
 }

 func TestAbsoluteIs(t *testing.T) {
+	t.Parallel()
+
 	testCases := []struct {
 		name string
 		a, v *Absolute
@@ -114,6 +136,8 @@ func TestAbsoluteIs(t *testing.T) {
 	}
 	for _, tc := range testCases {
 		t.Run(tc.name, func(t *testing.T) {
+			t.Parallel()
+
 			if got := tc.a.Is(tc.v); got != tc.want {
 				t.Errorf("Is: %v, want %v", got, tc.want)
 			}
@@ -127,6 +151,8 @@ type sCheck struct {
 }

 func TestCodecAbsolute(t *testing.T) {
+	t.Parallel()
+
 	testCases := []struct {
 		name string
 		a    *Absolute
@@ -147,7 +173,7 @@ func TestCodecAbsolute(t *testing.T) {

 			`"/etc"`, `{"val":"/etc","magic":3236757504}`},
 		{"not absolute", nil,
-			&AbsoluteError{"etc"},
+			&AbsoluteError{Pathname: "etc"},
 			"\t\x7f\x05\x01\x02\xff\x82\x00\x00\x00\a\xff\x80\x00\x03etc",
 			",\xff\x83\x03\x01\x01\x06sCheck\x01\xff\x84\x00\x01\x02\x01\bPathname\x01\xff\x80\x00\x01\x05Magic\x01\x04\x00\x00\x00\t\x7f\x05\x01\x02\xff\x82\x00\x00\x00\x0f\xff\x84\x01\x03etc\x01\xfb\x01\x81\xda\x00\x00\x00",

@@ -161,13 +187,18 @@ func TestCodecAbsolute(t *testing.T) {

 	for _, tc := range testCases {
 		t.Run(tc.name, func(t *testing.T) {
+			t.Parallel()
+
 			t.Run("gob", func(t *testing.T) {
 				if tc.gob == "\x00" && tc.sGob == "\x00" {
 					// these values mark the current test to skip gob
 					return
 				}
+				t.Parallel()

 				t.Run("encode", func(t *testing.T) {
+					t.Parallel()
+
 					// encode is unchecked
 					if errors.Is(tc.wantErr, syscall.EINVAL) {
 						return
@@ -204,6 +235,8 @@ func TestCodecAbsolute(t *testing.T) {
 				})

 				t.Run("decode", func(t *testing.T) {
+					t.Parallel()
+
 					{
 						var gotA *Absolute
 						err := gob.NewDecoder(strings.NewReader(tc.gob)).Decode(&gotA)
@@ -238,7 +271,11 @@ func TestCodecAbsolute(t *testing.T) {
 			})

 			t.Run("json", func(t *testing.T) {
+				t.Parallel()
+
 				t.Run("marshal", func(t *testing.T) {
+					t.Parallel()
+
 					// marshal is unchecked
 					if errors.Is(tc.wantErr, syscall.EINVAL) {
 						return
@@ -273,6 +310,8 @@ func TestCodecAbsolute(t *testing.T) {
 				})

 				t.Run("unmarshal", func(t *testing.T) {
+					t.Parallel()
+
 					{
 						var gotA *Absolute
 						err := json.Unmarshal([]byte(tc.json), &gotA)
@@ -308,6 +347,8 @@ func TestCodecAbsolute(t *testing.T) {
 	}

 	t.Run("json passthrough", func(t *testing.T) {
+		t.Parallel()
+
 		wantErr := "invalid character ':' looking for beginning of value"
 		if err := new(Absolute).UnmarshalJSON([]byte(":3")); err == nil || err.Error() != wantErr {
 			t.Errorf("UnmarshalJSON: error = %v, want %s", err, wantErr)
@@ -316,7 +357,11 @@ func TestCodecAbsolute(t *testing.T) {
 }

 func TestAbsoluteWrap(t *testing.T) {
+	t.Parallel()
+
 	t.Run("join", func(t *testing.T) {
+		t.Parallel()
+
 		want := "/etc/nix/nix.conf"
 		if got := MustAbs("/etc").Append("nix", "nix.conf"); got.String() != want {
 			t.Errorf("Append: %q, want %q", got, want)
@@ -324,6 +369,8 @@ func TestAbsoluteWrap(t *testing.T) {
 	})

 	t.Run("dir", func(t *testing.T) {
+		t.Parallel()
+
 		want := "/"
 		if got := MustAbs("/etc").Dir(); got.String() != want {
 			t.Errorf("Dir: %q, want %q", got, want)
@@ -331,6 +378,8 @@ func TestAbsoluteWrap(t *testing.T) {
 	})

 	t.Run("sort", func(t *testing.T) {
+		t.Parallel()
+
 		want := []*Absolute{MustAbs("/etc"), MustAbs("/proc"), MustAbs("/sys")}
 		got := []*Absolute{MustAbs("/proc"), MustAbs("/sys"), MustAbs("/etc")}
 		SortAbs(got)
@@ -340,6 +389,8 @@ func TestAbsoluteWrap(t *testing.T) {
 	})

 	t.Run("compact", func(t *testing.T) {
+		t.Parallel()
+
 		want := []*Absolute{MustAbs("/etc"), MustAbs("/proc"), MustAbs("/sys")}
 		if got := CompactAbs([]*Absolute{MustAbs("/etc"), MustAbs("/proc"), MustAbs("/proc"), MustAbs("/sys")}); !reflect.DeepEqual(got, want) {
 			t.Errorf("CompactAbs: %#v, want %#v", got, want)
--- a/container/check/overlay.go
+++ b/container/check/overlay.go
@@ -0,0 +1,29 @@
+package check
+
+import "strings"
+
+const (
+	// SpecialOverlayEscape is the escape string for overlay mount options.
+	SpecialOverlayEscape = `\`
+	// SpecialOverlayOption is the separator string between overlay mount options.
+	SpecialOverlayOption = ","
+	// SpecialOverlayPath is the separator string between overlay paths.
+	SpecialOverlayPath = ":"
+)
+
+// EscapeOverlayDataSegment escapes a string for formatting into the data argument of an overlay mount call.
+func EscapeOverlayDataSegment(s string) string {
+	if s == "" {
+		return ""
+	}
+
+	if f := strings.SplitN(s, "\x00", 2); len(f) > 0 {
+		s = f[0]
+	}
+
+	return strings.NewReplacer(
+		SpecialOverlayEscape, SpecialOverlayEscape+SpecialOverlayEscape,
+		SpecialOverlayOption, SpecialOverlayEscape+SpecialOverlayOption,
+		SpecialOverlayPath, SpecialOverlayEscape+SpecialOverlayPath,
+	).Replace(s)
+}
--- a/container/check/overlay_test.go
+++ b/container/check/overlay_test.go
@@ -0,0 +1,31 @@
+package check_test
+
+import (
+	"testing"
+
+	"hakurei.app/container/check"
+)
+
+func TestEscapeOverlayDataSegment(t *testing.T) {
+	t.Parallel()
+
+	testCases := []struct {
+		name string
+		s    string
+		want string
+	}{
+		{"zero", "", ""},
+		{"multi", `\\\:,:,\\\`, `\\\\\\\:\,\:\,\\\\\\`},
+		{"bwrap", `/path :,\`, `/path \:\,\\`},
+	}
+
+	for _, tc := range testCases {
+		t.Run(tc.name, func(t *testing.T) {
+			t.Parallel()
+
+			if got := check.EscapeOverlayDataSegment(tc.s); got != tc.want {
+				t.Errorf("escapeOverlayDataSegment: %s, want %s", got, tc.want)
+			}
+		})
+	}
+}
--- a/container/container.go
+++ b/container/container.go
@@ -14,7 +14,11 @@ import (
 	. "syscall"
 	"time"

+	"hakurei.app/container/bits"
+	"hakurei.app/container/check"
+	"hakurei.app/container/fhs"
 	"hakurei.app/container/seccomp"
+	"hakurei.app/message"
 )

 const (
@@ -49,22 +53,23 @@ type (

 		cmd *exec.Cmd
 		ctx context.Context
+		msg message.Msg
 		Params
 	}

 	// Params holds container configuration and is safe to serialise.
 	Params struct {
 		// Working directory in the container.
-		Dir *Absolute
+		Dir *check.Absolute
 		// Initial process environment.
 		Env []string
 		// Pathname of initial process in the container.
-		Path *Absolute
+		Path *check.Absolute
 		// Initial process argv.
 		Args []string
 		// Deliver SIGINT to the initial process on context cancellation.
 		ForwardCancel bool
-		// time to wait for linger processes after death of initial process
+		// Time to wait for processes lingering after the initial process terminates.
 		AdoptWaitDelay time.Duration

 		// Mapped Uid in user namespace.
@@ -81,7 +86,7 @@ type (
 		// Extra seccomp flags.
 		SeccompFlags seccomp.ExportFlag
 		// Seccomp presets. Has no effect unless SeccompRules is zero-length.
-		SeccompPresets seccomp.FilterPreset
+		SeccompPresets bits.FilterPreset
 		// Do not load seccomp program.
 		SeccompDisable bool

@@ -99,28 +104,77 @@ type (
 	}
 )

-// Start starts the container init. The init process blocks until Serve is called.
-func (p *Container) Start() error {
-	if p.cmd != nil {
-		return errors.New("container: already started")
+// A StartError contains additional information on a container startup failure.
+type StartError struct {
+	// Fatal suggests whether this error should be considered fatal for the entire program.
+	Fatal bool
+	// Step refers to the part of the setup this error is returned from.
+	Step string
+	// Err is the underlying error.
+	Err error
+	// Origin is whether this error originated from the [Container.Start] method.
+	Origin bool
+	// Passthrough is whether the Error method is passed through to Err.
+	Passthrough bool
+}
+
+func (e *StartError) Unwrap() error { return e.Err }
+func (e *StartError) Error() string {
+	if e.Passthrough {
+		return e.Err.Error()
 	}
-	if p.Ops == nil || len(*p.Ops) == 0 {
-		return errors.New("container: starting an empty container")
+	if e.Origin {
+		return e.Step
 	}

-	ctx, cancel := context.WithCancel(p.ctx)
-	p.cancel = cancel
+	{
+		var syscallError *os.SyscallError
+		if errors.As(e.Err, &syscallError) && syscallError != nil {
+			return e.Step + " " + syscallError.Error()
+		}
+	}
+
+	return e.Step + ": " + e.Err.Error()
+}
+
+// Message returns a user-facing error message.
+func (e *StartError) Message() string {
+	if e.Passthrough {
+		switch {
+		case errors.As(e.Err, new(*os.PathError)),
+			errors.As(e.Err, new(*os.SyscallError)):
+			return "cannot " + e.Err.Error()
+
+		default:
+			return e.Err.Error()
+		}
+	}
+	if e.Origin {
+		return e.Step
+	}
+	return "cannot " + e.Error()
+}
+
+// Start starts the container init. The init process blocks until Serve is called.
+func (p *Container) Start() error {
+	if p == nil || p.cmd == nil ||
+		p.Ops == nil || len(*p.Ops) == 0 {
+		return errors.New("container: starting an invalid container")
+	}
+	if p.cmd.Process != nil {
+		return errors.New("container: already started")
+	}

 	// map to overflow id to work around ownership checks
 	if p.Uid < 1 {
-		p.Uid = OverflowUid()
+		p.Uid = OverflowUid(p.msg)
 	}
 	if p.Gid < 1 {
-		p.Gid = OverflowGid()
+		p.Gid = OverflowGid(p.msg)
 	}

 	if !p.RetainSession {
-		p.SeccompPresets |= seccomp.PresetDenyTTY
+		p.SeccompPresets |= bits.PresetDenyTTY
 	}

 	if p.AdoptWaitDelay == 0 {
@@ -131,16 +185,24 @@ func (p *Container) Start() error {
 		p.AdoptWaitDelay = 0
 	}

-	p.cmd = exec.CommandContext(ctx, MustExecutable())
+	if p.cmd.Stdin == nil {
+		p.cmd.Stdin = p.Stdin
+	}
+	if p.cmd.Stdout == nil {
+		p.cmd.Stdout = p.Stdout
+	}
+	if p.cmd.Stderr == nil {
+		p.cmd.Stderr = p.Stderr
+	}
+
 	p.cmd.Args = []string{initName}
-	p.cmd.Stdin, p.cmd.Stdout, p.cmd.Stderr = p.Stdin, p.Stdout, p.Stderr
 	p.cmd.WaitDelay = p.WaitDelay
 	if p.Cancel != nil {
 		p.cmd.Cancel = func() error { return p.Cancel(p.cmd) }
 	} else {
 		p.cmd.Cancel = func() error { return p.cmd.Process.Signal(CancelSignal) }
 	}
-	p.cmd.Dir = FHSRoot
+	p.cmd.Dir = fhs.Root
 	p.cmd.SysProcAttr = &SysProcAttr{
 		Setsid:    !p.RetainSession,
 		Pdeathsig: SIGKILL,
@@ -167,8 +229,7 @@ func (p *Container) Start() error {

 	// place setup pipe before user supplied extra files, this is later restored by init
 	if fd, e, err := Setup(&p.cmd.ExtraFiles); err != nil {
-		return wrapErrSuffix(err,
-			"cannot create shim setup pipe:")
+		return &StartError{true, "set up params stream", err, false, false}
 	} else {
 		p.setup = e
 		p.cmd.Env = []string{setupEnv + "=" + strconv.Itoa(fd)}
@@ -183,8 +244,7 @@ func (p *Container) Start() error {
 		done <- func() error { // setup depending on per-thread state must happen here
 			// PR_SET_NO_NEW_PRIVS: depends on per-thread state but acts on all processes created from that thread
 			if err := SetNoNewPrivs(); err != nil {
-				return wrapErrSuffix(err,
-					"prctl(PR_SET_NO_NEW_PRIVS):")
+				return &StartError{true, "prctl(PR_SET_NO_NEW_PRIVS)", err, false, false}
 			}

 			// landlock: depends on per-thread state but acts on a process group
@@ -200,31 +260,27 @@ func (p *Container) Start() error {
 						// already covered by namespaces (pid)
 						goto landlockOut
 					}
-					return wrapErrSuffix(err,
-						"landlock does not appear to be enabled:")
+					return &StartError{false, "get landlock ABI", err, false, false}
 				} else if abi < 6 {
 					if p.HostAbstract {
 						// see above comment
 						goto landlockOut
 					}
-					return msg.WrapErr(ENOSYS,
-						"kernel version too old for LANDLOCK_SCOPE_ABSTRACT_UNIX_SOCKET")
+					return &StartError{false, "kernel version too old for LANDLOCK_SCOPE_ABSTRACT_UNIX_SOCKET", ENOSYS, true, false}
 				} else {
-					msg.Verbosef("landlock abi version %d", abi)
+					p.msg.Verbosef("landlock abi version %d", abi)
 				}

 				if rulesetFd, err := rulesetAttr.Create(0); err != nil {
-					return wrapErrSuffix(err,
-						"cannot create landlock ruleset:")
+					return &StartError{true, "create landlock ruleset", err, false, false}
 				} else {
-					msg.Verbosef("enforcing landlock ruleset %s", rulesetAttr)
+					p.msg.Verbosef("enforcing landlock ruleset %s", rulesetAttr)
 					if err = LandlockRestrictSelf(rulesetFd, 0); err != nil {
 						_ = Close(rulesetFd)
-						return wrapErrSuffix(err,
-							"cannot enforce landlock ruleset:")
+						return &StartError{true, "enforce landlock ruleset", err, false, false}
 					}
 					if err = Close(rulesetFd); err != nil {
-						msg.Verbosef("cannot close landlock ruleset: %v", err)
+						p.msg.Verbosef("cannot close landlock ruleset: %v", err)
 						// not fatal
 					}
 				}
@@ -232,9 +288,9 @@ func (p *Container) Start() error {
 			landlockOut:
 			}

-			msg.Verbose("starting container init")
+			p.msg.Verbose("starting container init")
 			if err := p.cmd.Start(); err != nil {
-				return msg.WrapErr(err, err.Error())
+				return &StartError{false, "start container init", err, false, true}
 			}
 			return nil
 		}()
@@ -257,12 +313,12 @@ func (p *Container) Serve() error {

 	if p.Path == nil {
 		p.cancel()
-		return msg.WrapErr(EINVAL, "invalid executable pathname")
+		return &StartError{false, "invalid executable pathname", EINVAL, true, false}
 	}

 	// do not transmit nil
 	if p.Dir == nil {
-		p.Dir = AbsFHSRoot
+		p.Dir = fhs.AbsRoot
 	}
 	if p.SeccompRules == nil {
 		p.SeccompRules = make([]seccomp.NativeRule, 0)
@@ -274,7 +330,7 @@ func (p *Container) Serve() error {
 			Getuid(),
 			Getgid(),
 			len(p.ExtraFiles),
-			msg.IsVerbose(),
+			p.msg.IsVerbose(),
 		},
 	)
 	if err != nil {
@@ -285,7 +341,7 @@ func (p *Container) Serve() error {

 // Wait waits for the container init process to exit and releases any resources associated with the [Container].
 func (p *Container) Wait() error {
-	if p.cmd == nil {
+	if p.cmd == nil || p.cmd.Process == nil {
 		return EINVAL
 	}

@@ -297,6 +353,36 @@ func (p *Container) Wait() error {
 	return err
 }

+// StdinPipe calls the [exec.Cmd] method with the same name.
+func (p *Container) StdinPipe() (w io.WriteCloser, err error) {
+	if p.Stdin != nil {
+		return nil, errors.New("container: Stdin already set")
+	}
+	w, err = p.cmd.StdinPipe()
+	p.Stdin = p.cmd.Stdin
+	return
+}
+
+// StdoutPipe calls the [exec.Cmd] method with the same name.
+func (p *Container) StdoutPipe() (r io.ReadCloser, err error) {
+	if p.Stdout != nil {
+		return nil, errors.New("container: Stdout already set")
+	}
+	r, err = p.cmd.StdoutPipe()
+	p.Stdout = p.cmd.Stdout
+	return
+}
+
+// StderrPipe calls the [exec.Cmd] method with the same name.
+func (p *Container) StderrPipe() (r io.ReadCloser, err error) {
+	if p.Stderr != nil {
+		return nil, errors.New("container: Stderr already set")
+	}
+	r, err = p.cmd.StderrPipe()
+	p.Stderr = p.cmd.Stderr
+	return
+}
+
 func (p *Container) String() string {
 	return fmt.Sprintf("argv: %q, filter: %v, rules: %d, flags: %#x, presets: %#x",
 		p.Args, !p.SeccompDisable, len(p.SeccompRules), int(p.SeccompFlags), int(p.SeccompPresets))
@@ -311,13 +397,21 @@ func (p *Container) ProcessState() *os.ProcessState {
 }

 // New returns the address to a new instance of [Container] that requires further initialisation before use.
-func New(ctx context.Context) *Container {
-	return &Container{ctx: ctx, Params: Params{Ops: new(Ops)}}
+func New(ctx context.Context, msg message.Msg) *Container {
+	if msg == nil {
+		msg = message.NewMsg(nil)
+	}
+
+	p := &Container{ctx: ctx, msg: msg, Params: Params{Ops: new(Ops)}}
+	c, cancel := context.WithCancel(ctx)
+	p.cancel = cancel
+	p.cmd = exec.CommandContext(c, MustExecutable(msg))
+	return p
 }

 // NewCommand calls [New] and initialises the [Params.Path] and [Params.Args] fields.
-func NewCommand(ctx context.Context, pathname *Absolute, name string, args ...string) *Container {
-	z := New(ctx)
+func NewCommand(ctx context.Context, msg message.Msg, pathname *check.Absolute, name string, args ...string) *Container {
+	z := New(ctx, msg)
 	z.Path = pathname
 	z.Args = append([]string{name}, args...)
 	return z
--- a/container/container_test.go
+++ b/container/container_test.go
@@ -7,9 +7,11 @@ import (
 	"errors"
 	"fmt"
 	"log"
+	"net"
 	"os"
 	"os/exec"
 	"os/signal"
+	"reflect"
 	"strconv"
 	"strings"
 	"syscall"
@@ -18,14 +20,156 @@ import (

 	"hakurei.app/command"
 	"hakurei.app/container"
+	"hakurei.app/container/bits"
+	"hakurei.app/container/check"
 	"hakurei.app/container/seccomp"
 	"hakurei.app/container/vfs"
 	"hakurei.app/hst"
-	"hakurei.app/internal"
-	"hakurei.app/internal/hlog"
 	"hakurei.app/ldd"
+	"hakurei.app/message"
 )

+func TestStartError(t *testing.T) {
+	t.Parallel()
+
+	testCases := []struct {
+		name string
+		err  error
+		s    string
+		is   error
+		isF  error
+		msg  string
+	}{
+		{"params env", &container.StartError{
+			Fatal: true,
+			Step:  "set up params stream",
+			Err:   container.ErrReceiveEnv,
+		},
+			"set up params stream: environment variable not set",
+			container.ErrReceiveEnv, syscall.EBADF,
+			"cannot set up params stream: environment variable not set"},
+
+		{"params", &container.StartError{
+			Fatal: true,
+			Step:  "set up params stream",
+			Err:   &os.SyscallError{Syscall: "pipe2", Err: syscall.EBADF},
+		},
+			"set up params stream pipe2: bad file descriptor",
+			syscall.EBADF, os.ErrInvalid,
+			"cannot set up params stream pipe2: bad file descriptor"},
+
+		{"PR_SET_NO_NEW_PRIVS", &container.StartError{
+			Fatal: true,
+			Step:  "prctl(PR_SET_NO_NEW_PRIVS)",
+			Err:   syscall.EPERM,
+		},
+			"prctl(PR_SET_NO_NEW_PRIVS): operation not permitted",
+			syscall.EPERM, syscall.EACCES,
+			"cannot prctl(PR_SET_NO_NEW_PRIVS): operation not permitted"},
+
+		{"landlock abi", &container.StartError{
+			Step: "get landlock ABI",
+			Err:  syscall.ENOSYS,
+		},
+			"get landlock ABI: function not implemented",
+			syscall.ENOSYS, syscall.ENOEXEC,
+			"cannot get landlock ABI: function not implemented"},
+
+		{"landlock old", &container.StartError{
+			Step:   "kernel version too old for LANDLOCK_SCOPE_ABSTRACT_UNIX_SOCKET",
+			Err:    syscall.ENOSYS,
+			Origin: true,
+		},
+			"kernel version too old for LANDLOCK_SCOPE_ABSTRACT_UNIX_SOCKET",
+			syscall.ENOSYS, syscall.ENOSPC,
+			"kernel version too old for LANDLOCK_SCOPE_ABSTRACT_UNIX_SOCKET"},
+
+		{"landlock create", &container.StartError{
+			Fatal: true,
+			Step:  "create landlock ruleset",
+			Err:   syscall.EBADFD,
+		},
+			"create landlock ruleset: file descriptor in bad state",
+			syscall.EBADFD, syscall.EBADF,
+			"cannot create landlock ruleset: file descriptor in bad state"},
+
+		{"landlock enforce", &container.StartError{
+			Fatal: true,
+			Step:  "enforce landlock ruleset",
+			Err:   syscall.ENOTRECOVERABLE,
+		},
+			"enforce landlock ruleset: state not recoverable",
+			syscall.ENOTRECOVERABLE, syscall.ETIMEDOUT,
+			"cannot enforce landlock ruleset: state not recoverable"},
+
+		{"start", &container.StartError{
+			Step: "start container init",
+			Err: &os.PathError{
+				Op:   "fork/exec",
+				Path: "/proc/nonexistent",
+				Err:  syscall.ENOENT,
+			}, Passthrough: true,
+		},
+			"fork/exec /proc/nonexistent: no such file or directory",
+			syscall.ENOENT, syscall.ENOSYS,
+			"cannot fork/exec /proc/nonexistent: no such file or directory"},
+
+		{"start syscall", &container.StartError{
+			Step: "start container init",
+			Err: &os.SyscallError{
+				Syscall: "open",
+				Err:     syscall.ENOSYS,
+			}, Passthrough: true,
+		},
+			"open: function not implemented",
+			syscall.ENOSYS, syscall.ENOENT,
+			"cannot open: function not implemented"},
+
+		{"start other", &container.StartError{
+			Step: "start container init",
+			Err: &net.OpError{
+				Op:  "dial",
+				Net: "unix",
+				Err: syscall.ECONNREFUSED,
+			}, Passthrough: true,
+		},
+			"dial unix: connection refused",
+			syscall.ECONNREFUSED, syscall.ECONNABORTED,
+			"dial unix: connection refused"},
+	}
+	for _, tc := range testCases {
+		t.Run(tc.name, func(t *testing.T) {
+			t.Parallel()
+
+			t.Run("error", func(t *testing.T) {
+				if got := tc.err.Error(); got != tc.s {
+					t.Errorf("Error: %q, want %q", got, tc.s)
+				}
+			})
+
+			t.Run("is", func(t *testing.T) {
+				if !errors.Is(tc.err, tc.is) {
+					t.Error("Is: unexpected false")
+				}
+				if errors.Is(tc.err, tc.isF) {
+					t.Errorf("Is: unexpected true")
+				}
+			})
+
+			t.Run("msg", func(t *testing.T) {
+				if got, ok := message.GetMessage(tc.err); !ok {
+					if tc.msg != "" {
+						t.Errorf("GetMessage: err does not implement MessageError")
+					}
+					return
+				} else if got != tc.msg {
+					t.Errorf("GetMessage: %q, want %q", got, tc.msg)
+				}
+			})
+		})
+	}
+}
+
 const (
 	ignore  = "\x00"
 	ignoreV = -1
@@ -62,33 +206,33 @@ var containerTestCases = []struct {

 	rules   []seccomp.NativeRule
 	flags   seccomp.ExportFlag
-	presets seccomp.FilterPreset
+	presets bits.FilterPreset
 }{
 	{"minimal", true, false, false, true,
 		emptyOps, emptyMnt,
-		1000, 100, nil, 0, seccomp.PresetStrict},
+		1000, 100, nil, 0, bits.PresetStrict},
 	{"allow", true, true, true, false,
 		emptyOps, emptyMnt,
-		1000, 100, nil, 0, seccomp.PresetExt | seccomp.PresetDenyDevel},
+		1000, 100, nil, 0, bits.PresetExt | bits.PresetDenyDevel},
 	{"no filter", false, true, true, true,
 		emptyOps, emptyMnt,
-		1000, 100, nil, 0, seccomp.PresetExt},
+		1000, 100, nil, 0, bits.PresetExt},
 	{"custom rules", true, true, true, false,
 		emptyOps, emptyMnt,
-		1, 31, []seccomp.NativeRule{{seccomp.ScmpSyscall(syscall.SYS_SETUID), seccomp.ScmpErrno(syscall.EPERM), nil}}, 0, seccomp.PresetExt},
+		1, 31, []seccomp.NativeRule{{Syscall: seccomp.ScmpSyscall(syscall.SYS_SETUID), Errno: seccomp.ScmpErrno(syscall.EPERM)}}, 0, bits.PresetExt},

 	{"tmpfs", true, false, false, true,
 		earlyOps(new(container.Ops).
-			Tmpfs(hst.AbsTmp, 0, 0755),
+			Tmpfs(hst.AbsPrivateTmp, 0, 0755),
 		),
 		earlyMnt(
-			ent("/", hst.Tmp, "rw,nosuid,nodev,relatime", "tmpfs", "ephemeral", ignore),
+			ent("/", hst.PrivateTmp, "rw,nosuid,nodev,relatime", "tmpfs", "ephemeral", ignore),
 		),
-		9, 9, nil, 0, seccomp.PresetStrict},
+		9, 9, nil, 0, bits.PresetStrict},

 	{"dev", true, true /* go test output is not a tty */, false, false,
 		earlyOps(new(container.Ops).
-			Dev(container.MustAbs("/dev"), true),
+			Dev(check.MustAbs("/dev"), true),
 		),
 		earlyMnt(
 			ent("/", "/dev", "ro,nosuid,nodev,relatime", "tmpfs", "devtmpfs", ignore),
@@ -100,12 +244,13 @@ var containerTestCases = []struct {
 			ent("/tty", "/dev/tty", "rw,nosuid", "devtmpfs", "devtmpfs", ignore),
 			ent("/", "/dev/pts", "rw,nosuid,noexec,relatime", "devpts", "devpts", "rw,mode=620,ptmxmode=666"),
 			ent("/", "/dev/mqueue", "rw,nosuid,nodev,noexec,relatime", "mqueue", "mqueue", "rw"),
+			ent("/", "/dev/shm", "rw,nosuid,nodev,relatime", "tmpfs", "tmpfs", ignore),
 		),
-		1971, 100, nil, 0, seccomp.PresetStrict},
+		1971, 100, nil, 0, bits.PresetStrict},

 	{"dev no mqueue", true, true /* go test output is not a tty */, false, false,
 		earlyOps(new(container.Ops).
-			Dev(container.MustAbs("/dev"), false),
+			Dev(check.MustAbs("/dev"), false),
 		),
 		earlyMnt(
 			ent("/", "/dev", "ro,nosuid,nodev,relatime", "tmpfs", "devtmpfs", ignore),
@@ -116,25 +261,26 @@ var containerTestCases = []struct {
 			ent("/urandom", "/dev/urandom", "rw,nosuid", "devtmpfs", "devtmpfs", ignore),
 			ent("/tty", "/dev/tty", "rw,nosuid", "devtmpfs", "devtmpfs", ignore),
 			ent("/", "/dev/pts", "rw,nosuid,noexec,relatime", "devpts", "devpts", "rw,mode=620,ptmxmode=666"),
+			ent("/", "/dev/shm", "rw,nosuid,nodev,relatime", "tmpfs", "tmpfs", ignore),
 		),
-		1971, 100, nil, 0, seccomp.PresetStrict},
+		1971, 100, nil, 0, bits.PresetStrict},

 	{"overlay", true, false, false, true,
 		func(t *testing.T) (*container.Ops, context.Context) {
-			tempDir := container.MustAbs(t.TempDir())
+			tempDir := check.MustAbs(t.TempDir())
 			lower0, lower1, upper, work :=
 				tempDir.Append("lower0"),
 				tempDir.Append("lower1"),
 				tempDir.Append("upper"),
 				tempDir.Append("work")
-			for _, a := range []*container.Absolute{lower0, lower1, upper, work} {
+			for _, a := range []*check.Absolute{lower0, lower1, upper, work} {
 				if err := os.Mkdir(a.String(), 0755); err != nil {
 					t.Fatalf("Mkdir: error = %v", err)
 				}
 			}

 			return new(container.Ops).
-					Overlay(hst.AbsTmp, upper, work, lower0, lower1),
+					Overlay(hst.AbsPrivateTmp, upper, work, lower0, lower1),
 				context.WithValue(context.WithValue(context.WithValue(context.WithValue(t.Context(),
 					testVal("lower1"), lower1),
 					testVal("lower0"), lower0),
@@ -143,81 +289,83 @@ var containerTestCases = []struct {
 		},
 		func(t *testing.T, ctx context.Context) []*vfs.MountInfoEntry {
 			return []*vfs.MountInfoEntry{
-				ent("/", hst.Tmp, "rw", "overlay", "overlay",
+				ent("/", hst.PrivateTmp, "rw", "overlay", "overlay",
 					"rw,lowerdir="+
-						container.InternalToHostOvlEscape(ctx.Value(testVal("lower0")).(*container.Absolute).String())+":"+
-						container.InternalToHostOvlEscape(ctx.Value(testVal("lower1")).(*container.Absolute).String())+
+						container.InternalToHostOvlEscape(ctx.Value(testVal("lower0")).(*check.Absolute).String())+":"+
+						container.InternalToHostOvlEscape(ctx.Value(testVal("lower1")).(*check.Absolute).String())+
 						",upperdir="+
-						container.InternalToHostOvlEscape(ctx.Value(testVal("upper")).(*container.Absolute).String())+
+						container.InternalToHostOvlEscape(ctx.Value(testVal("upper")).(*check.Absolute).String())+
 						",workdir="+
-						container.InternalToHostOvlEscape(ctx.Value(testVal("work")).(*container.Absolute).String())+
+						container.InternalToHostOvlEscape(ctx.Value(testVal("work")).(*check.Absolute).String())+
 						",redirect_dir=nofollow,uuid=on,userxattr"),
 			}
 		},
-		1 << 3, 1 << 14, nil, 0, seccomp.PresetStrict},
+		1 << 3, 1 << 14, nil, 0, bits.PresetStrict},

 	{"overlay ephemeral", true, false, false, true,
 		func(t *testing.T) (*container.Ops, context.Context) {
-			tempDir := container.MustAbs(t.TempDir())
+			tempDir := check.MustAbs(t.TempDir())
 			lower0, lower1 :=
 				tempDir.Append("lower0"),
 				tempDir.Append("lower1")
-			for _, a := range []*container.Absolute{lower0, lower1} {
+			for _, a := range []*check.Absolute{lower0, lower1} {
 				if err := os.Mkdir(a.String(), 0755); err != nil {
 					t.Fatalf("Mkdir: error = %v", err)
 				}
 			}

 			return new(container.Ops).
-					OverlayEphemeral(hst.AbsTmp, lower0, lower1),
+					OverlayEphemeral(hst.AbsPrivateTmp, lower0, lower1),
 				t.Context()
 		},
 		func(t *testing.T, ctx context.Context) []*vfs.MountInfoEntry {
 			return []*vfs.MountInfoEntry{
 				// contains random suffix
-				ent("/", hst.Tmp, "rw", "overlay", "overlay", ignore),
+				ent("/", hst.PrivateTmp, "rw", "overlay", "overlay", ignore),
 			}
 		},
-		1 << 3, 1 << 14, nil, 0, seccomp.PresetStrict},
+		1 << 3, 1 << 14, nil, 0, bits.PresetStrict},

 	{"overlay readonly", true, false, false, true,
 		func(t *testing.T) (*container.Ops, context.Context) {
-			tempDir := container.MustAbs(t.TempDir())
+			tempDir := check.MustAbs(t.TempDir())
 			lower0, lower1 :=
 				tempDir.Append("lower0"),
 				tempDir.Append("lower1")
-			for _, a := range []*container.Absolute{lower0, lower1} {
+			for _, a := range []*check.Absolute{lower0, lower1} {
 				if err := os.Mkdir(a.String(), 0755); err != nil {
 					t.Fatalf("Mkdir: error = %v", err)
 				}
 			}
 			return new(container.Ops).
-					OverlayReadonly(hst.AbsTmp, lower0, lower1),
+					OverlayReadonly(hst.AbsPrivateTmp, lower0, lower1),
 				context.WithValue(context.WithValue(t.Context(),
 					testVal("lower1"), lower1),
 					testVal("lower0"), lower0)
 		},
 		func(t *testing.T, ctx context.Context) []*vfs.MountInfoEntry {
 			return []*vfs.MountInfoEntry{
-				ent("/", hst.Tmp, "rw", "overlay", "overlay",
+				ent("/", hst.PrivateTmp, "rw", "overlay", "overlay",
 					"ro,lowerdir="+
-						container.InternalToHostOvlEscape(ctx.Value(testVal("lower0")).(*container.Absolute).String())+":"+
-						container.InternalToHostOvlEscape(ctx.Value(testVal("lower1")).(*container.Absolute).String())+
+						container.InternalToHostOvlEscape(ctx.Value(testVal("lower0")).(*check.Absolute).String())+":"+
+						container.InternalToHostOvlEscape(ctx.Value(testVal("lower1")).(*check.Absolute).String())+
 						",redirect_dir=nofollow,userxattr"),
 			}
 		},
-		1 << 3, 1 << 14, nil, 0, seccomp.PresetStrict},
+		1 << 3, 1 << 14, nil, 0, bits.PresetStrict},
 }

 func TestContainer(t *testing.T) {
-	replaceOutput(t)
+	t.Parallel()

 	t.Run("cancel", testContainerCancel(nil, func(t *testing.T, c *container.Container) {
 		wantErr := context.Canceled
 		wantExitCode := 0
-		if err := c.Wait(); !errors.Is(err, wantErr) {
-			container.GetOutput().PrintBaseErr(err, "wait:")
-			t.Errorf("Wait: error = %v, want %v", err, wantErr)
+		if err := c.Wait(); !reflect.DeepEqual(err, wantErr) {
+			if m, ok := container.InternalMessageFromError(err); ok {
+				t.Error(m)
+			}
+			t.Errorf("Wait: error = %#v, want %#v", err, wantErr)
 		}
 		if ps := c.ProcessState(); ps == nil {
 			t.Errorf("ProcessState unexpectedly returned nil")
@@ -231,7 +379,9 @@ func TestContainer(t *testing.T) {
 	}, func(t *testing.T, c *container.Container) {
 		var exitError *exec.ExitError
 		if err := c.Wait(); !errors.As(err, &exitError) {
-			container.GetOutput().PrintBaseErr(err, "wait:")
+			if m, ok := container.InternalMessageFromError(err); ok {
+				t.Error(m)
+			}
 			t.Errorf("Wait: error = %v", err)
 		}
 		if code := exitError.ExitCode(); code != blockExitCodeInterrupt {
@@ -241,13 +391,15 @@ func TestContainer(t *testing.T) {

 	for i, tc := range containerTestCases {
 		t.Run(tc.name, func(t *testing.T) {
+			t.Parallel()
+
 			wantOps, wantOpsCtx := tc.ops(t)
 			wantMnt := tc.mnt(t, wantOpsCtx)

 			ctx, cancel := context.WithTimeout(t.Context(), helperDefaultTimeout)
 			defer cancel()

-			var libPaths []*container.Absolute
+			var libPaths []*check.Absolute
 			c := helperNewContainerLibPaths(ctx, &libPaths, "container", strconv.Itoa(i))
 			c.Uid = tc.uid
 			c.Gid = tc.gid
@@ -268,11 +420,11 @@ func TestContainer(t *testing.T) {
 			c.HostNet = tc.net

 			c.
-				Readonly(container.MustAbs(pathReadonly), 0755).
-				Tmpfs(container.MustAbs("/tmp"), 0, 0755).
-				Place(container.MustAbs("/etc/hostname"), []byte(c.Hostname))
+				Readonly(check.MustAbs(pathReadonly), 0755).
+				Tmpfs(check.MustAbs("/tmp"), 0, 0755).
+				Place(check.MustAbs("/etc/hostname"), []byte(c.Hostname))
 			// needs /proc to check mountinfo
-			c.Proc(container.MustAbs("/proc"))
+			c.Proc(check.MustAbs("/proc"))

 			// mountinfo cannot be resolved directly by helper due to libPaths nondeterminism
 			mnt := make([]*vfs.MountInfoEntry, 0, 3+len(libPaths))
@@ -303,25 +455,34 @@ func TestContainer(t *testing.T) {
 				_, _ = output.WriteTo(os.Stdout)
 				t.Fatalf("cannot serialise expected mount points: %v", err)
 			}
-			c.Place(container.MustAbs(pathWantMnt), want.Bytes())
+			c.Place(check.MustAbs(pathWantMnt), want.Bytes())

 			if tc.ro {
-				c.Remount(container.MustAbs("/"), syscall.MS_RDONLY)
+				c.Remount(check.MustAbs("/"), syscall.MS_RDONLY)
 			}

 			if err := c.Start(); err != nil {
 				_, _ = output.WriteTo(os.Stdout)
-				container.GetOutput().PrintBaseErr(err, "start:")
-				t.Fatalf("cannot start container: %v", err)
+				if m, ok := container.InternalMessageFromError(err); ok {
+					t.Fatal(m)
+				} else {
+					t.Fatalf("cannot start container: %v", err)
+				}
 			} else if err = c.Serve(); err != nil {
 				_, _ = output.WriteTo(os.Stdout)
-				container.GetOutput().PrintBaseErr(err, "serve:")
-				t.Errorf("cannot serve setup params: %v", err)
+				if m, ok := container.InternalMessageFromError(err); ok {
+					t.Error(m)
+				} else {
+					t.Errorf("cannot serve setup params: %v", err)
+				}
 			}
 			if err := c.Wait(); err != nil {
 				_, _ = output.WriteTo(os.Stdout)
-				container.GetOutput().PrintBaseErr(err, "wait:")
-				t.Fatalf("wait: %v", err)
+				if m, ok := container.InternalMessageFromError(err); ok {
+					t.Fatal(m)
+				} else {
+					t.Fatalf("wait: %v", err)
+				}
 			}
 		})
 	}
@@ -351,6 +512,7 @@ func testContainerCancel(
 	waitCheck func(t *testing.T, c *container.Container),
 ) func(t *testing.T) {
 	return func(t *testing.T) {
+		t.Parallel()
 		ctx, cancel := context.WithTimeout(t.Context(), helperDefaultTimeout)

 		c := helperNewContainer(ctx, "block")
@@ -374,11 +536,17 @@ func testContainerCancel(
 		}

 		if err := c.Start(); err != nil {
-			container.GetOutput().PrintBaseErr(err, "start:")
-			t.Fatalf("cannot start container: %v", err)
+			if m, ok := container.InternalMessageFromError(err); ok {
+				t.Fatal(m)
+			} else {
+				t.Fatalf("cannot start container: %v", err)
+			}
 		} else if err = c.Serve(); err != nil {
-			container.GetOutput().PrintBaseErr(err, "serve:")
-			t.Errorf("cannot serve setup params: %v", err)
+			if m, ok := container.InternalMessageFromError(err); ok {
+				t.Error(m)
+			} else {
+				t.Errorf("cannot serve setup params: %v", err)
+			}
 		}
 		<-ready
 		cancel()
@@ -387,12 +555,14 @@ func testContainerCancel(
 }

 func TestContainerString(t *testing.T) {
-	c := container.NewCommand(t.Context(), container.MustAbs("/run/current-system/sw/bin/ldd"), "ldd", "/usr/bin/env")
+	t.Parallel()
+	msg := message.NewMsg(nil)
+	c := container.NewCommand(t.Context(), msg, check.MustAbs("/run/current-system/sw/bin/ldd"), "ldd", "/usr/bin/env")
 	c.SeccompFlags |= seccomp.AllowMultiarch
 	c.SeccompRules = seccomp.Preset(
-		seccomp.PresetExt|seccomp.PresetDenyNS|seccomp.PresetDenyTTY,
+		bits.PresetExt|bits.PresetDenyNS|bits.PresetDenyTTY,
 		c.SeccompFlags)
-	c.SeccompPresets = seccomp.PresetStrict
+	c.SeccompPresets = bits.PresetStrict
 	want := `argv: ["ldd" "/usr/bin/env"], filter: true, rules: 65, flags: 0x1, presets: 0xf`
 	if got := c.String(); got != want {
 		t.Errorf("String: %s, want %s", got, want)
@@ -523,13 +693,13 @@ const (
 )

 var (
-	absHelperInnerPath = container.MustAbs(helperInnerPath)
+	absHelperInnerPath = check.MustAbs(helperInnerPath)
 )

 var helperCommands []func(c command.Command)

 func TestMain(m *testing.M) {
-	container.TryArgv0(hlog.Output{}, hlog.Prepare, internal.InstallOutput)
+	container.TryArgv0(nil)

 	if os.Getenv(envDoCheck) == "1" {
 		c := command.New(os.Stderr, log.Printf, "helper", func(args []string) error {
@@ -551,13 +721,14 @@ func TestMain(m *testing.M) {
 	os.Exit(m.Run())
 }

-func helperNewContainerLibPaths(ctx context.Context, libPaths *[]*container.Absolute, args ...string) (c *container.Container) {
-	c = container.NewCommand(ctx, absHelperInnerPath, "helper", args...)
+func helperNewContainerLibPaths(ctx context.Context, libPaths *[]*check.Absolute, args ...string) (c *container.Container) {
+	msg := message.NewMsg(nil)
+	c = container.NewCommand(ctx, msg, absHelperInnerPath, "helper", args...)
 	c.Env = append(c.Env, envDoCheck+"=1")
-	c.Bind(container.MustAbs(os.Args[0]), absHelperInnerPath, 0)
+	c.Bind(check.MustAbs(os.Args[0]), absHelperInnerPath, 0)

 	// in case test has cgo enabled
-	if entries, err := ldd.Exec(ctx, os.Args[0]); err != nil {
+	if entries, err := ldd.Exec(ctx, msg, os.Args[0]); err != nil {
 		log.Fatalf("ldd: %v", err)
 	} else {
 		*libPaths = ldd.Path(entries)
@@ -570,5 +741,5 @@ func helperNewContainerLibPaths(ctx context.Context, libPaths *[]*container.Abso
 }

 func helperNewContainer(ctx context.Context, args ...string) (c *container.Container) {
-	return helperNewContainerLibPaths(ctx, new([]*container.Absolute), args...)
+	return helperNewContainerLibPaths(ctx, new([]*check.Absolute), args...)
 }
--- a/container/dispatcher.go
+++ b/container/dispatcher.go
@@ -3,7 +3,6 @@ package container
 import (
 	"io"
 	"io/fs"
-	"log"
 	"os"
 	"os/exec"
 	"os/signal"
@@ -12,6 +11,7 @@ import (
 	"syscall"

 	"hakurei.app/container/seccomp"
+	"hakurei.app/message"
 )

 type osFile interface {
@@ -38,7 +38,7 @@ type syscallDispatcher interface {
 	setNoNewPrivs() error

 	// lastcap provides [LastCap].
-	lastcap() uintptr
+	lastcap(msg message.Msg) uintptr
 	// capset provides capset.
 	capset(hdrp *capHeader, datap *[2]capData) error
 	// capBoundingSetDrop provides capBoundingSetDrop.
@@ -53,9 +53,9 @@ type syscallDispatcher interface {
 	receive(key string, e any, fdp *uintptr) (closeFunc func() error, err error)

 	// bindMount provides procPaths.bindMount.
-	bindMount(source, target string, flags uintptr, eq bool) error
+	bindMount(msg message.Msg, source, target string, flags uintptr) error
 	// remount provides procPaths.remount.
-	remount(target string, flags uintptr) error
+	remount(msg message.Msg, target string, flags uintptr) error
 	// mountTmpfs provides mountTmpfs.
 	mountTmpfs(fsname, target string, flags uintptr, size int, perm os.FileMode) error
 	// ensureFile provides ensureFile.
@@ -122,24 +122,12 @@ type syscallDispatcher interface {
 	// wait4 provides syscall.Wait4
 	wait4(pid int, wstatus *syscall.WaitStatus, options int, rusage *syscall.Rusage) (wpid int, err error)

-	// printf provides [log.Printf].
-	printf(format string, v ...any)
-	// fatal provides [log.Fatal]
-	fatal(v ...any)
-	// fatalf provides [log.Fatalf]
-	fatalf(format string, v ...any)
-	// verbose provides [Msg.Verbose].
-	verbose(v ...any)
-	// verbosef provides [Msg.Verbosef].
-	verbosef(format string, v ...any)
-	// suspend provides [Msg.Suspend].
-	suspend()
-	// resume provides [Msg.Resume].
-	resume() bool
-	// beforeExit provides [Msg.BeforeExit].
-	beforeExit()
-	// printBaseErr provides [Msg.PrintBaseErr].
-	printBaseErr(err error, fallback string)
+	// printf provides the Printf method of [log.Logger].
+	printf(msg message.Msg, format string, v ...any)
+	// fatal provides the Fatal method of [log.Logger]
+	fatal(msg message.Msg, v ...any)
+	// fatalf provides the Fatalf method of [log.Logger]
+	fatalf(msg message.Msg, format string, v ...any)
 }

 // direct implements syscallDispatcher on the current kernel.
@@ -153,7 +141,7 @@ func (direct) setPtracer(pid uintptr) error       { return SetPtracer(pid) }
 func (direct) setDumpable(dumpable uintptr) error { return SetDumpable(dumpable) }
 func (direct) setNoNewPrivs() error               { return SetNoNewPrivs() }

-func (direct) lastcap() uintptr                                { return LastCap() }
+func (direct) lastcap(msg message.Msg) uintptr                 { return LastCap(msg) }
 func (direct) capset(hdrp *capHeader, datap *[2]capData) error { return capset(hdrp, datap) }
 func (direct) capBoundingSetDrop(cap uintptr) error            { return capBoundingSetDrop(cap) }
 func (direct) capAmbientClearAll() error                       { return capAmbientClearAll() }
@@ -163,11 +151,11 @@ func (direct) receive(key string, e any, fdp *uintptr) (func() error, error) {
 	return Receive(key, e, fdp)
 }

-func (direct) bindMount(source, target string, flags uintptr, eq bool) error {
-	return hostProc.bindMount(source, target, flags, eq)
+func (direct) bindMount(msg message.Msg, source, target string, flags uintptr) error {
+	return hostProc.bindMount(msg, source, target, flags)
 }
-func (direct) remount(target string, flags uintptr) error {
-	return hostProc.remount(target, flags)
+func (direct) remount(msg message.Msg, target string, flags uintptr) error {
+	return hostProc.remount(msg, target, flags)
 }
 func (k direct) mountTmpfs(fsname, target string, flags uintptr, size int, perm os.FileMode) error {
 	return mountTmpfs(k, fsname, target, flags, size, perm)
@@ -225,7 +213,7 @@ func (direct) pivotRoot(newroot, putold string) (err error) {
 	return syscall.PivotRoot(newroot, putold)
 }
 func (direct) mount(source, target, fstype string, flags uintptr, data string) (err error) {
-	return syscall.Mount(source, target, fstype, flags, data)
+	return mount(source, target, fstype, flags, data)
 }
 func (direct) unmount(target string, flags int) (err error) {
 	return syscall.Unmount(target, flags)
@@ -234,12 +222,6 @@ func (direct) wait4(pid int, wstatus *syscall.WaitStatus, options int, rusage *s
 	return syscall.Wait4(pid, wstatus, options, rusage)
 }

-func (direct) printf(format string, v ...any)          { log.Printf(format, v...) }
-func (direct) fatal(v ...any)                          { log.Fatal(v...) }
-func (direct) fatalf(format string, v ...any)          { log.Fatalf(format, v...) }
-func (direct) verbose(v ...any)                        { msg.Verbose(v...) }
-func (direct) verbosef(format string, v ...any)        { msg.Verbosef(format, v...) }
-func (direct) suspend()                                { msg.Suspend() }
-func (direct) resume() bool                            { return msg.Resume() }
-func (direct) beforeExit()                             { msg.BeforeExit() }
-func (direct) printBaseErr(err error, fallback string) { msg.PrintBaseErr(err, fallback) }
+func (direct) printf(msg message.Msg, format string, v ...any) { msg.GetLogger().Printf(format, v...) }
+func (direct) fatal(msg message.Msg, v ...any)                 { msg.GetLogger().Fatal(v...) }
+func (direct) fatalf(msg message.Msg, format string, v ...any) { msg.GetLogger().Fatalf(format, v...) }
--- a/container/dispatcher_test.go
+++ b/container/dispatcher_test.go
@@ -2,25 +2,25 @@ package container

 import (
 	"bytes"
-	"errors"
+	"fmt"
 	"io"
 	"io/fs"
+	"log"
 	"os"
 	"os/exec"
 	"reflect"
 	"runtime"
 	"slices"
 	"strings"
-	"sync"
 	"syscall"
 	"testing"
 	"time"

 	"hakurei.app/container/seccomp"
+	"hakurei.app/container/stub"
+	"hakurei.app/message"
 )

-var errUnique = errors.New("unique error injected by the test suite")
-
 type opValidTestCase struct {
 	name string
 	op   Op
@@ -28,9 +28,17 @@ type opValidTestCase struct {
 }

 func checkOpsValid(t *testing.T, testCases []opValidTestCase) {
+	t.Helper()
+
 	t.Run("valid", func(t *testing.T) {
+		t.Helper()
+		t.Parallel()
+
 		for _, tc := range testCases {
 			t.Run(tc.name, func(t *testing.T) {
+				t.Helper()
+				t.Parallel()
+
 				if got := tc.op.Valid(); got != tc.want {
 					t.Errorf("Valid: %v, want %v", got, tc.want)
 				}
@@ -46,9 +54,17 @@ type opsBuilderTestCase struct {
 }

 func checkOpsBuilder(t *testing.T, testCases []opsBuilderTestCase) {
+	t.Helper()
+
 	t.Run("build", func(t *testing.T) {
+		t.Helper()
+		t.Parallel()
+
 		for _, tc := range testCases {
 			t.Run(tc.name, func(t *testing.T) {
+				t.Helper()
+				t.Parallel()
+
 				if !slices.EqualFunc(*tc.ops, tc.want, func(op Op, v Op) bool { return op.Is(v) }) {
 					t.Errorf("Ops: %#v, want %#v", tc.ops, tc.want)
 				}
@@ -64,9 +80,17 @@ type opIsTestCase struct {
 }

 func checkOpIs(t *testing.T, testCases []opIsTestCase) {
+	t.Helper()
+
 	t.Run("is", func(t *testing.T) {
+		t.Helper()
+		t.Parallel()
+
 		for _, tc := range testCases {
 			t.Run(tc.name, func(t *testing.T) {
+				t.Helper()
+				t.Parallel()
+
 				if got := tc.op.Is(tc.v); got != tc.want {
 					t.Errorf("Is: %v, want %v", got, tc.want)
 				}
@@ -84,16 +108,28 @@ type opMetaTestCase struct {
 }

 func checkOpMeta(t *testing.T, testCases []opMetaTestCase) {
+	t.Helper()
+
 	t.Run("meta", func(t *testing.T) {
+		t.Helper()
+		t.Parallel()
+
 		for _, tc := range testCases {
 			t.Run(tc.name, func(t *testing.T) {
+				t.Helper()
+				t.Parallel()
+
 				t.Run("prefix", func(t *testing.T) {
-					if got := tc.op.prefix(); got != tc.wantPrefix {
+					t.Helper()
+
+					if got, _ := tc.op.prefix(); got != tc.wantPrefix {
 						t.Errorf("prefix: %q, want %q", got, tc.wantPrefix)
 					}
 				})

 				t.Run("string", func(t *testing.T) {
+					t.Helper()
+
 					if got := tc.op.String(); got != tc.wantString {
 						t.Errorf("String: %s, want %s", got, tc.wantString)
 					}
@@ -103,23 +139,37 @@ func checkOpMeta(t *testing.T, testCases []opMetaTestCase) {
 	})
 }

+// call initialises a [stub.Call].
+// This keeps composites analysis happy without making the test cases too bloated.
+func call(name string, args stub.ExpectArgs, ret any, err error) stub.Call {
+	return stub.Call{Name: name, Args: args, Ret: ret, Err: err}
+}
+
 type simpleTestCase struct {
 	name    string
-	f       func(k syscallDispatcher) error
-	want    [][]kexpect
+	f       func(k *kstub) error
+	want    stub.Expect
 	wantErr error
 }

 func checkSimple(t *testing.T, fname string, testCases []simpleTestCase) {
+	t.Helper()
+
 	for _, tc := range testCases {
 		t.Run(tc.name, func(t *testing.T) {
-			defer handleExitStub()
-			k := &kstub{t: t, want: tc.want, wg: new(sync.WaitGroup)}
-			if err := tc.f(k); !errors.Is(err, tc.wantErr) {
+			t.Helper()
+			t.Parallel()
+
+			wait4signal := make(chan struct{})
+			k := &kstub{wait4signal, stub.New(t, func(s *stub.Stub[syscallDispatcher]) syscallDispatcher { return &kstub{wait4signal, s} }, tc.want)}
+			defer stub.HandleExit(t)
+			if err := tc.f(k); !reflect.DeepEqual(err, tc.wantErr) {
 				t.Errorf("%s: error = %v, want %v", fname, err, tc.wantErr)
 			}
-			k.handleIncomplete(func(k *kstub) {
-				t.Errorf("%s: %d calls, want %d (track %d)", fname, k.pos, len(k.want[k.track]), k.track)
+			k.VisitIncomplete(func(s *stub.Stub[syscallDispatcher]) {
+				t.Helper()
+
+				t.Errorf("%s: %d calls, want %d", fname, s.Pos(), s.Len())
 			})
 		})
 	}
@@ -130,36 +180,47 @@ type opBehaviourTestCase struct {
 	params *Params
 	op     Op

-	early        []kexpect
+	early        []stub.Call
 	wantErrEarly error

-	apply        []kexpect
+	apply        []stub.Call
 	wantErrApply error
 }

 func checkOpBehaviour(t *testing.T, testCases []opBehaviourTestCase) {
+	t.Helper()
+
 	t.Run("behaviour", func(t *testing.T) {
+		t.Helper()
+		t.Parallel()
+
 		for _, tc := range testCases {
 			t.Run(tc.name, func(t *testing.T) {
-				defer handleExitStub()
-				state := &setupState{Params: tc.params}
-				k := &kstub{t: t, want: [][]kexpect{slices.Concat(tc.early, []kexpect{{name: "\x00"}}, tc.apply)}, wg: new(sync.WaitGroup)}
+				t.Helper()
+				t.Parallel()
+
+				k := &kstub{nil, stub.New(t,
+					func(s *stub.Stub[syscallDispatcher]) syscallDispatcher { return &kstub{nil, s} },
+					stub.Expect{Calls: slices.Concat(tc.early, []stub.Call{{Name: stub.CallSeparator}}, tc.apply)},
+				)}
+				state := &setupState{Params: tc.params, Msg: k}
+				defer stub.HandleExit(t)
 				errEarly := tc.op.early(state, k)
-				k.expect("\x00")
-				if !errors.Is(errEarly, tc.wantErrEarly) {
+				k.Expects(stub.CallSeparator)
+				if !reflect.DeepEqual(errEarly, tc.wantErrEarly) {
 					t.Errorf("early: error = %v, want %v", errEarly, tc.wantErrEarly)
 				}
 				if errEarly != nil {
 					goto out
 				}

-				if err := tc.op.apply(state, k); !errors.Is(err, tc.wantErrApply) {
+				if err := tc.op.apply(state, k); !reflect.DeepEqual(err, tc.wantErrApply) {
 					t.Errorf("apply: error = %v, want %v", err, tc.wantErrApply)
 				}

 			out:
-				k.handleIncomplete(func(k *kstub) {
-					count := k.pos - 1 // separator
+				k.VisitIncomplete(func(s *stub.Stub[syscallDispatcher]) {
+					count := k.Pos() - 1 // separator
 					if count < len(tc.early) {
 						t.Errorf("early: %d calls, want %d", count, len(tc.early))
 					} else {
@@ -226,8 +287,6 @@ func (writeErrOsFile) Stat() (fs.FileInfo, error)  { panic("unreachable") }
 func (writeErrOsFile) Read([]byte) (int, error)    { panic("unreachable") }
 func (writeErrOsFile) Close() error                { panic("unreachable") }

-type expectArgs = [5]any
-
 type isDirFi bool

 func (isDirFi) Name() string       { panic("unreachable") }
@@ -252,184 +311,98 @@ func (nameDentry) IsDir() bool                { panic("unreachable") }
 func (nameDentry) Type() fs.FileMode          { panic("unreachable") }
 func (nameDentry) Info() (fs.FileInfo, error) { panic("unreachable") }

-type kexpect struct {
-	name string
-	args expectArgs
-	ret  any
-	err  error
-}
-
-func (k *kexpect) error(ok ...bool) error {
-	if !slices.Contains(ok, false) {
-		return k.err
-	}
-	return syscall.ENOTRECOVERABLE
-}
-
-func handleExitStub() {
-	r := recover()
-	if r == 0xdeadbeef {
-		return
-	}
-	if r != nil {
-		panic(r)
-	}
-}
+const (
+	// magicWait4Signal must be used in a single pair of signal and wait4 calls across two goroutines
+	// originating from the same toplevel kstub.
+	// To enable this behaviour this value must be the last element of the args field in the wait4 call
+	// and the ret value of the signal call.
+	magicWait4Signal = 0xdef
+)

 type kstub struct {
-	t *testing.T
-
-	want [][]kexpect
-	// pos is the current position in want[track].
-	pos int
-	// track is the current active want.
-	track int
-	// sub stores addresses of kstub created by new.
-	sub []*kstub
-	// wg waits for all descendants to complete.
-	wg *sync.WaitGroup
+	wait4signal chan struct{}
+	*stub.Stub[syscallDispatcher]
 }

-// handleIncomplete calls f on an incomplete k and all its descendants.
-func (k *kstub) handleIncomplete(f func(k *kstub)) {
-	k.wg.Wait()
+func (k *kstub) new(f func(k syscallDispatcher)) { k.Helper(); k.New(f) }

-	if k.want != nil && len(k.want[k.track]) != k.pos {
-		f(k)
-	}
-	for _, sk := range k.sub {
-		sk.handleIncomplete(f)
-	}
-}
-
-// expect checks name and returns the current kexpect and advances pos.
-func (k *kstub) expect(name string) (expect *kexpect) {
-	if len(k.want[k.track]) == k.pos {
-		k.t.Fatal("expect: want too short")
-	}
-	expect = &k.want[k.track][k.pos]
-	if name != expect.name {
-		if expect.name == "\x00" {
-			k.t.Fatalf("expect: func = %s, separator overrun", name)
-		}
-		if name == "\x00" {
-			k.t.Fatalf("expect: separator, want %s", expect.name)
-		}
-		k.t.Fatalf("expect: func = %s, want %s", name, expect.name)
-	}
-	k.pos++
-	return
-}
-
-// checkArg checks an argument comparable with the == operator. Avoid using this with pointers.
-func checkArg[T comparable](k *kstub, arg string, got T, n int) bool {
-	if k.pos == 0 {
-		panic("invalid call to checkArg")
-	}
-	expect := k.want[k.track][k.pos-1]
-	want, ok := expect.args[n].(T)
-	if !ok || got != want {
-		k.t.Errorf("%s: %s = %#v, want %#v (%d)", expect.name, arg, got, want, k.pos-1)
-		return false
-	}
-	return true
-}
-
-// checkArgReflect checks an argument of any type.
-func checkArgReflect(k *kstub, arg string, got any, n int) bool {
-	if k.pos == 0 {
-		panic("invalid call to checkArgReflect")
-	}
-	expect := k.want[k.track][k.pos-1]
-	want := expect.args[n]
-	if !reflect.DeepEqual(got, want) {
-		k.t.Errorf("%s: %s = %#v, want %#v (%d)", expect.name, arg, got, want, k.pos-1)
-		return false
-	}
-	return true
-}
-
-func (k *kstub) new(f func(k syscallDispatcher)) {
-	k.expect("new")
-	if len(k.want) <= k.track+1 {
-		k.t.Fatalf("new: track overrun")
-	}
-	sk := &kstub{t: k.t, want: k.want, track: len(k.sub) + 1, wg: k.wg}
-	k.sub = append(k.sub, sk)
-	k.wg.Add(1)
-	go func() {
-		defer k.wg.Done()
-		defer handleExitStub()
-		f(sk)
-	}()
-}
-
-func (k *kstub) lockOSThread() { k.expect("lockOSThread") }
+func (k *kstub) lockOSThread() { k.Helper(); k.Expects("lockOSThread") }

 func (k *kstub) setPtracer(pid uintptr) error {
-	return k.expect("setPtracer").error(
-		checkArg(k, "pid", pid, 0))
+	k.Helper()
+	return k.Expects("setPtracer").Error(
+		stub.CheckArg(k.Stub, "pid", pid, 0))
 }

 func (k *kstub) setDumpable(dumpable uintptr) error {
-	return k.expect("setDumpable").error(
-		checkArg(k, "dumpable", dumpable, 0))
+	k.Helper()
+	return k.Expects("setDumpable").Error(
+		stub.CheckArg(k.Stub, "dumpable", dumpable, 0))
 }

-func (k *kstub) setNoNewPrivs() error { return k.expect("setNoNewPrivs").err }
-func (k *kstub) lastcap() uintptr     { return k.expect("lastcap").ret.(uintptr) }
+func (k *kstub) setNoNewPrivs() error { k.Helper(); return k.Expects("setNoNewPrivs").Err }
+func (k *kstub) lastcap(msg message.Msg) uintptr {
+	k.Helper()
+	k.checkMsg(msg)
+	return k.Expects("lastcap").Ret.(uintptr)
+}

 func (k *kstub) capset(hdrp *capHeader, datap *[2]capData) error {
-	return k.expect("capset").error(
-		checkArgReflect(k, "hdrp", hdrp, 0),
-		checkArgReflect(k, "datap", datap, 1))
+	k.Helper()
+	return k.Expects("capset").Error(
+		stub.CheckArgReflect(k.Stub, "hdrp", hdrp, 0),
+		stub.CheckArgReflect(k.Stub, "datap", datap, 1))
 }

 func (k *kstub) capBoundingSetDrop(cap uintptr) error {
-	return k.expect("capBoundingSetDrop").error(
-		checkArg(k, "cap", cap, 0))
+	k.Helper()
+	return k.Expects("capBoundingSetDrop").Error(
+		stub.CheckArg(k.Stub, "cap", cap, 0))
 }

-func (k *kstub) capAmbientClearAll() error { return k.expect("capAmbientClearAll").err }
+func (k *kstub) capAmbientClearAll() error { k.Helper(); return k.Expects("capAmbientClearAll").Err }

 func (k *kstub) capAmbientRaise(cap uintptr) error {
-	return k.expect("capAmbientRaise").error(
-		checkArg(k, "cap", cap, 0))
+	k.Helper()
+	return k.Expects("capAmbientRaise").Error(
+		stub.CheckArg(k.Stub, "cap", cap, 0))
 }

 func (k *kstub) isatty(fd int) bool {
-	expect := k.expect("isatty")
-	if !checkArg(k, "fd", fd, 0) {
-		k.t.FailNow()
+	k.Helper()
+	expect := k.Expects("isatty")
+	if !stub.CheckArg(k.Stub, "fd", fd, 0) {
+		k.FailNow()
 	}
-	return expect.ret.(bool)
+	return expect.Ret.(bool)
 }

 func (k *kstub) receive(key string, e any, fdp *uintptr) (closeFunc func() error, err error) {
-	expect := k.expect("receive")
+	k.Helper()
+	expect := k.Expects("receive")

 	var closed bool
 	closeFunc = func() error {
 		if closed {
-			k.t.Error("closeFunc called more than once")
+			k.Error("closeFunc called more than once")
 			return os.ErrClosed
 		}
 		closed = true

-		if expect.ret != nil {
+		if expect.Ret != nil {
 			// use return stored in kexpect for closeFunc instead
-			return expect.ret.(error)
+			return expect.Ret.(error)
 		}
 		return nil
 	}
-	err = expect.error(
-		checkArg(k, "key", key, 0),
-		checkArgReflect(k, "e", e, 1),
-		checkArgReflect(k, "fdp", fdp, 2))
+	err = expect.Error(
+		stub.CheckArg(k.Stub, "key", key, 0),
+		stub.CheckArgReflect(k.Stub, "e", e, 1),
+		stub.CheckArgReflect(k.Stub, "fdp", fdp, 2))

 	// 3 is unused so stores params
-	if expect.args[3] != nil {
-		if v, ok := expect.args[3].(*initParams); ok && v != nil {
+	if expect.Args[3] != nil {
+		if v, ok := expect.Args[3].(*initParams); ok && v != nil {
 			if p, ok0 := e.(*initParams); ok0 && p != nil {
 				*p = *v
 			}
@@ -437,8 +410,8 @@ func (k *kstub) receive(key string, e any, fdp *uintptr) (closeFunc func() error
 	}

 	// 4 is unused so stores fd
-	if expect.args[4] != nil {
-		if v, ok := expect.args[4].(uintptr); ok && v >= 3 {
+	if expect.Args[4] != nil {
+		if v, ok := expect.Args[4].(uintptr); ok && v >= 3 {
 			if fdp != nil {
 				*fdp = v
 			}
@@ -448,301 +421,372 @@ func (k *kstub) receive(key string, e any, fdp *uintptr) (closeFunc func() error
 	return
 }

-func (k *kstub) bindMount(source, target string, flags uintptr, eq bool) error {
-	return k.expect("bindMount").error(
-		checkArg(k, "source", source, 0),
-		checkArg(k, "target", target, 1),
-		checkArg(k, "flags", flags, 2),
-		checkArg(k, "eq", eq, 3))
+func (k *kstub) bindMount(msg message.Msg, source, target string, flags uintptr) error {
+	k.Helper()
+	k.checkMsg(msg)
+	return k.Expects("bindMount").Error(
+		stub.CheckArg(k.Stub, "source", source, 0),
+		stub.CheckArg(k.Stub, "target", target, 1),
+		stub.CheckArg(k.Stub, "flags", flags, 2))
 }

-func (k *kstub) remount(target string, flags uintptr) error {
-	return k.expect("remount").error(
-		checkArg(k, "target", target, 0),
-		checkArg(k, "flags", flags, 1))
+func (k *kstub) remount(msg message.Msg, target string, flags uintptr) error {
+	k.Helper()
+	k.checkMsg(msg)
+	return k.Expects("remount").Error(
+		stub.CheckArg(k.Stub, "target", target, 0),
+		stub.CheckArg(k.Stub, "flags", flags, 1))
 }

 func (k *kstub) mountTmpfs(fsname, target string, flags uintptr, size int, perm os.FileMode) error {
-	return k.expect("mountTmpfs").error(
-		checkArg(k, "fsname", fsname, 0),
-		checkArg(k, "target", target, 1),
-		checkArg(k, "flags", flags, 2),
-		checkArg(k, "size", size, 3),
-		checkArg(k, "perm", perm, 4))
+	k.Helper()
+	return k.Expects("mountTmpfs").Error(
+		stub.CheckArg(k.Stub, "fsname", fsname, 0),
+		stub.CheckArg(k.Stub, "target", target, 1),
+		stub.CheckArg(k.Stub, "flags", flags, 2),
+		stub.CheckArg(k.Stub, "size", size, 3),
+		stub.CheckArg(k.Stub, "perm", perm, 4))
 }

 func (k *kstub) ensureFile(name string, perm, pperm os.FileMode) error {
-
-	return k.expect("ensureFile").error(
-		checkArg(k, "name", name, 0),
-		checkArg(k, "perm", perm, 1),
-		checkArg(k, "pperm", pperm, 2))
+	k.Helper()
+	return k.Expects("ensureFile").Error(
+		stub.CheckArg(k.Stub, "name", name, 0),
+		stub.CheckArg(k.Stub, "perm", perm, 1),
+		stub.CheckArg(k.Stub, "pperm", pperm, 2))
 }

 func (k *kstub) seccompLoad(rules []seccomp.NativeRule, flags seccomp.ExportFlag) error {
-	return k.expect("seccompLoad").error(
-		checkArgReflect(k, "rules", rules, 0),
-		checkArg(k, "flags", flags, 1))
+	k.Helper()
+	return k.Expects("seccompLoad").Error(
+		stub.CheckArgReflect(k.Stub, "rules", rules, 0),
+		stub.CheckArg(k.Stub, "flags", flags, 1))
 }

 func (k *kstub) notify(c chan<- os.Signal, sig ...os.Signal) {
-	expect := k.expect("notify")
-	if c == nil || expect.error(
-		checkArgReflect(k, "sig", sig, 1)) != nil {
-		k.t.FailNow()
+	k.Helper()
+	expect := k.Expects("notify")
+	if c == nil || expect.Error(
+		stub.CheckArgReflect(k.Stub, "sig", sig, 1)) != nil {
+		k.FailNow()
 	}

 	// export channel for external instrumentation
-	if chanf, ok := expect.args[0].(func(c chan<- os.Signal)); ok && chanf != nil {
+	if chanf, ok := expect.Args[0].(func(c chan<- os.Signal)); ok && chanf != nil {
 		chanf(c)
 	}
 }

 func (k *kstub) start(c *exec.Cmd) error {
-	expect := k.expect("start")
-	err := expect.error(
-		checkArg(k, "c.Path", c.Path, 0),
-		checkArgReflect(k, "c.Args", c.Args, 1),
-		checkArgReflect(k, "c.Env", c.Env, 2),
-		checkArg(k, "c.Dir", c.Dir, 3))
+	k.Helper()
+	expect := k.Expects("start")
+	err := expect.Error(
+		stub.CheckArg(k.Stub, "c.Path", c.Path, 0),
+		stub.CheckArgReflect(k.Stub, "c.Args", c.Args, 1),
+		stub.CheckArgReflect(k.Stub, "c.Env", c.Env, 2),
+		stub.CheckArg(k.Stub, "c.Dir", c.Dir, 3))

-	if process, ok := expect.ret.(*os.Process); ok && process != nil {
+	if process, ok := expect.Ret.(*os.Process); ok && process != nil {
 		c.Process = process
 	}
 	return err
 }

 func (k *kstub) signal(c *exec.Cmd, sig os.Signal) error {
-	return k.expect("signal").error(
-		checkArg(k, "c.Path", c.Path, 0),
-		checkArgReflect(k, "c.Args", c.Args, 1),
-		checkArgReflect(k, "c.Env", c.Env, 2),
-		checkArg(k, "c.Dir", c.Dir, 3),
-		checkArg(k, "sig", sig, 4))
+	k.Helper()
+	expect := k.Expects("signal")
+	if v, ok := expect.Ret.(int); ok && v == magicWait4Signal {
+		if k.wait4signal == nil {
+			panic("kstub not initialised for wait4 simulation")
+		}
+		defer func() { close(k.wait4signal) }()
+	}
+	return expect.Error(
+		stub.CheckArg(k.Stub, "c.Path", c.Path, 0),
+		stub.CheckArgReflect(k.Stub, "c.Args", c.Args, 1),
+		stub.CheckArgReflect(k.Stub, "c.Env", c.Env, 2),
+		stub.CheckArg(k.Stub, "c.Dir", c.Dir, 3),
+		stub.CheckArg(k.Stub, "sig", sig, 4))
 }

 func (k *kstub) evalSymlinks(path string) (string, error) {
-	expect := k.expect("evalSymlinks")
-	return expect.ret.(string), expect.error(
-		checkArg(k, "path", path, 0))
+	k.Helper()
+	expect := k.Expects("evalSymlinks")
+	return expect.Ret.(string), expect.Error(
+		stub.CheckArg(k.Stub, "path", path, 0))
 }

 func (k *kstub) exit(code int) {
-	k.expect("exit")
-	if !checkArg(k, "code", code, 0) {
-		k.t.FailNow()
+	k.Helper()
+	k.Expects("exit")
+	if !stub.CheckArg(k.Stub, "code", code, 0) {
+		k.FailNow()
 	}
-	panic(0xdeadbeef)
+	panic(stub.PanicExit)
 }

-func (k *kstub) getpid() int { return k.expect("getpid").ret.(int) }
+func (k *kstub) getpid() int { k.Helper(); return k.Expects("getpid").Ret.(int) }

 func (k *kstub) stat(name string) (os.FileInfo, error) {
-	expect := k.expect("stat")
-	return expect.ret.(os.FileInfo), expect.error(
-		checkArg(k, "name", name, 0))
+	k.Helper()
+	expect := k.Expects("stat")
+	return expect.Ret.(os.FileInfo), expect.Error(
+		stub.CheckArg(k.Stub, "name", name, 0))
 }

 func (k *kstub) mkdir(name string, perm os.FileMode) error {
-	return k.expect("mkdir").error(
-		checkArg(k, "name", name, 0),
-		checkArg(k, "perm", perm, 1))
+	k.Helper()
+	return k.Expects("mkdir").Error(
+		stub.CheckArg(k.Stub, "name", name, 0),
+		stub.CheckArg(k.Stub, "perm", perm, 1))
 }

 func (k *kstub) mkdirTemp(dir, pattern string) (string, error) {
-	expect := k.expect("mkdirTemp")
-	return expect.ret.(string), expect.error(
-		checkArg(k, "dir", dir, 0),
-		checkArg(k, "pattern", pattern, 1))
+	k.Helper()
+	expect := k.Expects("mkdirTemp")
+	return expect.Ret.(string), expect.Error(
+		stub.CheckArg(k.Stub, "dir", dir, 0),
+		stub.CheckArg(k.Stub, "pattern", pattern, 1))
 }

 func (k *kstub) mkdirAll(path string, perm os.FileMode) error {
-	return k.expect("mkdirAll").error(
-		checkArg(k, "path", path, 0),
-		checkArg(k, "perm", perm, 1))
+	k.Helper()
+	return k.Expects("mkdirAll").Error(
+		stub.CheckArg(k.Stub, "path", path, 0),
+		stub.CheckArg(k.Stub, "perm", perm, 1))
 }

 func (k *kstub) readdir(name string) ([]os.DirEntry, error) {
-	expect := k.expect("readdir")
-	return expect.ret.([]os.DirEntry), expect.error(
-		checkArg(k, "name", name, 0))
+	k.Helper()
+	expect := k.Expects("readdir")
+	return expect.Ret.([]os.DirEntry), expect.Error(
+		stub.CheckArg(k.Stub, "name", name, 0))
 }

 func (k *kstub) openNew(name string) (osFile, error) {
-	expect := k.expect("openNew")
-	return expect.ret.(osFile), expect.error(
-		checkArg(k, "name", name, 0))
+	k.Helper()
+	expect := k.Expects("openNew")
+	return expect.Ret.(osFile), expect.Error(
+		stub.CheckArg(k.Stub, "name", name, 0))
 }

 func (k *kstub) writeFile(name string, data []byte, perm os.FileMode) error {
-	return k.expect("writeFile").error(
-		checkArg(k, "name", name, 0),
-		checkArgReflect(k, "data", data, 1),
-		checkArg(k, "perm", perm, 2))
+	k.Helper()
+	return k.Expects("writeFile").Error(
+		stub.CheckArg(k.Stub, "name", name, 0),
+		stub.CheckArgReflect(k.Stub, "data", data, 1),
+		stub.CheckArg(k.Stub, "perm", perm, 2))
 }

 func (k *kstub) createTemp(dir, pattern string) (osFile, error) {
-	expect := k.expect("createTemp")
-	return expect.ret.(osFile), expect.error(
-		checkArg(k, "dir", dir, 0),
-		checkArg(k, "pattern", pattern, 1))
+	k.Helper()
+	expect := k.Expects("createTemp")
+	return expect.Ret.(osFile), expect.Error(
+		stub.CheckArg(k.Stub, "dir", dir, 0),
+		stub.CheckArg(k.Stub, "pattern", pattern, 1))
 }

 func (k *kstub) remove(name string) error {
-	return k.expect("remove").error(
-		checkArg(k, "name", name, 0))
+	k.Helper()
+	return k.Expects("remove").Error(
+		stub.CheckArg(k.Stub, "name", name, 0))
 }

 func (k *kstub) newFile(fd uintptr, name string) *os.File {
-	expect := k.expect("newFile")
-	if expect.error(
-		checkArg(k, "fd", fd, 0),
-		checkArg(k, "name", name, 1)) != nil {
-		k.t.FailNow()
+	k.Helper()
+	expect := k.Expects("newFile")
+	if expect.Error(
+		stub.CheckArg(k.Stub, "fd", fd, 0),
+		stub.CheckArg(k.Stub, "name", name, 1)) != nil {
+		k.FailNow()
 	}
-	return expect.ret.(*os.File)
+	return expect.Ret.(*os.File)
 }

 func (k *kstub) symlink(oldname, newname string) error {
-	return k.expect("symlink").error(
-		checkArg(k, "oldname", oldname, 0),
-		checkArg(k, "newname", newname, 1))
+	k.Helper()
+	return k.Expects("symlink").Error(
+		stub.CheckArg(k.Stub, "oldname", oldname, 0),
+		stub.CheckArg(k.Stub, "newname", newname, 1))
 }

 func (k *kstub) readlink(name string) (string, error) {
-	expect := k.expect("readlink")
-	return expect.ret.(string), expect.error(
-		checkArg(k, "name", name, 0))
+	k.Helper()
+	expect := k.Expects("readlink")
+	return expect.Ret.(string), expect.Error(
+		stub.CheckArg(k.Stub, "name", name, 0))
 }

 func (k *kstub) umask(mask int) (oldmask int) {
-	expect := k.expect("umask")
-	if !checkArg(k, "mask", mask, 0) {
-		k.t.FailNow()
+	k.Helper()
+	expect := k.Expects("umask")
+	if !stub.CheckArg(k.Stub, "mask", mask, 0) {
+		k.FailNow()
 	}
-	return expect.ret.(int)
+	return expect.Ret.(int)
 }

 func (k *kstub) sethostname(p []byte) (err error) {
-	return k.expect("sethostname").error(
-		checkArgReflect(k, "p", p, 0))
+	k.Helper()
+	return k.Expects("sethostname").Error(
+		stub.CheckArgReflect(k.Stub, "p", p, 0))
 }

 func (k *kstub) chdir(path string) (err error) {
-	return k.expect("chdir").error(
-		checkArg(k, "path", path, 0))
+	k.Helper()
+	return k.Expects("chdir").Error(
+		stub.CheckArg(k.Stub, "path", path, 0))
 }

 func (k *kstub) fchdir(fd int) (err error) {
-	return k.expect("fchdir").error(
-		checkArg(k, "fd", fd, 0))
+	k.Helper()
+	return k.Expects("fchdir").Error(
+		stub.CheckArg(k.Stub, "fd", fd, 0))
 }

 func (k *kstub) open(path string, mode int, perm uint32) (fd int, err error) {
-	expect := k.expect("open")
-	return expect.ret.(int), expect.error(
-		checkArg(k, "path", path, 0),
-		checkArg(k, "mode", mode, 1),
-		checkArg(k, "perm", perm, 2))
+	k.Helper()
+	expect := k.Expects("open")
+	return expect.Ret.(int), expect.Error(
+		stub.CheckArg(k.Stub, "path", path, 0),
+		stub.CheckArg(k.Stub, "mode", mode, 1),
+		stub.CheckArg(k.Stub, "perm", perm, 2))
 }

 func (k *kstub) close(fd int) (err error) {
-	return k.expect("close").error(
-		checkArg(k, "fd", fd, 0))
+	k.Helper()
+	return k.Expects("close").Error(
+		stub.CheckArg(k.Stub, "fd", fd, 0))
 }

 func (k *kstub) pivotRoot(newroot, putold string) (err error) {
-	return k.expect("pivotRoot").error(
-		checkArg(k, "newroot", newroot, 0),
-		checkArg(k, "putold", putold, 1))
+	k.Helper()
+	return k.Expects("pivotRoot").Error(
+		stub.CheckArg(k.Stub, "newroot", newroot, 0),
+		stub.CheckArg(k.Stub, "putold", putold, 1))
 }

 func (k *kstub) mount(source, target, fstype string, flags uintptr, data string) (err error) {
-	return k.expect("mount").error(
-		checkArg(k, "source", source, 0),
-		checkArg(k, "target", target, 1),
-		checkArg(k, "fstype", fstype, 2),
-		checkArg(k, "flags", flags, 3),
-		checkArg(k, "data", data, 4))
+	k.Helper()
+	return k.Expects("mount").Error(
+		stub.CheckArg(k.Stub, "source", source, 0),
+		stub.CheckArg(k.Stub, "target", target, 1),
+		stub.CheckArg(k.Stub, "fstype", fstype, 2),
+		stub.CheckArg(k.Stub, "flags", flags, 3),
+		stub.CheckArg(k.Stub, "data", data, 4))
 }

 func (k *kstub) unmount(target string, flags int) (err error) {
-	return k.expect("unmount").error(
-		checkArg(k, "target", target, 0),
-		checkArg(k, "flags", flags, 1))
+	k.Helper()
+	return k.Expects("unmount").Error(
+		stub.CheckArg(k.Stub, "target", target, 0),
+		stub.CheckArg(k.Stub, "flags", flags, 1))
 }

 func (k *kstub) wait4(pid int, wstatus *syscall.WaitStatus, options int, rusage *syscall.Rusage) (wpid int, err error) {
-	expect := k.expect("wait4")
-	// special case to prevent leaking the wait4 goroutine when testing initEntrypoint
-	if v, ok := expect.args[4].(int); ok && v == 0xdeadbeef {
-		k.t.Log("terminating current goroutine as requested by kexpect")
-		panic(0xdeadbeef)
+	k.Helper()
+	expect := k.Expects("wait4")
+	if v, ok := expect.Args[4].(int); ok {
+		switch v {
+		case stub.PanicExit: // special case to prevent leaking the wait4 goroutine while testing initEntrypoint
+			panic(stub.PanicExit)
+
+		case magicWait4Signal: // block until corresponding signal call
+			if k.wait4signal == nil {
+				panic("kstub not initialised for wait4 simulation")
+			}
+			<-k.wait4signal
+		}
 	}

-	wpid = expect.ret.(int)
-	err = expect.error(
-		checkArg(k, "pid", pid, 0),
-		checkArg(k, "options", options, 2))
+	wpid = expect.Ret.(int)
+	err = expect.Error(
+		stub.CheckArg(k.Stub, "pid", pid, 0),
+		stub.CheckArg(k.Stub, "options", options, 2))

-	if wstatusV, ok := expect.args[1].(syscall.WaitStatus); wstatus != nil && ok {
+	if wstatusV, ok := expect.Args[1].(syscall.WaitStatus); wstatus != nil && ok {
 		*wstatus = wstatusV
 	}
-	if rusageV, ok := expect.args[3].(syscall.Rusage); rusage != nil && ok {
+	if rusageV, ok := expect.Args[3].(syscall.Rusage); rusage != nil && ok {
 		*rusage = rusageV
 	}

 	return
 }

-func (k *kstub) printf(format string, v ...any) {
-	if k.expect("printf").error(
-		checkArg(k, "format", format, 0),
-		checkArgReflect(k, "v", v, 1)) != nil {
-		k.t.FailNow()
+func (k *kstub) printf(_ message.Msg, format string, v ...any) {
+	k.Helper()
+	if k.Expects("printf").Error(
+		stub.CheckArg(k.Stub, "format", format, 0),
+		stub.CheckArgReflect(k.Stub, "v", v, 1)) != nil {
+		k.FailNow()
 	}
 }

-func (k *kstub) fatal(v ...any) {
-	if k.expect("fatal").error(
-		checkArgReflect(k, "v", v, 0)) != nil {
-		k.t.FailNow()
+func (k *kstub) fatal(_ message.Msg, v ...any) {
+	k.Helper()
+	if k.Expects("fatal").Error(
+		stub.CheckArgReflect(k.Stub, "v", v, 0)) != nil {
+		k.FailNow()
 	}
-	panic(0xdeadbeef)
+	panic(stub.PanicExit)
 }

-func (k *kstub) fatalf(format string, v ...any) {
-	if k.expect("fatalf").error(
-		checkArg(k, "format", format, 0),
-		checkArgReflect(k, "v", v, 1)) != nil {
-		k.t.FailNow()
+func (k *kstub) fatalf(_ message.Msg, format string, v ...any) {
+	k.Helper()
+	if k.Expects("fatalf").Error(
+		stub.CheckArg(k.Stub, "format", format, 0),
+		stub.CheckArgReflect(k.Stub, "v", v, 1)) != nil {
+		k.FailNow()
 	}
-	panic(0xdeadbeef)
+	panic(stub.PanicExit)
 }

-func (k *kstub) verbose(v ...any) {
-	if k.expect("verbose").error(
-		checkArgReflect(k, "v", v, 0)) != nil {
-		k.t.FailNow()
+func (k *kstub) checkMsg(msg message.Msg) {
+	k.Helper()
+	var target *kstub
+
+	if state, ok := msg.(*setupState); ok {
+		target = state.Msg.(*kstub)
+	} else {
+		target = msg.(*kstub)
+	}
+
+	if k != target {
+		panic(fmt.Sprintf("unexpected Msg: %#v", msg))
 	}
 }

-func (k *kstub) verbosef(format string, v ...any) {
-	if k.expect("verbosef").error(
-		checkArg(k, "format", format, 0),
-		checkArgReflect(k, "v", v, 1)) != nil {
-		k.t.FailNow()
+func (k *kstub) GetLogger() *log.Logger { panic("unreachable") }
+func (k *kstub) IsVerbose() bool        { panic("unreachable") }
+
+func (k *kstub) SwapVerbose(verbose bool) bool {
+	k.Helper()
+	expect := k.Expects("swapVerbose")
+	if expect.Error(
+		stub.CheckArg(k.Stub, "verbose", verbose, 0)) != nil {
+		k.FailNow()
+	}
+	return expect.Ret.(bool)
+}
+
+func (k *kstub) Verbose(v ...any) {
+	k.Helper()
+	if k.Expects("verbose").Error(
+		stub.CheckArgReflect(k.Stub, "v", v, 0)) != nil {
+		k.FailNow()
 	}
 }

-func (k *kstub) suspend()     { k.expect("suspend") }
-func (k *kstub) resume() bool { return k.expect("resume").ret.(bool) }
-func (k *kstub) beforeExit()  { k.expect("beforeExit") }
-
-func (k *kstub) printBaseErr(err error, fallback string) {
-	if k.expect("printBaseErr").error(
-		checkArgReflect(k, "err", err, 0),
-		checkArg(k, "fallback", fallback, 1)) != nil {
-		k.t.FailNow()
+func (k *kstub) Verbosef(format string, v ...any) {
+	k.Helper()
+	if k.Expects("verbosef").Error(
+		stub.CheckArg(k.Stub, "format", format, 0),
+		stub.CheckArgReflect(k.Stub, "v", v, 1)) != nil {
+		k.FailNow()
 	}
 }
+
+func (k *kstub) Suspend() bool { k.Helper(); return k.Expects("suspend").Ret.(bool) }
+func (k *kstub) Resume() bool  { k.Helper(); return k.Expects("resume").Ret.(bool) }
+func (k *kstub) BeforeExit()   { k.Helper(); k.Expects("beforeExit") }
--- a/container/errors.go
+++ b/container/errors.go
@@ -0,0 +1,124 @@
+package container
+
+import (
+	"errors"
+	"os"
+	"syscall"
+
+	"hakurei.app/container/check"
+	"hakurei.app/container/vfs"
+)
+
+// messageFromError returns a printable error message for a supported concrete type.
+func messageFromError(err error) (string, bool) {
+	if m, ok := messagePrefixP[MountError]("cannot ", err); ok {
+		return m, ok
+	}
+	if m, ok := messagePrefixP[os.PathError]("cannot ", err); ok {
+		return m, ok
+	}
+	if m, ok := messagePrefixP[check.AbsoluteError]("", err); ok {
+		return m, ok
+	}
+	if m, ok := messagePrefix[OpRepeatError]("", err); ok {
+		return m, ok
+	}
+	if m, ok := messagePrefix[OpStateError]("", err); ok {
+		return m, ok
+	}
+
+	if m, ok := messagePrefixP[vfs.DecoderError]("cannot ", err); ok {
+		return m, ok
+	}
+	if m, ok := messagePrefix[TmpfsSizeError]("", err); ok {
+		return m, ok
+	}
+
+	return zeroString, false
+}
+
+// messagePrefix checks and prefixes the error message of a non-pointer error.
+// While this is usable for pointer errors, such use should be avoided as nil check is omitted.
+func messagePrefix[T error](prefix string, err error) (string, bool) {
+	var targetError T
+	if errors.As(err, &targetError) {
+		return prefix + targetError.Error(), true
+	}
+	return zeroString, false
+}
+
+// messagePrefixP checks and prefixes the error message of a pointer error.
+func messagePrefixP[V any, T interface {
+	*V
+	error
+}](prefix string, err error) (string, bool) {
+	var targetError T
+	if errors.As(err, &targetError) && targetError != nil {
+		return prefix + targetError.Error(), true
+	}
+	return zeroString, false
+}
+
+// MountError wraps errors returned by syscall.Mount.
+type MountError struct {
+	Source, Target, Fstype string
+
+	Flags uintptr
+	Data  string
+	syscall.Errno
+}
+
+func (e *MountError) Unwrap() error {
+	if e.Errno == 0 {
+		return nil
+	}
+	return e.Errno
+}
+
+func (e *MountError) Message() string { return "cannot " + e.Error() }
+func (e *MountError) Error() string {
+	if e.Flags&syscall.MS_BIND != 0 {
+		if e.Flags&syscall.MS_REMOUNT != 0 {
+			return "remount " + e.Target + ": " + e.Errno.Error()
+		}
+		return "bind " + e.Source + " on " + e.Target + ": " + e.Errno.Error()
+	}
+
+	if e.Fstype != FstypeNULL {
+		return "mount " + e.Fstype + " on " + e.Target + ": " + e.Errno.Error()
+	}
+
+	// fallback case: if this is reached, the conditions for it to occur should be handled above
+	return "mount " + e.Target + ": " + e.Errno.Error()
+}
+
+// optionalErrorUnwrap calls [errors.Unwrap] and returns the resulting value
+// if it is not nil, or the original value if it is.
+func optionalErrorUnwrap(err error) error {
+	if underlyingErr := errors.Unwrap(err); underlyingErr != nil {
+		return underlyingErr
+	}
+	return err
+}
+
+// errnoFallback returns the concrete errno from an error, or a [os.PathError] fallback.
+func errnoFallback(op, path string, err error) (syscall.Errno, *os.PathError) {
+	var errno syscall.Errno
+	if !errors.As(err, &errno) {
+		return 0, &os.PathError{Op: op, Path: path, Err: err}
+	}
+	return errno, nil
+}
+
+// mount wraps syscall.Mount for error handling.
+func mount(source, target, fstype string, flags uintptr, data string) error {
+	err := syscall.Mount(source, target, fstype, flags, data)
+	if err == nil {
+		return nil
+	}
+	if errno, pathError := errnoFallback("mount", target, err); pathError != nil {
+		return pathError
+	} else {
+		return &MountError{source, target, fstype, flags, data, errno}
+	}
+}
--- a/container/errors_test.go
+++ b/container/errors_test.go
@@ -0,0 +1,179 @@
+package container
+
+import (
+	"errors"
+	"os"
+	"reflect"
+	"strconv"
+	"syscall"
+	"testing"
+
+	"hakurei.app/container/check"
+	"hakurei.app/container/stub"
+	"hakurei.app/container/vfs"
+)
+
+func TestMessageFromError(t *testing.T) {
+	t.Parallel()
+
+	testCases := []struct {
+		name   string
+		err    error
+		want   string
+		wantOk bool
+	}{
+		{"mount", &MountError{
+			Source: SourceTmpfsEphemeral,
+			Target: "/sysroot/tmp",
+			Fstype: FstypeTmpfs,
+			Flags:  syscall.MS_NOSUID | syscall.MS_NODEV,
+			Data:   zeroString,
+			Errno:  syscall.EINVAL,
+		}, "cannot mount tmpfs on /sysroot/tmp: invalid argument", true},
+
+		{"path", &os.PathError{
+			Op:   "mount",
+			Path: "/sysroot",
+			Err:  stub.UniqueError(0xdeadbeef),
+		}, "cannot mount /sysroot: unique error 3735928559 injected by the test suite", true},
+
+		{"absolute", &check.AbsoluteError{Pathname: "etc/mtab"},
+			`path "etc/mtab" is not absolute`, true},
+
+		{"repeat", OpRepeatError("autoetc"),
+			"autoetc is not repeatable", true},
+
+		{"state", OpStateError("overlay"),
+			"impossible overlay state reached", true},
+
+		{"vfs parse", &vfs.DecoderError{Op: "parse", Line: 0xdeadbeef, Err: &strconv.NumError{Func: "Atoi", Num: "meow", Err: strconv.ErrSyntax}},
+			`cannot parse mountinfo at line 3735928559: numeric field "meow" invalid syntax`, true},
+
+		{"tmpfs", TmpfsSizeError(-1),
+			"tmpfs size -1 out of bounds", true},
+
+		{"unsupported", stub.UniqueError(0xdeadbeef), zeroString, false},
+	}
+	for _, tc := range testCases {
+		t.Run(tc.name, func(t *testing.T) {
+			t.Parallel()
+			got, ok := messageFromError(tc.err)
+			if got != tc.want {
+				t.Errorf("messageFromError: %q, want %q", got, tc.want)
+			}
+			if ok != tc.wantOk {
+				t.Errorf("messageFromError: ok = %v, want %v", ok, tc.wantOk)
+			}
+		})
+	}
+}
+
+func TestMountError(t *testing.T) {
+	t.Parallel()
+
+	testCases := []struct {
+		name  string
+		err   error
+		errno syscall.Errno
+		want  string
+	}{
+		{"bind", &MountError{
+			Source: "/host/nix/store",
+			Target: "/sysroot/nix/store",
+			Fstype: FstypeNULL,
+			Flags:  syscall.MS_SILENT | syscall.MS_BIND | syscall.MS_REC,
+			Data:   zeroString,
+			Errno:  syscall.ENOSYS,
+		}, syscall.ENOSYS,
+			"bind /host/nix/store on /sysroot/nix/store: function not implemented"},
+
+		{"remount", &MountError{
+			Source: SourceNone,
+			Target: "/sysroot/nix/store",
+			Fstype: FstypeNULL,
+			Flags:  syscall.MS_SILENT | syscall.MS_BIND | syscall.MS_REMOUNT,
+			Data:   zeroString,
+			Errno:  syscall.EPERM,
+		}, syscall.EPERM,
+			"remount /sysroot/nix/store: operation not permitted"},
+
+		{"overlay", &MountError{
+			Source: SourceOverlay,
+			Target: sysrootPath,
+			Fstype: FstypeOverlay,
+			Data:   `lowerdir=/host/var/lib/planterette/base/debian\:f92c9052`,
+			Errno:  syscall.EINVAL,
+		}, syscall.EINVAL,
+			"mount overlay on /sysroot: invalid argument"},
+
+		{"fallback", &MountError{
+			Source: SourceNone,
+			Target: sysrootPath,
+			Fstype: FstypeNULL,
+			Errno:  syscall.ENOTRECOVERABLE,
+		}, syscall.ENOTRECOVERABLE,
+			"mount /sysroot: state not recoverable"},
+	}
+	for _, tc := range testCases {
+		t.Run(tc.name, func(t *testing.T) {
+			t.Parallel()
+			t.Run("is", func(t *testing.T) {
+				if !errors.Is(tc.err, tc.errno) {
+					t.Errorf("Is: %#v is not %v", tc.err, tc.errno)
+				}
+			})
+			t.Run("error", func(t *testing.T) {
+				if got := tc.err.Error(); got != tc.want {
+					t.Errorf("Error: %q, want %q", got, tc.want)
+				}
+			})
+		})
+	}
+
+	t.Run("zero", func(t *testing.T) {
+		t.Parallel()
+		if errors.Is(new(MountError), syscall.Errno(0)) {
+			t.Errorf("Is: zero MountError unexpected true")
+		}
+	})
+}
+
+func TestErrnoFallback(t *testing.T) {
+	t.Parallel()
+
+	testCases := []struct {
+		name      string
+		err       error
+		wantErrno syscall.Errno
+		wantPath  *os.PathError
+	}{
+		{"mount", &MountError{
+			Errno: syscall.ENOTRECOVERABLE,
+		}, syscall.ENOTRECOVERABLE, nil},
+
+		{"path errno", &os.PathError{
+			Err: syscall.ETIMEDOUT,
+		}, syscall.ETIMEDOUT, nil},
+
+		{"fallback", stub.UniqueError(0xcafebabe), 0, &os.PathError{
+			Op:   "fallback",
+			Path: "/proc/nonexistent",
+			Err:  stub.UniqueError(0xcafebabe),
+		}},
+	}
+	for _, tc := range testCases {
+		t.Run(tc.name, func(t *testing.T) {
+			t.Parallel()
+			errno, err := errnoFallback(tc.name, Nonexistent, tc.err)
+			if errno != tc.wantErrno {
+				t.Errorf("errnoFallback: errno = %v, want %v", errno, tc.wantErrno)
+			}
+			if !reflect.DeepEqual(err, tc.wantPath) {
+				t.Errorf("errnoFallback: pathError = %#v, want %#v", err, tc.wantPath)
+			}
+		})
+	}
+}
+
+// InternalMessageFromError exports messageFromError for other tests.
+func InternalMessageFromError(err error) (string, bool) { return messageFromError(err) }
--- a/container/executable.go
+++ b/container/executable.go
@@ -1,9 +1,10 @@
 package container

 import (
-	"log"
 	"os"
 	"sync"
+
+	"hakurei.app/message"
 )

 var (
@@ -11,16 +12,16 @@ var (
 	executableOnce sync.Once
 )

-func copyExecutable() {
+func copyExecutable(msg message.Msg) {
 	if name, err := os.Executable(); err != nil {
 		msg.BeforeExit()
-		log.Fatalf("cannot read executable path: %v", err)
+		msg.GetLogger().Fatalf("cannot read executable path: %v", err)
 	} else {
 		executable = name
 	}
 }

-func MustExecutable() string {
-	executableOnce.Do(copyExecutable)
+func MustExecutable(msg message.Msg) string {
+	executableOnce.Do(func() { copyExecutable(msg) })
 	return executable
 }
--- a/container/executable_test.go
+++ b/container/executable_test.go
@@ -5,13 +5,14 @@ import (
 	"testing"

 	"hakurei.app/container"
+	"hakurei.app/message"
 )

 func TestExecutable(t *testing.T) {
+	t.Parallel()
 	for i := 0; i < 16; i++ {
-		if got := container.MustExecutable(); got != os.Args[0] {
-			t.Errorf("MustExecutable: %q, want %q",
-				got, os.Args[0])
+		if got := container.MustExecutable(message.NewMsg(nil)); got != os.Args[0] {
+			t.Errorf("MustExecutable: %q, want %q", got, os.Args[0])
 		}
 	}
 }
--- a/container/fhs/abs.go
+++ b/container/fhs/abs.go
@@ -0,0 +1,41 @@
+package fhs
+
+import (
+	_ "unsafe"
+
+	"hakurei.app/container/check"
+)
+
+/* constants in this file bypass abs check, be extremely careful when changing them! */
+
+//go:linkname unsafeAbs hakurei.app/container/check.unsafeAbs
+func unsafeAbs(_ string) *check.Absolute
+
+var (
+	// AbsRoot is [Root] as [check.Absolute].
+	AbsRoot = unsafeAbs(Root)
+	// AbsEtc is [Etc] as [check.Absolute].
+	AbsEtc = unsafeAbs(Etc)
+	// AbsTmp is [Tmp] as [check.Absolute].
+	AbsTmp = unsafeAbs(Tmp)
+
+	// AbsRun is [Run] as [check.Absolute].
+	AbsRun = unsafeAbs(Run)
+	// AbsRunUser is [RunUser] as [check.Absolute].
+	AbsRunUser = unsafeAbs(RunUser)
+
+	// AbsUsrBin is [UsrBin] as [check.Absolute].
+	AbsUsrBin = unsafeAbs(UsrBin)
+
+	// AbsVar is [Var] as [check.Absolute].
+	AbsVar = unsafeAbs(Var)
+	// AbsVarLib is [VarLib] as [check.Absolute].
+	AbsVarLib = unsafeAbs(VarLib)
+
+	// AbsDev is [Dev] as [check.Absolute].
+	AbsDev = unsafeAbs(Dev)
+	// AbsProc is [Proc] as [check.Absolute].
+	AbsProc = unsafeAbs(Proc)
+	// AbsSys is [Sys] as [check.Absolute].
+	AbsSys = unsafeAbs(Sys)
+)
--- a/container/fhs/fhs.go
+++ b/container/fhs/fhs.go
@@ -0,0 +1,38 @@
+// Package fhs provides constant and checked pathname values for common FHS paths.
+package fhs
+
+const (
+	// Root points to the file system root.
+	Root = "/"
+	// Etc points to the directory for system-specific configuration.
+	Etc = "/etc/"
+	// Tmp points to the place for small temporary files.
+	Tmp = "/tmp/"
+
+	// Run points to a "tmpfs" file system for system packages to place runtime data, socket files, and similar.
+	Run = "/run/"
+	// RunUser points to a directory containing per-user runtime directories,
+	// each usually individually mounted "tmpfs" instances.
+	RunUser = Run + "user/"
+
+	// Usr points to vendor-supplied operating system resources.
+	Usr = "/usr/"
+	// UsrBin points to binaries and executables for user commands that shall appear in the $PATH search path.
+	UsrBin = Usr + "bin/"
+
+	// Var points to persistent, variable system data. Writable during normal system operation.
+	Var = "/var/"
+	// VarLib points to persistent system data.
+	VarLib = Var + "lib/"
+	// VarEmpty points to a nonstandard directory that is usually empty.
+	VarEmpty = Var + "empty/"
+
+	// Dev points to the root directory for device nodes.
+	Dev = "/dev/"
+	// Proc points to a virtual kernel file system exposing the process list and other functionality.
+	Proc = "/proc/"
+	// ProcSys points to a hierarchy below /proc/ that exposes a number of kernel tunables.
+	ProcSys = Proc + "sys/"
+	// Sys points to a virtual kernel file system exposing discovered devices and other functionality.
+	Sys = "/sys/"
+)
--- a/container/init.go
+++ b/container/init.go
@@ -3,6 +3,7 @@ package container
 import (
 	"errors"
 	"fmt"
+	"log"
 	"os"
 	"os/exec"
 	"path"
@@ -11,7 +12,9 @@ import (
 	. "syscall"
 	"time"

+	"hakurei.app/container/fhs"
 	"hakurei.app/container/seccomp"
+	"hakurei.app/message"
 )

 const (
@@ -29,7 +32,7 @@ const (

 	it should be noted that none of this should become relevant at any point since the resulting
 	intermediate root tmpfs should be effectively anonymous */
-	intermediateHostPath = FHSProc + "self/fd"
+	intermediateHostPath = fhs.Proc + "self/fd"

 	// setup params file descriptor
 	setupEnv = "HAKUREI_SETUP"
@@ -47,7 +50,9 @@ type (
 		// apply is called in intermediate root.
 		apply(state *setupState, k syscallDispatcher) error

-		prefix() string
+		// prefix returns a log message prefix, and whether this Op prints no identifying message on its own.
+		prefix() (string, bool)
+
 		Is(op Op) bool
 		Valid() bool
 		fmt.Stringer
@@ -57,6 +62,7 @@ type (
 	setupState struct {
 		nonrepeatable uintptr
 		*Params
+		message.Msg
 	}
 )

@@ -68,6 +74,16 @@ const (
 	nrAutoRoot
 )

+// OpRepeatError is returned applying a repeated nonrepeatable [Op].
+type OpRepeatError string
+
+func (e OpRepeatError) Error() string { return string(e) + " is not repeatable" }
+
+// OpStateError indicates an impossible internal state has been reached in an [Op].
+type OpStateError string
+
+func (o OpStateError) Error() string { return "impossible " + string(o) + " state reached" }
+
 // initParams are params passed from parent.
 type initParams struct {
 	Params
@@ -79,20 +95,23 @@ type initParams struct {
 	Verbose bool
 }

-func Init(prepareLogger func(prefix string), setVerbose func(verbose bool)) {
-	initEntrypoint(direct{}, prepareLogger, setVerbose)
+// Init is called by [TryArgv0] if the current process is the container init.
+func Init(msg message.Msg) {
+	if msg == nil {
+		panic("attempting to call initEntrypoint with nil msg")
+	}
+	initEntrypoint(direct{}, msg)
 }

-func initEntrypoint(k syscallDispatcher, prepareLogger func(prefix string), setVerbose func(verbose bool)) {
+func initEntrypoint(k syscallDispatcher, msg message.Msg) {
 	k.lockOSThread()
-	prepareLogger("init")

 	if k.getpid() != 1 {
-		k.fatal("this process must run as pid 1")
+		k.fatal(msg, "this process must run as pid 1")
 	}

 	if err := k.setPtracer(0); err != nil {
-		k.verbosef("cannot enable ptrace protection via Yama LSM: %v", err)
+		msg.Verbosef("cannot enable ptrace protection via Yama LSM: %v", err)
 		// not fatal: this program has no additional privileges at initial program start
 	}

@@ -104,65 +123,65 @@ func initEntrypoint(k syscallDispatcher, prepareLogger func(prefix string), setV
 	)
 	if f, err := k.receive(setupEnv, &params, &setupFd); err != nil {
 		if errors.Is(err, EBADF) {
-			k.fatal("invalid setup descriptor")
+			k.fatal(msg, "invalid setup descriptor")
 		}
-		if errors.Is(err, ErrNotSet) {
-			k.fatal("HAKUREI_SETUP not set")
+		if errors.Is(err, ErrReceiveEnv) {
+			k.fatal(msg, setupEnv+" not set")
 		}

-		k.fatalf("cannot decode init setup payload: %v", err)
+		k.fatalf(msg, "cannot decode init setup payload: %v", err)
 	} else {
 		if params.Ops == nil {
-			k.fatal("invalid setup parameters")
+			k.fatal(msg, "invalid setup parameters")
 		}
 		if params.ParentPerm == 0 {
 			params.ParentPerm = 0755
 		}

-		setVerbose(params.Verbose)
-		k.verbose("received setup parameters")
+		msg.SwapVerbose(params.Verbose)
+		msg.Verbose("received setup parameters")
 		closeSetup = f
 		offsetSetup = int(setupFd + 1)
 	}

 	// write uid/gid map here so parent does not need to set dumpable
 	if err := k.setDumpable(SUID_DUMP_USER); err != nil {
-		k.fatalf("cannot set SUID_DUMP_USER: %v", err)
+		k.fatalf(msg, "cannot set SUID_DUMP_USER: %v", err)
 	}
-	if err := k.writeFile(FHSProc+"self/uid_map",
+	if err := k.writeFile(fhs.Proc+"self/uid_map",
 		append([]byte{}, strconv.Itoa(params.Uid)+" "+strconv.Itoa(params.HostUid)+" 1\n"...),
 		0); err != nil {
-		k.fatalf("%v", err)
+		k.fatalf(msg, "%v", err)
 	}
-	if err := k.writeFile(FHSProc+"self/setgroups",
+	if err := k.writeFile(fhs.Proc+"self/setgroups",
 		[]byte("deny\n"),
 		0); err != nil && !os.IsNotExist(err) {
-		k.fatalf("%v", err)
+		k.fatalf(msg, "%v", err)
 	}
-	if err := k.writeFile(FHSProc+"self/gid_map",
+	if err := k.writeFile(fhs.Proc+"self/gid_map",
 		append([]byte{}, strconv.Itoa(params.Gid)+" "+strconv.Itoa(params.HostGid)+" 1\n"...),
 		0); err != nil {
-		k.fatalf("%v", err)
+		k.fatalf(msg, "%v", err)
 	}
 	if err := k.setDumpable(SUID_DUMP_DISABLE); err != nil {
-		k.fatalf("cannot set SUID_DUMP_DISABLE: %v", err)
+		k.fatalf(msg, "cannot set SUID_DUMP_DISABLE: %v", err)
 	}

 	oldmask := k.umask(0)
 	if params.Hostname != "" {
 		if err := k.sethostname([]byte(params.Hostname)); err != nil {
-			k.fatalf("cannot set hostname: %v", err)
+			k.fatalf(msg, "cannot set hostname: %v", err)
 		}
 	}

 	// cache sysctl before pivot_root
-	lastcap := k.lastcap()
+	lastcap := k.lastcap(msg)

-	if err := k.mount(zeroString, FHSRoot, zeroString, MS_SILENT|MS_SLAVE|MS_REC, zeroString); err != nil {
-		k.fatalf("cannot make / rslave: %v", err)
+	if err := k.mount(zeroString, fhs.Root, zeroString, MS_SILENT|MS_SLAVE|MS_REC, zeroString); err != nil {
+		k.fatalf(msg, "cannot make / rslave: %v", optionalErrorUnwrap(err))
 	}

-	state := &setupState{Params: &params.Params}
+	state := &setupState{Params: &params.Params, Msg: msg}

 	/* early is called right before pivot_root into intermediate root;
 	this step is mostly for gathering information that would otherwise be difficult to obtain
@@ -170,40 +189,41 @@ func initEntrypoint(k syscallDispatcher, prepareLogger func(prefix string), setV
 	the state of the mount namespace */
 	for i, op := range *params.Ops {
 		if op == nil || !op.Valid() {
-			k.fatalf("invalid op at index %d", i)
+			k.fatalf(msg, "invalid op at index %d", i)
 		}

 		if err := op.early(state, k); err != nil {
-			k.printBaseErr(err,
-				fmt.Sprintf("cannot prepare op at index %d:", i))
-			k.beforeExit()
-			k.exit(1)
+			if m, ok := messageFromError(err); ok {
+				k.fatal(msg, m)
+			} else {
+				k.fatalf(msg, "cannot prepare op at index %d: %v", i, err)
+			}
 		}
 	}

 	if err := k.mount(SourceTmpfsRootfs, intermediateHostPath, FstypeTmpfs, MS_NODEV|MS_NOSUID, zeroString); err != nil {
-		k.fatalf("cannot mount intermediate root: %v", err)
+		k.fatalf(msg, "cannot mount intermediate root: %v", optionalErrorUnwrap(err))
 	}
 	if err := k.chdir(intermediateHostPath); err != nil {
-		k.fatalf("cannot enter intermediate host path: %v", err)
+		k.fatalf(msg, "cannot enter intermediate host path: %v", err)
 	}

 	if err := k.mkdir(sysrootDir, 0755); err != nil {
-		k.fatalf("%v", err)
+		k.fatalf(msg, "%v", err)
 	}
 	if err := k.mount(sysrootDir, sysrootDir, zeroString, MS_SILENT|MS_BIND|MS_REC, zeroString); err != nil {
-		k.fatalf("cannot bind sysroot: %v", err)
+		k.fatalf(msg, "cannot bind sysroot: %v", optionalErrorUnwrap(err))
 	}

 	if err := k.mkdir(hostDir, 0755); err != nil {
-		k.fatalf("%v", err)
+		k.fatalf(msg, "%v", err)
 	}
 	// pivot_root uncovers intermediateHostPath in hostDir
 	if err := k.pivotRoot(intermediateHostPath, hostDir); err != nil {
-		k.fatalf("cannot pivot into intermediate root: %v", err)
+		k.fatalf(msg, "cannot pivot into intermediate root: %v", err)
 	}
-	if err := k.chdir(FHSRoot); err != nil {
-		k.fatalf("cannot enter intermediate root: %v", err)
+	if err := k.chdir(fhs.Root); err != nil {
+		k.fatalf(msg, "cannot enter intermediate root: %v", err)
 	}

 	/* apply is called right after pivot_root and entering the new root;
@@ -212,62 +232,65 @@ func initEntrypoint(k syscallDispatcher, prepareLogger func(prefix string), setV
 	chdir is allowed but discouraged */
 	for i, op := range *params.Ops {
 		// ops already checked during early setup
-		k.verbosef("%s %s", op.prefix(), op)
+		if prefix, ok := op.prefix(); ok {
+			msg.Verbosef("%s %s", prefix, op)
+		}
 		if err := op.apply(state, k); err != nil {
-			k.printBaseErr(err,
-				fmt.Sprintf("cannot apply op at index %d:", i))
-			k.beforeExit()
-			k.exit(1)
+			if m, ok := messageFromError(err); ok {
+				k.fatal(msg, m)
+			} else {
+				k.fatalf(msg, "cannot apply op at index %d: %v", i, err)
+			}
 		}
 	}

 	// setup requiring host root complete at this point
 	if err := k.mount(hostDir, hostDir, zeroString, MS_SILENT|MS_REC|MS_PRIVATE, zeroString); err != nil {
-		k.fatalf("cannot make host root rprivate: %v", err)
+		k.fatalf(msg, "cannot make host root rprivate: %v", optionalErrorUnwrap(err))
 	}
 	if err := k.unmount(hostDir, MNT_DETACH); err != nil {
-		k.fatalf("cannot unmount host root: %v", err)
+		k.fatalf(msg, "cannot unmount host root: %v", err)
 	}

 	{
 		var fd int
 		if err := IgnoringEINTR(func() (err error) {
-			fd, err = k.open(FHSRoot, O_DIRECTORY|O_RDONLY, 0)
+			fd, err = k.open(fhs.Root, O_DIRECTORY|O_RDONLY, 0)
 			return
 		}); err != nil {
-			k.fatalf("cannot open intermediate root: %v", err)
+			k.fatalf(msg, "cannot open intermediate root: %v", err)
 		}
 		if err := k.chdir(sysrootPath); err != nil {
-			k.fatalf("cannot enter sysroot: %v", err)
+			k.fatalf(msg, "cannot enter sysroot: %v", err)
 		}

 		if err := k.pivotRoot(".", "."); err != nil {
-			k.fatalf("cannot pivot into sysroot: %v", err)
+			k.fatalf(msg, "cannot pivot into sysroot: %v", err)
 		}
 		if err := k.fchdir(fd); err != nil {
-			k.fatalf("cannot re-enter intermediate root: %v", err)
+			k.fatalf(msg, "cannot re-enter intermediate root: %v", err)
 		}
 		if err := k.unmount(".", MNT_DETACH); err != nil {
-			k.fatalf("cannot unmount intermediate root: %v", err)
+			k.fatalf(msg, "cannot unmount intermediate root: %v", err)
 		}
-		if err := k.chdir(FHSRoot); err != nil {
-			k.fatalf("cannot enter root: %v", err)
+		if err := k.chdir(fhs.Root); err != nil {
+			k.fatalf(msg, "cannot enter root: %v", err)
 		}

 		if err := k.close(fd); err != nil {
-			k.fatalf("cannot close intermediate root: %v", err)
+			k.fatalf(msg, "cannot close intermediate root: %v", err)
 		}
 	}

 	if err := k.capAmbientClearAll(); err != nil {
-		k.fatalf("cannot clear the ambient capability set: %v", err)
+		k.fatalf(msg, "cannot clear the ambient capability set: %v", err)
 	}
 	for i := uintptr(0); i <= lastcap; i++ {
 		if params.Privileged && i == CAP_SYS_ADMIN {
 			continue
 		}
 		if err := k.capBoundingSetDrop(i); err != nil {
-			k.fatalf("cannot drop capability from bounding set: %v", err)
+			k.fatalf(msg, "cannot drop capability from bounding set: %v", err)
 		}
 	}

@@ -276,29 +299,29 @@ func initEntrypoint(k syscallDispatcher, prepareLogger func(prefix string), setV
 		keep[capToIndex(CAP_SYS_ADMIN)] |= capToMask(CAP_SYS_ADMIN)

 		if err := k.capAmbientRaise(CAP_SYS_ADMIN); err != nil {
-			k.fatalf("cannot raise CAP_SYS_ADMIN: %v", err)
+			k.fatalf(msg, "cannot raise CAP_SYS_ADMIN: %v", err)
 		}
 	}
 	if err := k.capset(
 		&capHeader{_LINUX_CAPABILITY_VERSION_3, 0},
 		&[2]capData{{0, keep[0], keep[0]}, {0, keep[1], keep[1]}},
 	); err != nil {
-		k.fatalf("cannot capset: %v", err)
+		k.fatalf(msg, "cannot capset: %v", err)
 	}

 	if !params.SeccompDisable {
 		rules := params.SeccompRules
 		if len(rules) == 0 { // non-empty rules slice always overrides presets
-			k.verbosef("resolving presets %#x", params.SeccompPresets)
+			msg.Verbosef("resolving presets %#x", params.SeccompPresets)
 			rules = seccomp.Preset(params.SeccompPresets, params.SeccompFlags)
 		}
 		if err := k.seccompLoad(rules, params.SeccompFlags); err != nil {
 			// this also indirectly asserts PR_SET_NO_NEW_PRIVS
-			k.fatalf("cannot load syscall filter: %v", err)
+			k.fatalf(msg, "cannot load syscall filter: %v", err)
 		}
-		k.verbosef("%d filter rules loaded", len(rules))
+		msg.Verbosef("%d filter rules loaded", len(rules))
 	} else {
-		k.verbose("syscall filter not configured")
+		msg.Verbose("syscall filter not configured")
 	}

 	extraFiles := make([]*os.File, params.Count)
@@ -315,14 +338,14 @@ func initEntrypoint(k syscallDispatcher, prepareLogger func(prefix string), setV
 	cmd.ExtraFiles = extraFiles
 	cmd.Dir = params.Dir.String()

-	k.verbosef("starting initial program %s", params.Path)
+	msg.Verbosef("starting initial program %s", params.Path)
 	if err := k.start(cmd); err != nil {
-		k.fatalf("%v", err)
+		k.fatalf(msg, "%v", err)
 	}
-	k.suspend()
+	msg.Suspend()

 	if err := closeSetup(); err != nil {
-		k.printf("cannot close setup pipe: %v", err)
+		k.printf(msg, "cannot close setup pipe: %v", err)
 		// not fatal
 	}

@@ -356,7 +379,7 @@ func initEntrypoint(k syscallDispatcher, prepareLogger func(prefix string), setV
 			}
 		}
 		if !errors.Is(err, ECHILD) {
-			k.printf("unexpected wait4 response: %v", err)
+			k.printf(msg, "unexpected wait4 response: %v", err)
 		}

 		close(done)
@@ -373,50 +396,50 @@ func initEntrypoint(k syscallDispatcher, prepareLogger func(prefix string), setV
 	for {
 		select {
 		case s := <-sig:
-			if k.resume() {
-				k.verbosef("%s after process start", s.String())
+			if msg.Resume() {
+				msg.Verbosef("%s after process start", s.String())
 			} else {
-				k.verbosef("got %s", s.String())
+				msg.Verbosef("got %s", s.String())
 			}
 			if s == CancelSignal && params.ForwardCancel && cmd.Process != nil {
-				k.verbose("forwarding context cancellation")
+				msg.Verbose("forwarding context cancellation")
 				if err := k.signal(cmd, os.Interrupt); err != nil {
-					k.printf("cannot forward cancellation: %v", err)
+					k.printf(msg, "cannot forward cancellation: %v", err)
 				}
 				continue
 			}
-			k.beforeExit()
+			msg.BeforeExit()
 			k.exit(0)

 		case w := <-info:
 			if w.wpid == cmd.Process.Pid {
 				// initial process exited, output is most likely available again
-				k.resume()
+				msg.Resume()

 				switch {
 				case w.wstatus.Exited():
 					r = w.wstatus.ExitStatus()
-					k.verbosef("initial process exited with code %d", w.wstatus.ExitStatus())
+					msg.Verbosef("initial process exited with code %d", w.wstatus.ExitStatus())

 				case w.wstatus.Signaled():
 					r = 128 + int(w.wstatus.Signal())
-					k.verbosef("initial process exited with signal %s", w.wstatus.Signal())
+					msg.Verbosef("initial process exited with signal %s", w.wstatus.Signal())

 				default:
 					r = 255
-					k.verbosef("initial process exited with status %#x", w.wstatus)
+					msg.Verbosef("initial process exited with status %#x", w.wstatus)
 				}

 				go func() { time.Sleep(params.AdoptWaitDelay); close(timeout) }()
 			}

 		case <-done:
-			k.beforeExit()
+			msg.BeforeExit()
 			k.exit(r)

 		case <-timeout:
-			k.printf("timeout exceeded waiting for lingering processes")
-			k.beforeExit()
+			k.printf(msg, "timeout exceeded waiting for lingering processes")
+			msg.BeforeExit()
 			k.exit(r)
 		}
 	}
@@ -425,10 +448,16 @@ func initEntrypoint(k syscallDispatcher, prepareLogger func(prefix string), setV
 const initName = "init"

 // TryArgv0 calls [Init] if the last element of argv0 is "init".
-func TryArgv0(v Msg, prepare func(prefix string), setVerbose func(verbose bool)) {
+// If a nil msg is passed, the system logger is used instead.
+func TryArgv0(msg message.Msg) {
+	if msg == nil {
+		log.SetPrefix(initName + ": ")
+		log.SetFlags(0)
+		msg = message.NewMsg(log.Default())
+	}
+
 	if len(os.Args) > 0 && path.Base(os.Args[0]) == initName {
-		msg = v
-		Init(prepare, setVerbose)
+		Init(msg)
 		msg.BeforeExit()
 		os.Exit(0)
 	}
--- a/container/init_test.go
+++ b/container/init_test.go
--- a/container/initbind.go
+++ b/container/initbind.go
@@ -5,12 +5,15 @@ import (
 	"fmt"
 	"os"
 	"syscall"
+
+	"hakurei.app/container/bits"
+	"hakurei.app/container/check"
 )

 func init() { gob.Register(new(BindMountOp)) }

 // Bind appends an [Op] that bind mounts host path [BindMountOp.Source] on container path [BindMountOp.Target].
-func (f *Ops) Bind(source, target *Absolute, flags int) *Ops {
+func (f *Ops) Bind(source, target *check.Absolute, flags int) *Ops {
 	*f = append(*f, &BindMountOp{nil, source, target, flags})
 	return f
 }
@@ -18,52 +21,41 @@ func (f *Ops) Bind(source, target *Absolute, flags int) *Ops {
 // BindMountOp bind mounts host path Source on container path Target.
 // Note that Flags uses bits declared in this package and should not be set with constants in [syscall].
 type BindMountOp struct {
-	sourceFinal, Source, Target *Absolute
+	sourceFinal, Source, Target *check.Absolute

 	Flags int
 }

-const (
-	// BindOptional skips nonexistent host paths.
-	BindOptional = 1 << iota
-	// BindWritable mounts filesystem read-write.
-	BindWritable
-	// BindDevice allows access to devices (special files) on this filesystem.
-	BindDevice
-	// BindEnsure attempts to create the host path if it does not exist.
-	BindEnsure
-)
-
 func (b *BindMountOp) Valid() bool {
 	return b != nil &&
 		b.Source != nil && b.Target != nil &&
-		b.Flags&(BindOptional|BindEnsure) != (BindOptional|BindEnsure)
+		b.Flags&(bits.BindOptional|bits.BindEnsure) != (bits.BindOptional|bits.BindEnsure)
 }

 func (b *BindMountOp) early(_ *setupState, k syscallDispatcher) error {
-	if b.Flags&BindEnsure != 0 {
+	if b.Flags&bits.BindEnsure != 0 {
 		if err := k.mkdirAll(b.Source.String(), 0700); err != nil {
-			return wrapErrSelf(err)
+			return err
 		}
 	}

 	if pathname, err := k.evalSymlinks(b.Source.String()); err != nil {
-		if os.IsNotExist(err) && b.Flags&BindOptional != 0 {
+		if os.IsNotExist(err) && b.Flags&bits.BindOptional != 0 {
 			// leave sourceFinal as nil
 			return nil
 		}
-		return wrapErrSelf(err)
+		return err
 	} else {
-		b.sourceFinal, err = NewAbs(pathname)
+		b.sourceFinal, err = check.NewAbs(pathname)
 		return err
 	}
 }

-func (b *BindMountOp) apply(_ *setupState, k syscallDispatcher) error {
+func (b *BindMountOp) apply(state *setupState, k syscallDispatcher) error {
 	if b.sourceFinal == nil {
-		if b.Flags&BindOptional == 0 {
+		if b.Flags&bits.BindOptional == 0 {
 			// unreachable
-			return msg.WrapErr(os.ErrClosed, "impossible bind state reached")
+			return OpStateError("bind")
 		}
 		return nil
 	}
@@ -74,24 +66,29 @@ func (b *BindMountOp) apply(_ *setupState, k syscallDispatcher) error {
 	// this perm value emulates bwrap behaviour as it clears bits from 0755 based on
 	// op->perms which is never set for any bind setup op so always results in 0700
 	if fi, err := k.stat(source); err != nil {
-		return wrapErrSelf(err)
+		return err
 	} else if fi.IsDir() {
 		if err = k.mkdirAll(target, 0700); err != nil {
-			return wrapErrSelf(err)
+			return err
 		}
 	} else if err = k.ensureFile(target, 0444, 0700); err != nil {
 		return err
 	}

 	var flags uintptr = syscall.MS_REC
-	if b.Flags&BindWritable == 0 {
+	if b.Flags&bits.BindWritable == 0 {
 		flags |= syscall.MS_RDONLY
 	}
-	if b.Flags&BindDevice == 0 {
+	if b.Flags&bits.BindDevice == 0 {
 		flags |= syscall.MS_NODEV
 	}

-	return k.bindMount(source, target, flags, b.sourceFinal == b.Target)
+	if b.sourceFinal.String() == b.Target.String() {
+		state.Verbosef("mounting %q flags %#x", target, flags)
+	} else {
+		state.Verbosef("mounting %q on %q flags %#x", source, target, flags)
+	}
+	return k.bindMount(state, source, target, flags)
 }

 func (b *BindMountOp) Is(op Op) bool {
@@ -101,7 +98,7 @@ func (b *BindMountOp) Is(op Op) bool {
 		b.Target.Is(vb.Target) &&
 		b.Flags == vb.Flags
 }
-func (*BindMountOp) prefix() string { return "mounting" }
+func (*BindMountOp) prefix() (string, bool) { return "mounting", false }
 func (b *BindMountOp) String() string {
 	if b.Source == nil || b.Target == nil {
 		return "<invalid>"
--- a/container/initbind_test.go
+++ b/container/initbind_test.go
@@ -5,145 +5,172 @@ import (
 	"os"
 	"syscall"
 	"testing"
+
+	"hakurei.app/container/bits"
+	"hakurei.app/container/check"
+	"hakurei.app/container/stub"
 )

 func TestBindMountOp(t *testing.T) {
+	t.Parallel()
+
 	checkOpBehaviour(t, []opBehaviourTestCase{
 		{"ENOENT not optional", new(Params), &BindMountOp{
-			Source: MustAbs("/bin/"),
-			Target: MustAbs("/bin/"),
-		}, []kexpect{
-			{"evalSymlinks", expectArgs{"/bin/"}, "", syscall.ENOENT},
-		}, wrapErrSelf(syscall.ENOENT), nil, nil},
+			Source: check.MustAbs("/bin/"),
+			Target: check.MustAbs("/bin/"),
+		}, []stub.Call{
+			call("evalSymlinks", stub.ExpectArgs{"/bin/"}, "", syscall.ENOENT),
+		}, syscall.ENOENT, nil, nil},

 		{"skip optional", new(Params), &BindMountOp{
-			Source: MustAbs("/bin/"),
-			Target: MustAbs("/bin/"),
-			Flags:  BindOptional,
-		}, []kexpect{
-			{"evalSymlinks", expectArgs{"/bin/"}, "", syscall.ENOENT},
+			Source: check.MustAbs("/bin/"),
+			Target: check.MustAbs("/bin/"),
+			Flags:  bits.BindOptional,
+		}, []stub.Call{
+			call("evalSymlinks", stub.ExpectArgs{"/bin/"}, "", syscall.ENOENT),
 		}, nil, nil, nil},

 		{"success optional", new(Params), &BindMountOp{
-			Source: MustAbs("/bin/"),
-			Target: MustAbs("/bin/"),
-			Flags:  BindOptional,
-		}, []kexpect{
-			{"evalSymlinks", expectArgs{"/bin/"}, "/usr/bin", nil},
-		}, nil, []kexpect{
-			{"stat", expectArgs{"/host/usr/bin"}, isDirFi(true), nil},
-			{"mkdirAll", expectArgs{"/sysroot/bin", os.FileMode(0700)}, nil, nil},
-			{"bindMount", expectArgs{"/host/usr/bin", "/sysroot/bin", uintptr(0x4005), false}, nil, nil},
+			Source: check.MustAbs("/bin/"),
+			Target: check.MustAbs("/bin/"),
+			Flags:  bits.BindOptional,
+		}, []stub.Call{
+			call("evalSymlinks", stub.ExpectArgs{"/bin/"}, "/usr/bin", nil),
+		}, nil, []stub.Call{
+			call("stat", stub.ExpectArgs{"/host/usr/bin"}, isDirFi(true), nil),
+			call("mkdirAll", stub.ExpectArgs{"/sysroot/bin", os.FileMode(0700)}, nil, nil),
+			call("verbosef", stub.ExpectArgs{"mounting %q on %q flags %#x", []any{"/host/usr/bin", "/sysroot/bin", uintptr(0x4005)}}, nil, nil),
+			call("bindMount", stub.ExpectArgs{"/host/usr/bin", "/sysroot/bin", uintptr(0x4005), false}, nil, nil),
 		}, nil},

 		{"ensureFile device", new(Params), &BindMountOp{
-			Source: MustAbs("/dev/null"),
-			Target: MustAbs("/dev/null"),
-			Flags:  BindWritable | BindDevice,
-		}, []kexpect{
-			{"evalSymlinks", expectArgs{"/dev/null"}, "/dev/null", nil},
-		}, nil, []kexpect{
-			{"stat", expectArgs{"/host/dev/null"}, isDirFi(false), nil},
-			{"ensureFile", expectArgs{"/sysroot/dev/null", os.FileMode(0444), os.FileMode(0700)}, nil, errUnique},
-		}, errUnique},
+			Source: check.MustAbs("/dev/null"),
+			Target: check.MustAbs("/dev/null"),
+			Flags:  bits.BindWritable | bits.BindDevice,
+		}, []stub.Call{
+			call("evalSymlinks", stub.ExpectArgs{"/dev/null"}, "/dev/null", nil),
+		}, nil, []stub.Call{
+			call("stat", stub.ExpectArgs{"/host/dev/null"}, isDirFi(false), nil),
+			call("ensureFile", stub.ExpectArgs{"/sysroot/dev/null", os.FileMode(0444), os.FileMode(0700)}, nil, stub.UniqueError(5)),
+		}, stub.UniqueError(5)},

 		{"mkdirAll ensure", new(Params), &BindMountOp{
-			Source: MustAbs("/bin/"),
-			Target: MustAbs("/bin/"),
-			Flags:  BindEnsure,
-		}, []kexpect{
-			{"mkdirAll", expectArgs{"/bin/", os.FileMode(0700)}, nil, errUnique},
-		}, wrapErrSelf(errUnique), nil, nil},
+			Source: check.MustAbs("/bin/"),
+			Target: check.MustAbs("/bin/"),
+			Flags:  bits.BindEnsure,
+		}, []stub.Call{
+			call("mkdirAll", stub.ExpectArgs{"/bin/", os.FileMode(0700)}, nil, stub.UniqueError(4)),
+		}, stub.UniqueError(4), nil, nil},

 		{"success ensure", new(Params), &BindMountOp{
-			Source: MustAbs("/bin/"),
-			Target: MustAbs("/usr/bin/"),
-			Flags:  BindEnsure,
-		}, []kexpect{
-			{"mkdirAll", expectArgs{"/bin/", os.FileMode(0700)}, nil, nil},
-			{"evalSymlinks", expectArgs{"/bin/"}, "/usr/bin", nil},
-		}, nil, []kexpect{
-			{"stat", expectArgs{"/host/usr/bin"}, isDirFi(true), nil},
-			{"mkdirAll", expectArgs{"/sysroot/usr/bin", os.FileMode(0700)}, nil, nil},
-			{"bindMount", expectArgs{"/host/usr/bin", "/sysroot/usr/bin", uintptr(0x4005), false}, nil, nil},
+			Source: check.MustAbs("/bin/"),
+			Target: check.MustAbs("/usr/bin/"),
+			Flags:  bits.BindEnsure,
+		}, []stub.Call{
+			call("mkdirAll", stub.ExpectArgs{"/bin/", os.FileMode(0700)}, nil, nil),
+			call("evalSymlinks", stub.ExpectArgs{"/bin/"}, "/usr/bin", nil),
+		}, nil, []stub.Call{
+			call("stat", stub.ExpectArgs{"/host/usr/bin"}, isDirFi(true), nil),
+			call("mkdirAll", stub.ExpectArgs{"/sysroot/usr/bin", os.FileMode(0700)}, nil, nil),
+			call("verbosef", stub.ExpectArgs{"mounting %q on %q flags %#x", []any{"/host/usr/bin", "/sysroot/usr/bin", uintptr(0x4005)}}, nil, nil),
+			call("bindMount", stub.ExpectArgs{"/host/usr/bin", "/sysroot/usr/bin", uintptr(0x4005), false}, nil, nil),
 		}, nil},

 		{"success device ro", new(Params), &BindMountOp{
-			Source: MustAbs("/dev/null"),
-			Target: MustAbs("/dev/null"),
-			Flags:  BindDevice,
-		}, []kexpect{
-			{"evalSymlinks", expectArgs{"/dev/null"}, "/dev/null", nil},
-		}, nil, []kexpect{
-			{"stat", expectArgs{"/host/dev/null"}, isDirFi(false), nil},
-			{"ensureFile", expectArgs{"/sysroot/dev/null", os.FileMode(0444), os.FileMode(0700)}, nil, nil},
-			{"bindMount", expectArgs{"/host/dev/null", "/sysroot/dev/null", uintptr(0x4001), false}, nil, nil},
+			Source: check.MustAbs("/dev/null"),
+			Target: check.MustAbs("/dev/null"),
+			Flags:  bits.BindDevice,
+		}, []stub.Call{
+			call("evalSymlinks", stub.ExpectArgs{"/dev/null"}, "/dev/null", nil),
+		}, nil, []stub.Call{
+			call("stat", stub.ExpectArgs{"/host/dev/null"}, isDirFi(false), nil),
+			call("ensureFile", stub.ExpectArgs{"/sysroot/dev/null", os.FileMode(0444), os.FileMode(0700)}, nil, nil),
+			call("verbosef", stub.ExpectArgs{"mounting %q flags %#x", []any{"/sysroot/dev/null", uintptr(0x4001)}}, nil, nil),
+			call("bindMount", stub.ExpectArgs{"/host/dev/null", "/sysroot/dev/null", uintptr(0x4001), false}, nil, nil),
 		}, nil},

 		{"success device", new(Params), &BindMountOp{
-			Source: MustAbs("/dev/null"),
-			Target: MustAbs("/dev/null"),
-			Flags:  BindWritable | BindDevice,
-		}, []kexpect{
-			{"evalSymlinks", expectArgs{"/dev/null"}, "/dev/null", nil},
-		}, nil, []kexpect{
-			{"stat", expectArgs{"/host/dev/null"}, isDirFi(false), nil},
-			{"ensureFile", expectArgs{"/sysroot/dev/null", os.FileMode(0444), os.FileMode(0700)}, nil, nil},
-			{"bindMount", expectArgs{"/host/dev/null", "/sysroot/dev/null", uintptr(0x4000), false}, nil, nil},
+			Source: check.MustAbs("/dev/null"),
+			Target: check.MustAbs("/dev/null"),
+			Flags:  bits.BindWritable | bits.BindDevice,
+		}, []stub.Call{
+			call("evalSymlinks", stub.ExpectArgs{"/dev/null"}, "/dev/null", nil),
+		}, nil, []stub.Call{
+			call("stat", stub.ExpectArgs{"/host/dev/null"}, isDirFi(false), nil),
+			call("ensureFile", stub.ExpectArgs{"/sysroot/dev/null", os.FileMode(0444), os.FileMode(0700)}, nil, nil),
+			call("verbosef", stub.ExpectArgs{"mounting %q flags %#x", []any{"/sysroot/dev/null", uintptr(0x4000)}}, nil, nil),
+			call("bindMount", stub.ExpectArgs{"/host/dev/null", "/sysroot/dev/null", uintptr(0x4000), false}, nil, nil),
 		}, nil},

 		{"evalSymlinks", new(Params), &BindMountOp{
-			Source: MustAbs("/bin/"),
-			Target: MustAbs("/bin/"),
-		}, []kexpect{
-			{"evalSymlinks", expectArgs{"/bin/"}, "/usr/bin", errUnique},
-		}, wrapErrSelf(errUnique), nil, nil},
+			Source: check.MustAbs("/bin/"),
+			Target: check.MustAbs("/bin/"),
+		}, []stub.Call{
+			call("evalSymlinks", stub.ExpectArgs{"/bin/"}, "/usr/bin", stub.UniqueError(3)),
+		}, stub.UniqueError(3), nil, nil},

 		{"stat", new(Params), &BindMountOp{
-			Source: MustAbs("/bin/"),
-			Target: MustAbs("/bin/"),
-		}, []kexpect{
-			{"evalSymlinks", expectArgs{"/bin/"}, "/usr/bin", nil},
-		}, nil, []kexpect{
-			{"stat", expectArgs{"/host/usr/bin"}, isDirFi(true), errUnique},
-		}, wrapErrSelf(errUnique)},
+			Source: check.MustAbs("/bin/"),
+			Target: check.MustAbs("/bin/"),
+		}, []stub.Call{
+			call("evalSymlinks", stub.ExpectArgs{"/bin/"}, "/usr/bin", nil),
+		}, nil, []stub.Call{
+			call("stat", stub.ExpectArgs{"/host/usr/bin"}, isDirFi(true), stub.UniqueError(2)),
+		}, stub.UniqueError(2)},

 		{"mkdirAll", new(Params), &BindMountOp{
-			Source: MustAbs("/bin/"),
-			Target: MustAbs("/bin/"),
-		}, []kexpect{
-			{"evalSymlinks", expectArgs{"/bin/"}, "/usr/bin", nil},
-		}, nil, []kexpect{
-			{"stat", expectArgs{"/host/usr/bin"}, isDirFi(true), nil},
-			{"mkdirAll", expectArgs{"/sysroot/bin", os.FileMode(0700)}, nil, errUnique},
-		}, wrapErrSelf(errUnique)},
+			Source: check.MustAbs("/bin/"),
+			Target: check.MustAbs("/bin/"),
+		}, []stub.Call{
+			call("evalSymlinks", stub.ExpectArgs{"/bin/"}, "/usr/bin", nil),
+		}, nil, []stub.Call{
+			call("stat", stub.ExpectArgs{"/host/usr/bin"}, isDirFi(true), nil),
+			call("mkdirAll", stub.ExpectArgs{"/sysroot/bin", os.FileMode(0700)}, nil, stub.UniqueError(1)),
+		}, stub.UniqueError(1)},

 		{"bindMount", new(Params), &BindMountOp{
-			Source: MustAbs("/bin/"),
-			Target: MustAbs("/bin/"),
-		}, []kexpect{
-			{"evalSymlinks", expectArgs{"/bin/"}, "/usr/bin", nil},
-		}, nil, []kexpect{
-			{"stat", expectArgs{"/host/usr/bin"}, isDirFi(true), nil},
-			{"mkdirAll", expectArgs{"/sysroot/bin", os.FileMode(0700)}, nil, nil},
-			{"bindMount", expectArgs{"/host/usr/bin", "/sysroot/bin", uintptr(0x4005), false}, nil, errUnique},
-		}, errUnique},
+			Source: check.MustAbs("/bin/"),
+			Target: check.MustAbs("/bin/"),
+		}, []stub.Call{
+			call("evalSymlinks", stub.ExpectArgs{"/bin/"}, "/usr/bin", nil),
+		}, nil, []stub.Call{
+			call("stat", stub.ExpectArgs{"/host/usr/bin"}, isDirFi(true), nil),
+			call("mkdirAll", stub.ExpectArgs{"/sysroot/bin", os.FileMode(0700)}, nil, nil),
+			call("verbosef", stub.ExpectArgs{"mounting %q on %q flags %#x", []any{"/host/usr/bin", "/sysroot/bin", uintptr(0x4005)}}, nil, nil),
+			call("bindMount", stub.ExpectArgs{"/host/usr/bin", "/sysroot/bin", uintptr(0x4005), false}, nil, stub.UniqueError(0)),
+		}, stub.UniqueError(0)},
+
+		{"success eval equals", new(Params), &BindMountOp{
+			Source: check.MustAbs("/bin/"),
+			Target: check.MustAbs("/bin/"),
+		}, []stub.Call{
+			call("evalSymlinks", stub.ExpectArgs{"/bin/"}, "/bin", nil),
+		}, nil, []stub.Call{
+			call("stat", stub.ExpectArgs{"/host/bin"}, isDirFi(true), nil),
+			call("mkdirAll", stub.ExpectArgs{"/sysroot/bin", os.FileMode(0700)}, nil, nil),
+			call("verbosef", stub.ExpectArgs{"mounting %q on %q flags %#x", []any{"/host/bin", "/sysroot/bin", uintptr(0x4005)}}, nil, nil),
+			call("bindMount", stub.ExpectArgs{"/host/bin", "/sysroot/bin", uintptr(0x4005), false}, nil, nil),
+		}, nil},

 		{"success", new(Params), &BindMountOp{
-			Source: MustAbs("/bin/"),
-			Target: MustAbs("/bin/"),
-		}, []kexpect{
-			{"evalSymlinks", expectArgs{"/bin/"}, "/usr/bin", nil},
-		}, nil, []kexpect{
-			{"stat", expectArgs{"/host/usr/bin"}, isDirFi(true), nil},
-			{"mkdirAll", expectArgs{"/sysroot/bin", os.FileMode(0700)}, nil, nil},
-			{"bindMount", expectArgs{"/host/usr/bin", "/sysroot/bin", uintptr(0x4005), false}, nil, nil},
+			Source: check.MustAbs("/bin/"),
+			Target: check.MustAbs("/bin/"),
+		}, []stub.Call{
+			call("evalSymlinks", stub.ExpectArgs{"/bin/"}, "/usr/bin", nil),
+		}, nil, []stub.Call{
+			call("stat", stub.ExpectArgs{"/host/usr/bin"}, isDirFi(true), nil),
+			call("mkdirAll", stub.ExpectArgs{"/sysroot/bin", os.FileMode(0700)}, nil, nil),
+			call("verbosef", stub.ExpectArgs{"mounting %q on %q flags %#x", []any{"/host/usr/bin", "/sysroot/bin", uintptr(0x4005)}}, nil, nil),
+			call("bindMount", stub.ExpectArgs{"/host/usr/bin", "/sysroot/bin", uintptr(0x4005), false}, nil, nil),
 		}, nil},
 	})

 	t.Run("unreachable", func(t *testing.T) {
+		t.Parallel()
+
 		t.Run("nil sourceFinal not optional", func(t *testing.T) {
-			wantErr := msg.WrapErr(os.ErrClosed, "impossible bind state reached")
+			t.Parallel()
+			wantErr := OpStateError("bind")
 			if err := new(BindMountOp).apply(nil, nil); !errors.Is(err, wantErr) {
 				t.Errorf("apply: error = %v, want %v", err, wantErr)
 			}
@@ -153,21 +180,21 @@ func TestBindMountOp(t *testing.T) {
 	checkOpsValid(t, []opValidTestCase{
 		{"nil", (*BindMountOp)(nil), false},
 		{"zero", new(BindMountOp), false},
-		{"nil source", &BindMountOp{Target: MustAbs("/")}, false},
-		{"nil target", &BindMountOp{Source: MustAbs("/")}, false},
-		{"flag optional ensure", &BindMountOp{Source: MustAbs("/"), Target: MustAbs("/"), Flags: BindOptional | BindEnsure}, false},
-		{"valid", &BindMountOp{Source: MustAbs("/"), Target: MustAbs("/")}, true},
+		{"nil source", &BindMountOp{Target: check.MustAbs("/")}, false},
+		{"nil target", &BindMountOp{Source: check.MustAbs("/")}, false},
+		{"flag optional ensure", &BindMountOp{Source: check.MustAbs("/"), Target: check.MustAbs("/"), Flags: bits.BindOptional | bits.BindEnsure}, false},
+		{"valid", &BindMountOp{Source: check.MustAbs("/"), Target: check.MustAbs("/")}, true},
 	})

 	checkOpsBuilder(t, []opsBuilderTestCase{
 		{"autoetc", new(Ops).Bind(
-			MustAbs("/etc/"),
-			MustAbs("/etc/.host/048090b6ed8f9ebb10e275ff5d8c0659"),
+			check.MustAbs("/etc/"),
+			check.MustAbs("/etc/.host/048090b6ed8f9ebb10e275ff5d8c0659"),
 			0,
 		), Ops{
 			&BindMountOp{
-				Source: MustAbs("/etc/"),
-				Target: MustAbs("/etc/.host/048090b6ed8f9ebb10e275ff5d8c0659"),
+				Source: check.MustAbs("/etc/"),
+				Target: check.MustAbs("/etc/.host/048090b6ed8f9ebb10e275ff5d8c0659"),
 			},
 		}},
 	})
@@ -176,45 +203,45 @@ func TestBindMountOp(t *testing.T) {
 		{"zero", new(BindMountOp), new(BindMountOp), false},

 		{"internal ne", &BindMountOp{
-			Source: MustAbs("/etc/"),
-			Target: MustAbs("/etc/.host/048090b6ed8f9ebb10e275ff5d8c0659"),
+			Source: check.MustAbs("/etc/"),
+			Target: check.MustAbs("/etc/.host/048090b6ed8f9ebb10e275ff5d8c0659"),
 		}, &BindMountOp{
-			Source:      MustAbs("/etc/"),
-			Target:      MustAbs("/etc/.host/048090b6ed8f9ebb10e275ff5d8c0659"),
-			sourceFinal: MustAbs("/etc/"),
+			Source:      check.MustAbs("/etc/"),
+			Target:      check.MustAbs("/etc/.host/048090b6ed8f9ebb10e275ff5d8c0659"),
+			sourceFinal: check.MustAbs("/etc/"),
 		}, true},

 		{"flags differs", &BindMountOp{
-			Source: MustAbs("/etc/"),
-			Target: MustAbs("/etc/.host/048090b6ed8f9ebb10e275ff5d8c0659"),
+			Source: check.MustAbs("/etc/"),
+			Target: check.MustAbs("/etc/.host/048090b6ed8f9ebb10e275ff5d8c0659"),
 		}, &BindMountOp{
-			Source: MustAbs("/etc/"),
-			Target: MustAbs("/etc/.host/048090b6ed8f9ebb10e275ff5d8c0659"),
-			Flags:  BindOptional,
+			Source: check.MustAbs("/etc/"),
+			Target: check.MustAbs("/etc/.host/048090b6ed8f9ebb10e275ff5d8c0659"),
+			Flags:  bits.BindOptional,
 		}, false},

 		{"source differs", &BindMountOp{
-			Source: MustAbs("/.hakurei/etc/"),
-			Target: MustAbs("/etc/.host/048090b6ed8f9ebb10e275ff5d8c0659"),
+			Source: check.MustAbs("/.hakurei/etc/"),
+			Target: check.MustAbs("/etc/.host/048090b6ed8f9ebb10e275ff5d8c0659"),
 		}, &BindMountOp{
-			Source: MustAbs("/etc/"),
-			Target: MustAbs("/etc/.host/048090b6ed8f9ebb10e275ff5d8c0659"),
+			Source: check.MustAbs("/etc/"),
+			Target: check.MustAbs("/etc/.host/048090b6ed8f9ebb10e275ff5d8c0659"),
 		}, false},

 		{"target differs", &BindMountOp{
-			Source: MustAbs("/etc/"),
-			Target: MustAbs("/etc/.host/048090b6ed8f9ebb10e275ff5d8c0659"),
+			Source: check.MustAbs("/etc/"),
+			Target: check.MustAbs("/etc/.host/048090b6ed8f9ebb10e275ff5d8c0659"),
 		}, &BindMountOp{
-			Source: MustAbs("/etc/"),
-			Target: MustAbs("/etc/"),
+			Source: check.MustAbs("/etc/"),
+			Target: check.MustAbs("/etc/"),
 		}, false},

 		{"equals", &BindMountOp{
-			Source: MustAbs("/etc/"),
-			Target: MustAbs("/etc/.host/048090b6ed8f9ebb10e275ff5d8c0659"),
+			Source: check.MustAbs("/etc/"),
+			Target: check.MustAbs("/etc/.host/048090b6ed8f9ebb10e275ff5d8c0659"),
 		}, &BindMountOp{
-			Source: MustAbs("/etc/"),
-			Target: MustAbs("/etc/.host/048090b6ed8f9ebb10e275ff5d8c0659"),
+			Source: check.MustAbs("/etc/"),
+			Target: check.MustAbs("/etc/.host/048090b6ed8f9ebb10e275ff5d8c0659"),
 		}, true},
 	})

@@ -222,14 +249,14 @@ func TestBindMountOp(t *testing.T) {
 		{"invalid", new(BindMountOp), "mounting", "<invalid>"},

 		{"autoetc", &BindMountOp{
-			Source: MustAbs("/etc/"),
-			Target: MustAbs("/etc/.host/048090b6ed8f9ebb10e275ff5d8c0659"),
+			Source: check.MustAbs("/etc/"),
+			Target: check.MustAbs("/etc/.host/048090b6ed8f9ebb10e275ff5d8c0659"),
 		}, "mounting", `"/etc/" on "/etc/.host/048090b6ed8f9ebb10e275ff5d8c0659" flags 0x0`},

 		{"hostdev", &BindMountOp{
-			Source: MustAbs("/dev/"),
-			Target: MustAbs("/dev/"),
-			Flags:  BindWritable | BindDevice,
+			Source: check.MustAbs("/dev/"),
+			Target: check.MustAbs("/dev/"),
+			Flags:  bits.BindWritable | bits.BindDevice,
 		}, "mounting", `"/dev/" flags 0x6`},
 	})
 }
--- a/container/initdev.go
+++ b/container/initdev.go
@@ -5,19 +5,22 @@ import (
 	"fmt"
 	"path"
 	. "syscall"
+
+	"hakurei.app/container/check"
+	"hakurei.app/container/fhs"
 )

 func init() { gob.Register(new(MountDevOp)) }

 // Dev appends an [Op] that mounts a subset of host /dev.
-func (f *Ops) Dev(target *Absolute, mqueue bool) *Ops {
+func (f *Ops) Dev(target *check.Absolute, mqueue bool) *Ops {
 	*f = append(*f, &MountDevOp{target, mqueue, false})
 	return f
 }

 // DevWritable appends an [Op] that mounts a writable subset of host /dev.
 // There is usually no good reason to write to /dev, so this should always be followed by a [RemountOp].
-func (f *Ops) DevWritable(target *Absolute, mqueue bool) *Ops {
+func (f *Ops) DevWritable(target *check.Absolute, mqueue bool) *Ops {
 	*f = append(*f, &MountDevOp{target, mqueue, true})
 	return f
 }
@@ -26,7 +29,7 @@ func (f *Ops) DevWritable(target *Absolute, mqueue bool) *Ops {
 // If Mqueue is true, a private instance of [FstypeMqueue] is mounted.
 // If Write is true, the resulting mount point is left writable.
 type MountDevOp struct {
-	Target *Absolute
+	Target *check.Absolute
 	Mqueue bool
 	Write  bool
 }
@@ -46,43 +49,43 @@ func (d *MountDevOp) apply(state *setupState, k syscallDispatcher) error {
 			return err
 		}
 		if err := k.bindMount(
-			toHost(FHSDev+name),
+			state,
+			toHost(fhs.Dev+name),
 			targetPath,
 			0,
-			true,
 		); err != nil {
 			return err
 		}
 	}
 	for i, name := range []string{"stdin", "stdout", "stderr"} {
 		if err := k.symlink(
-			FHSProc+"self/fd/"+string(rune(i+'0')),
+			fhs.Proc+"self/fd/"+string(rune(i+'0')),
 			path.Join(target, name),
 		); err != nil {
-			return wrapErrSelf(err)
+			return err
 		}
 	}
 	for _, pair := range [][2]string{
-		{FHSProc + "self/fd", "fd"},
-		{FHSProc + "kcore", "core"},
+		{fhs.Proc + "self/fd", "fd"},
+		{fhs.Proc + "kcore", "core"},
 		{"pts/ptmx", "ptmx"},
 	} {
 		if err := k.symlink(pair[0], path.Join(target, pair[1])); err != nil {
-			return wrapErrSelf(err)
+			return err
 		}
 	}

+	devShmPath := path.Join(target, "shm")
 	devPtsPath := path.Join(target, "pts")
-	for _, name := range []string{path.Join(target, "shm"), devPtsPath} {
+	for _, name := range []string{devShmPath, devPtsPath} {
 		if err := k.mkdir(name, state.ParentPerm); err != nil {
-			return wrapErrSelf(err)
+			return err
 		}
 	}

 	if err := k.mount(SourceDevpts, devPtsPath, FstypeDevpts, MS_NOSUID|MS_NOEXEC,
 		"newinstance,ptmxmode=0666,mode=620"); err != nil {
-		return wrapErrSuffix(err,
-			fmt.Sprintf("cannot mount devpts on %q:", devPtsPath))
+		return err
 	}

 	if state.RetainSession {
@@ -92,12 +95,12 @@ func (d *MountDevOp) apply(state *setupState, k syscallDispatcher) error {
 				return err
 			}
 			if name, err := k.readlink(hostProc.stdout()); err != nil {
-				return wrapErrSelf(err)
+				return err
 			} else if err = k.bindMount(
+				state,
 				toHost(name),
 				consolePath,
 				0,
-				false,
 			); err != nil {
 				return err
 			}
@@ -107,18 +110,21 @@ func (d *MountDevOp) apply(state *setupState, k syscallDispatcher) error {
 	if d.Mqueue {
 		mqueueTarget := path.Join(target, "mqueue")
 		if err := k.mkdir(mqueueTarget, state.ParentPerm); err != nil {
-			return wrapErrSelf(err)
+			return err
 		}
 		if err := k.mount(SourceMqueue, mqueueTarget, FstypeMqueue, MS_NOSUID|MS_NOEXEC|MS_NODEV, zeroString); err != nil {
-			return wrapErrSuffix(err, "cannot mount mqueue:")
+			return err
 		}
 	}

 	if d.Write {
 		return nil
 	}
-	return wrapErrSuffix(k.remount(target, MS_RDONLY),
-		fmt.Sprintf("cannot remount %q:", target))
+
+	if err := k.remount(state, target, MS_RDONLY); err != nil {
+		return err
+	}
+	return k.mountTmpfs(SourceTmpfs, devShmPath, MS_NOSUID|MS_NODEV, 0, 01777)
 }

 func (d *MountDevOp) Is(op Op) bool {
@@ -128,7 +134,7 @@ func (d *MountDevOp) Is(op Op) bool {
 		d.Mqueue == vd.Mqueue &&
 		d.Write == vd.Write
 }
-func (*MountDevOp) prefix() string { return "mounting" }
+func (*MountDevOp) prefix() (string, bool) { return "mounting", true }
 func (d *MountDevOp) String() string {
 	if d.Mqueue {
 		return fmt.Sprintf("dev on %q with mqueue", d.Target)
--- a/container/initdev_test.go
+++ b/container/initdev_test.go
--- a/container/initmkdir.go
+++ b/container/initmkdir.go
@@ -4,26 +4,28 @@ import (
 	"encoding/gob"
 	"fmt"
 	"os"
+
+	"hakurei.app/container/check"
 )

 func init() { gob.Register(new(MkdirOp)) }

 // Mkdir appends an [Op] that creates a directory in the container filesystem.
-func (f *Ops) Mkdir(name *Absolute, perm os.FileMode) *Ops {
+func (f *Ops) Mkdir(name *check.Absolute, perm os.FileMode) *Ops {
 	*f = append(*f, &MkdirOp{name, perm})
 	return f
 }

 // MkdirOp creates a directory at container Path with permission bits set to Perm.
 type MkdirOp struct {
-	Path *Absolute
+	Path *check.Absolute
 	Perm os.FileMode
 }

 func (m *MkdirOp) Valid() bool                                { return m != nil && m.Path != nil }
 func (m *MkdirOp) early(*setupState, syscallDispatcher) error { return nil }
 func (m *MkdirOp) apply(_ *setupState, k syscallDispatcher) error {
-	return wrapErrSelf(k.mkdirAll(toSysroot(m.Path.String()), m.Perm))
+	return k.mkdirAll(toSysroot(m.Path.String()), m.Perm)
 }

 func (m *MkdirOp) Is(op Op) bool {
@@ -32,5 +34,5 @@ func (m *MkdirOp) Is(op Op) bool {
 		m.Path.Is(vm.Path) &&
 		m.Perm == vm.Perm
 }
-func (*MkdirOp) prefix() string   { return "creating" }
-func (m *MkdirOp) String() string { return fmt.Sprintf("directory %q perm %s", m.Path, m.Perm) }
+func (*MkdirOp) prefix() (string, bool) { return "creating", true }
+func (m *MkdirOp) String() string       { return fmt.Sprintf("directory %q perm %s", m.Path, m.Perm) }
--- a/container/initmkdir_test.go
+++ b/container/initmkdir_test.go
@@ -3,40 +3,45 @@ package container
 import (
 	"os"
 	"testing"
+
+	"hakurei.app/container/check"
+	"hakurei.app/container/stub"
 )

 func TestMkdirOp(t *testing.T) {
+	t.Parallel()
+
 	checkOpBehaviour(t, []opBehaviourTestCase{
 		{"success", new(Params), &MkdirOp{
-			Path: MustAbs("/.hakurei"),
+			Path: check.MustAbs("/.hakurei"),
 			Perm: 0500,
-		}, nil, nil, []kexpect{
-			{"mkdirAll", expectArgs{"/sysroot/.hakurei", os.FileMode(0500)}, nil, nil},
+		}, nil, nil, []stub.Call{
+			call("mkdirAll", stub.ExpectArgs{"/sysroot/.hakurei", os.FileMode(0500)}, nil, nil),
 		}, nil},
 	})

 	checkOpsValid(t, []opValidTestCase{
 		{"nil", (*MkdirOp)(nil), false},
 		{"zero", new(MkdirOp), false},
-		{"valid", &MkdirOp{Path: MustAbs("/.hakurei")}, true},
+		{"valid", &MkdirOp{Path: check.MustAbs("/.hakurei")}, true},
 	})

 	checkOpsBuilder(t, []opsBuilderTestCase{
-		{"etc", new(Ops).Mkdir(MustAbs("/etc/"), 0), Ops{
-			&MkdirOp{Path: MustAbs("/etc/")},
+		{"etc", new(Ops).Mkdir(check.MustAbs("/etc/"), 0), Ops{
+			&MkdirOp{Path: check.MustAbs("/etc/")},
 		}},
 	})

 	checkOpIs(t, []opIsTestCase{
 		{"zero", new(MkdirOp), new(MkdirOp), false},
-		{"path differs", &MkdirOp{Path: MustAbs("/"), Perm: 0755}, &MkdirOp{Path: MustAbs("/etc/"), Perm: 0755}, false},
-		{"perm differs", &MkdirOp{Path: MustAbs("/")}, &MkdirOp{Path: MustAbs("/"), Perm: 0755}, false},
-		{"equals", &MkdirOp{Path: MustAbs("/")}, &MkdirOp{Path: MustAbs("/")}, true},
+		{"path differs", &MkdirOp{Path: check.MustAbs("/"), Perm: 0755}, &MkdirOp{Path: check.MustAbs("/etc/"), Perm: 0755}, false},
+		{"perm differs", &MkdirOp{Path: check.MustAbs("/")}, &MkdirOp{Path: check.MustAbs("/"), Perm: 0755}, false},
+		{"equals", &MkdirOp{Path: check.MustAbs("/")}, &MkdirOp{Path: check.MustAbs("/")}, true},
 	})

 	checkOpMeta(t, []opMetaTestCase{
 		{"etc", &MkdirOp{
-			Path: MustAbs("/etc/"),
+			Path: check.MustAbs("/etc/"),
 		}, "creating", `directory "/etc/" perm ----------`},
 	})
 }
--- a/container/initoverlay.go
+++ b/container/initoverlay.go
@@ -3,9 +3,11 @@ package container
 import (
 	"encoding/gob"
 	"fmt"
-	"io/fs"
 	"slices"
 	"strings"
+
+	"hakurei.app/container/check"
+	"hakurei.app/container/fhs"
 )

 const (
@@ -19,8 +21,41 @@ const (

 func init() { gob.Register(new(MountOverlayOp)) }

+const (
+	// OverlayEphemeralUnexpectedUpper is set when [MountOverlayOp.Work] is nil
+	// and [MountOverlayOp.Upper] holds an unexpected value.
+	OverlayEphemeralUnexpectedUpper = iota
+	// OverlayReadonlyLower is set when [MountOverlayOp.Lower] contains less than
+	// two entries when mounting readonly.
+	OverlayReadonlyLower
+	// OverlayEmptyLower is set when [MountOverlayOp.Lower] has length of zero.
+	OverlayEmptyLower
+)
+
+// OverlayArgumentError is returned for [MountOverlayOp] supplied with invalid argument.
+type OverlayArgumentError struct {
+	Type  uintptr
+	Value string
+}
+
+func (e *OverlayArgumentError) Error() string {
+	switch e.Type {
+	case OverlayEphemeralUnexpectedUpper:
+		return fmt.Sprintf("upperdir has unexpected value %q", e.Value)
+
+	case OverlayReadonlyLower:
+		return "readonly overlay requires at least two lowerdir"
+
+	case OverlayEmptyLower:
+		return "overlay requires at least one lowerdir"
+
+	default:
+		return fmt.Sprintf("invalid overlay argument error %#x", e.Type)
+	}
+}
+
 // Overlay appends an [Op] that mounts the overlay pseudo filesystem on [MountOverlayOp.Target].
-func (f *Ops) Overlay(target, state, work *Absolute, layers ...*Absolute) *Ops {
+func (f *Ops) Overlay(target, state, work *check.Absolute, layers ...*check.Absolute) *Ops {
 	*f = append(*f, &MountOverlayOp{
 		Target: target,
 		Lower:  layers,
@@ -32,34 +67,34 @@ func (f *Ops) Overlay(target, state, work *Absolute, layers ...*Absolute) *Ops {

 // OverlayEphemeral appends an [Op] that mounts the overlay pseudo filesystem on [MountOverlayOp.Target]
 // with an ephemeral upperdir and workdir.
-func (f *Ops) OverlayEphemeral(target *Absolute, layers ...*Absolute) *Ops {
-	return f.Overlay(target, AbsFHSRoot, nil, layers...)
+func (f *Ops) OverlayEphemeral(target *check.Absolute, layers ...*check.Absolute) *Ops {
+	return f.Overlay(target, fhs.AbsRoot, nil, layers...)
 }

 // OverlayReadonly appends an [Op] that mounts the overlay pseudo filesystem readonly on [MountOverlayOp.Target]
-func (f *Ops) OverlayReadonly(target *Absolute, layers ...*Absolute) *Ops {
+func (f *Ops) OverlayReadonly(target *check.Absolute, layers ...*check.Absolute) *Ops {
 	return f.Overlay(target, nil, nil, layers...)
 }

 // MountOverlayOp mounts [FstypeOverlay] on container path Target.
 type MountOverlayOp struct {
-	Target *Absolute
+	Target *check.Absolute

 	// Any filesystem, does not need to be on a writable filesystem.
-	Lower []*Absolute
+	Lower []*check.Absolute
 	// formatted for [OptionOverlayLowerdir], resolved, prefixed and escaped during early
 	lower []string
 	// The upperdir is normally on a writable filesystem.
 	//
-	// If Work is nil and Upper holds the special value [AbsFHSRoot],
+	// If Work is nil and Upper holds the special value [fhs.AbsRoot],
 	// an ephemeral upperdir and workdir will be set up.
 	//
 	// If both Work and Upper are nil, upperdir and workdir is omitted and the overlay is mounted readonly.
-	Upper *Absolute
+	Upper *check.Absolute
 	// formatted for [OptionOverlayUpperdir], resolved, prefixed and escaped during early
 	upper string
 	// The workdir needs to be an empty directory on the same filesystem as upperdir.
-	Work *Absolute
+	Work *check.Absolute
 	// formatted for [OptionOverlayWorkdir], resolved, prefixed and escaped during early
 	work string

@@ -85,11 +120,11 @@ func (o *MountOverlayOp) Valid() bool {
 func (o *MountOverlayOp) early(_ *setupState, k syscallDispatcher) error {
 	if o.Work == nil && o.Upper != nil {
 		switch o.Upper.String() {
-		case FHSRoot: // ephemeral
+		case fhs.Root: // ephemeral
 			o.ephemeral = true // intermediate root not yet available

 		default:
-			return msg.WrapErr(fs.ErrInvalid, fmt.Sprintf("upperdir has unexpected value %q", o.Upper))
+			return &OverlayArgumentError{OverlayEphemeralUnexpectedUpper, o.Upper.String()}
 		}
 	}
 	// readonly handled in apply
@@ -97,22 +132,22 @@ func (o *MountOverlayOp) early(_ *setupState, k syscallDispatcher) error {
 	if !o.ephemeral {
 		if o.Upper != o.Work && (o.Upper == nil || o.Work == nil) {
 			// unreachable
-			return msg.WrapErr(fs.ErrClosed, "impossible overlay state reached")
+			return OpStateError("overlay")
 		}

 		if o.Upper != nil {
 			if v, err := k.evalSymlinks(o.Upper.String()); err != nil {
-				return wrapErrSelf(err)
+				return err
 			} else {
-				o.upper = EscapeOverlayDataSegment(toHost(v))
+				o.upper = check.EscapeOverlayDataSegment(toHost(v))
 			}
 		}

 		if o.Work != nil {
 			if v, err := k.evalSymlinks(o.Work.String()); err != nil {
-				return wrapErrSelf(err)
+				return err
 			} else {
-				o.work = EscapeOverlayDataSegment(toHost(v))
+				o.work = check.EscapeOverlayDataSegment(toHost(v))
 			}
 		}
 	}
@@ -120,9 +155,9 @@ func (o *MountOverlayOp) early(_ *setupState, k syscallDispatcher) error {
 	o.lower = make([]string, len(o.Lower))
 	for i, a := range o.Lower { // nil checked in Valid
 		if v, err := k.evalSymlinks(a.String()); err != nil {
-			return wrapErrSelf(err)
+			return err
 		} else {
-			o.lower[i] = EscapeOverlayDataSegment(toHost(v))
+			o.lower[i] = check.EscapeOverlayDataSegment(toHost(v))
 		}
 	}
 	return nil
@@ -134,17 +169,17 @@ func (o *MountOverlayOp) apply(state *setupState, k syscallDispatcher) error {
 		target = toSysroot(target)
 	}
 	if err := k.mkdirAll(target, state.ParentPerm); err != nil {
-		return wrapErrSelf(err)
+		return err
 	}

 	if o.ephemeral {
 		var err error
 		// these directories are created internally, therefore early (absolute, symlink, prefix, escape) is bypassed
-		if o.upper, err = k.mkdirTemp(FHSRoot, intermediatePatternOverlayUpper); err != nil {
-			return wrapErrSelf(err)
+		if o.upper, err = k.mkdirTemp(fhs.Root, intermediatePatternOverlayUpper); err != nil {
+			return err
 		}
-		if o.work, err = k.mkdirTemp(FHSRoot, intermediatePatternOverlayWork); err != nil {
-			return wrapErrSelf(err)
+		if o.work, err = k.mkdirTemp(fhs.Root, intermediatePatternOverlayWork); err != nil {
+			return err
 		}
 	}

@@ -152,33 +187,32 @@ func (o *MountOverlayOp) apply(state *setupState, k syscallDispatcher) error {

 	if o.upper == zeroString && o.work == zeroString { // readonly
 		if len(o.Lower) < 2 {
-			return msg.WrapErr(fs.ErrInvalid, "readonly overlay requires at least two lowerdir")
+			return &OverlayArgumentError{OverlayReadonlyLower, zeroString}
 		}
 		// "upperdir=" and "workdir=" may be omitted. In that case the overlay will be read-only
 	} else {
 		if len(o.Lower) == 0 {
-			return msg.WrapErr(fs.ErrInvalid, "overlay requires at least one lowerdir")
+			return &OverlayArgumentError{OverlayEmptyLower, zeroString}
 		}
 		options = append(options,
 			OptionOverlayUpperdir+"="+o.upper,
 			OptionOverlayWorkdir+"="+o.work)
 	}
 	options = append(options,
-		OptionOverlayLowerdir+"="+strings.Join(o.lower, SpecialOverlayPath),
+		OptionOverlayLowerdir+"="+strings.Join(o.lower, check.SpecialOverlayPath),
 		OptionOverlayUserxattr)

-	return wrapErrSuffix(k.mount(SourceOverlay, target, FstypeOverlay, 0, strings.Join(options, SpecialOverlayOption)),
-		fmt.Sprintf("cannot mount overlay on %q:", o.Target))
+	return k.mount(SourceOverlay, target, FstypeOverlay, 0, strings.Join(options, check.SpecialOverlayOption))
 }

 func (o *MountOverlayOp) Is(op Op) bool {
 	vo, ok := op.(*MountOverlayOp)
 	return ok && o.Valid() && vo.Valid() &&
 		o.Target.Is(vo.Target) &&
-		slices.EqualFunc(o.Lower, vo.Lower, func(a *Absolute, v *Absolute) bool { return a.Is(v) }) &&
+		slices.EqualFunc(o.Lower, vo.Lower, func(a, v *check.Absolute) bool { return a.Is(v) }) &&
 		o.Upper.Is(vo.Upper) && o.Work.Is(vo.Work)
 }
-func (*MountOverlayOp) prefix() string { return "mounting" }
+func (*MountOverlayOp) prefix() (string, bool) { return "mounting", true }
 func (o *MountOverlayOp) String() string {
 	return fmt.Sprintf("overlay on %q with %d layers", o.Target, len(o.Lower))
 }
--- a/container/initoverlay_test.go
+++ b/container/initoverlay_test.go
@@ -2,232 +2,266 @@ package container

 import (
 	"errors"
-	"io/fs"
 	"os"
 	"testing"
+
+	"hakurei.app/container/check"
+	"hakurei.app/container/stub"
 )

 func TestMountOverlayOp(t *testing.T) {
+	t.Parallel()
+
+	t.Run("argument error", func(t *testing.T) {
+		t.Parallel()
+
+		testCases := []struct {
+			name string
+			err  *OverlayArgumentError
+			want string
+		}{
+			{"unexpected upper", &OverlayArgumentError{OverlayEphemeralUnexpectedUpper, "/proc/"},
+				`upperdir has unexpected value "/proc/"`},
+
+			{"lower ro short", &OverlayArgumentError{OverlayReadonlyLower, zeroString},
+				"readonly overlay requires at least two lowerdir"},
+
+			{"lower short", &OverlayArgumentError{OverlayEmptyLower, zeroString},
+				"overlay requires at least one lowerdir"},
+
+			{"oob", &OverlayArgumentError{0xdeadbeef, zeroString},
+				"invalid overlay argument error 0xdeadbeef"},
+		}
+		for _, tc := range testCases {
+			t.Run(tc.name, func(t *testing.T) {
+				t.Parallel()
+				if got := tc.err.Error(); got != tc.want {
+					t.Errorf("Error: %q, want %q", got, tc.want)
+				}
+			})
+		}
+	})
+
 	checkOpBehaviour(t, []opBehaviourTestCase{
 		{"mkdirTemp invalid ephemeral", &Params{ParentPerm: 0705}, &MountOverlayOp{
-			Target: MustAbs("/"),
-			Lower: []*Absolute{
-				MustAbs("/var/lib/planterette/base/debian:f92c9052"),
-				MustAbs("/var/lib/planterette/app/org.chromium.Chromium@debian:f92c9052"),
+			Target: check.MustAbs("/"),
+			Lower: []*check.Absolute{
+				check.MustAbs("/var/lib/planterette/base/debian:f92c9052"),
+				check.MustAbs("/var/lib/planterette/app/org.chromium.Chromium@debian:f92c9052"),
 			},
-			Upper: MustAbs("/proc/"),
-		}, nil, msg.WrapErr(fs.ErrInvalid, `upperdir has unexpected value "/proc/"`), nil, nil},
+			Upper: check.MustAbs("/proc/"),
+		}, nil, &OverlayArgumentError{OverlayEphemeralUnexpectedUpper, "/proc/"}, nil, nil},

 		{"mkdirTemp upper ephemeral", &Params{ParentPerm: 0705}, &MountOverlayOp{
-			Target: MustAbs("/"),
-			Lower: []*Absolute{
-				MustAbs("/var/lib/planterette/base/debian:f92c9052"),
-				MustAbs("/var/lib/planterette/app/org.chromium.Chromium@debian:f92c9052"),
+			Target: check.MustAbs("/"),
+			Lower: []*check.Absolute{
+				check.MustAbs("/var/lib/planterette/base/debian:f92c9052"),
+				check.MustAbs("/var/lib/planterette/app/org.chromium.Chromium@debian:f92c9052"),
 			},
-			Upper: MustAbs("/"),
-		}, []kexpect{
-			{"evalSymlinks", expectArgs{"/var/lib/planterette/base/debian:f92c9052"}, "/var/lib/planterette/base/debian:f92c9052", nil},
-			{"evalSymlinks", expectArgs{"/var/lib/planterette/app/org.chromium.Chromium@debian:f92c9052"}, "/var/lib/planterette/app/org.chromium.Chromium@debian:f92c9052", nil},
-		}, nil, []kexpect{
-			{"mkdirAll", expectArgs{"/sysroot", os.FileMode(0705)}, nil, nil},
-			{"mkdirTemp", expectArgs{"/", "overlay.upper.*"}, "overlay.upper.32768", errUnique},
-		}, wrapErrSelf(errUnique)},
+			Upper: check.MustAbs("/"),
+		}, []stub.Call{
+			call("evalSymlinks", stub.ExpectArgs{"/var/lib/planterette/base/debian:f92c9052"}, "/var/lib/planterette/base/debian:f92c9052", nil),
+			call("evalSymlinks", stub.ExpectArgs{"/var/lib/planterette/app/org.chromium.Chromium@debian:f92c9052"}, "/var/lib/planterette/app/org.chromium.Chromium@debian:f92c9052", nil),
+		}, nil, []stub.Call{
+			call("mkdirAll", stub.ExpectArgs{"/sysroot", os.FileMode(0705)}, nil, nil),
+			call("mkdirTemp", stub.ExpectArgs{"/", "overlay.upper.*"}, "overlay.upper.32768", stub.UniqueError(6)),
+		}, stub.UniqueError(6)},

 		{"mkdirTemp work ephemeral", &Params{ParentPerm: 0705}, &MountOverlayOp{
-			Target: MustAbs("/"),
-			Lower: []*Absolute{
-				MustAbs("/var/lib/planterette/base/debian:f92c9052"),
-				MustAbs("/var/lib/planterette/app/org.chromium.Chromium@debian:f92c9052"),
+			Target: check.MustAbs("/"),
+			Lower: []*check.Absolute{
+				check.MustAbs("/var/lib/planterette/base/debian:f92c9052"),
+				check.MustAbs("/var/lib/planterette/app/org.chromium.Chromium@debian:f92c9052"),
 			},
-			Upper: MustAbs("/"),
-		}, []kexpect{
-			{"evalSymlinks", expectArgs{"/var/lib/planterette/base/debian:f92c9052"}, "/var/lib/planterette/base/debian:f92c9052", nil},
-			{"evalSymlinks", expectArgs{"/var/lib/planterette/app/org.chromium.Chromium@debian:f92c9052"}, "/var/lib/planterette/app/org.chromium.Chromium@debian:f92c9052", nil},
-		}, nil, []kexpect{
-			{"mkdirAll", expectArgs{"/sysroot", os.FileMode(0705)}, nil, nil},
-			{"mkdirTemp", expectArgs{"/", "overlay.upper.*"}, "overlay.upper.32768", nil},
-			{"mkdirTemp", expectArgs{"/", "overlay.work.*"}, "overlay.work.32768", errUnique},
-		}, wrapErrSelf(errUnique)},
+			Upper: check.MustAbs("/"),
+		}, []stub.Call{
+			call("evalSymlinks", stub.ExpectArgs{"/var/lib/planterette/base/debian:f92c9052"}, "/var/lib/planterette/base/debian:f92c9052", nil),
+			call("evalSymlinks", stub.ExpectArgs{"/var/lib/planterette/app/org.chromium.Chromium@debian:f92c9052"}, "/var/lib/planterette/app/org.chromium.Chromium@debian:f92c9052", nil),
+		}, nil, []stub.Call{
+			call("mkdirAll", stub.ExpectArgs{"/sysroot", os.FileMode(0705)}, nil, nil),
+			call("mkdirTemp", stub.ExpectArgs{"/", "overlay.upper.*"}, "overlay.upper.32768", nil),
+			call("mkdirTemp", stub.ExpectArgs{"/", "overlay.work.*"}, "overlay.work.32768", stub.UniqueError(5)),
+		}, stub.UniqueError(5)},

 		{"success ephemeral", &Params{ParentPerm: 0705}, &MountOverlayOp{
-			Target: MustAbs("/"),
-			Lower: []*Absolute{
-				MustAbs("/var/lib/planterette/base/debian:f92c9052"),
-				MustAbs("/var/lib/planterette/app/org.chromium.Chromium@debian:f92c9052"),
+			Target: check.MustAbs("/"),
+			Lower: []*check.Absolute{
+				check.MustAbs("/var/lib/planterette/base/debian:f92c9052"),
+				check.MustAbs("/var/lib/planterette/app/org.chromium.Chromium@debian:f92c9052"),
 			},
-			Upper: MustAbs("/"),
-		}, []kexpect{
-			{"evalSymlinks", expectArgs{"/var/lib/planterette/base/debian:f92c9052"}, "/var/lib/planterette/base/debian:f92c9052", nil},
-			{"evalSymlinks", expectArgs{"/var/lib/planterette/app/org.chromium.Chromium@debian:f92c9052"}, "/var/lib/planterette/app/org.chromium.Chromium@debian:f92c9052", nil},
-		}, nil, []kexpect{
-			{"mkdirAll", expectArgs{"/sysroot", os.FileMode(0705)}, nil, nil},
-			{"mkdirTemp", expectArgs{"/", "overlay.upper.*"}, "overlay.upper.32768", nil},
-			{"mkdirTemp", expectArgs{"/", "overlay.work.*"}, "overlay.work.32768", nil},
-			{"mount", expectArgs{"overlay", "/sysroot", "overlay", uintptr(0), "" +
+			Upper: check.MustAbs("/"),
+		}, []stub.Call{
+			call("evalSymlinks", stub.ExpectArgs{"/var/lib/planterette/base/debian:f92c9052"}, "/var/lib/planterette/base/debian:f92c9052", nil),
+			call("evalSymlinks", stub.ExpectArgs{"/var/lib/planterette/app/org.chromium.Chromium@debian:f92c9052"}, "/var/lib/planterette/app/org.chromium.Chromium@debian:f92c9052", nil),
+		}, nil, []stub.Call{
+			call("mkdirAll", stub.ExpectArgs{"/sysroot", os.FileMode(0705)}, nil, nil),
+			call("mkdirTemp", stub.ExpectArgs{"/", "overlay.upper.*"}, "overlay.upper.32768", nil),
+			call("mkdirTemp", stub.ExpectArgs{"/", "overlay.work.*"}, "overlay.work.32768", nil),
+			call("mount", stub.ExpectArgs{"overlay", "/sysroot", "overlay", uintptr(0), "" +
 				"upperdir=overlay.upper.32768," +
 				"workdir=overlay.work.32768," +
 				"lowerdir=" +
 				`/host/var/lib/planterette/base/debian\:f92c9052:` +
 				`/host/var/lib/planterette/app/org.chromium.Chromium@debian\:f92c9052,` +
-				"userxattr"}, nil, nil},
+				"userxattr"}, nil, nil),
 		}, nil},

 		{"short lower ro", &Params{ParentPerm: 0755}, &MountOverlayOp{
-			Target: MustAbs("/nix/store"),
-			Lower: []*Absolute{
-				MustAbs("/mnt-root/nix/.ro-store"),
+			Target: check.MustAbs("/nix/store"),
+			Lower: []*check.Absolute{
+				check.MustAbs("/mnt-root/nix/.ro-store"),
 			},
-		}, []kexpect{
-			{"evalSymlinks", expectArgs{"/mnt-root/nix/.ro-store"}, "/mnt-root/nix/.ro-store", nil},
-		}, nil, []kexpect{
-			{"mkdirAll", expectArgs{"/sysroot/nix/store", os.FileMode(0755)}, nil, nil},
-		}, msg.WrapErr(fs.ErrInvalid, "readonly overlay requires at least two lowerdir")},
+		}, []stub.Call{
+			call("evalSymlinks", stub.ExpectArgs{"/mnt-root/nix/.ro-store"}, "/mnt-root/nix/.ro-store", nil),
+		}, nil, []stub.Call{
+			call("mkdirAll", stub.ExpectArgs{"/sysroot/nix/store", os.FileMode(0755)}, nil, nil),
+		}, &OverlayArgumentError{OverlayReadonlyLower, zeroString}},

 		{"success ro noPrefix", &Params{ParentPerm: 0755}, &MountOverlayOp{
-			Target: MustAbs("/nix/store"),
-			Lower: []*Absolute{
-				MustAbs("/mnt-root/nix/.ro-store"),
-				MustAbs("/mnt-root/nix/.ro-store0"),
+			Target: check.MustAbs("/nix/store"),
+			Lower: []*check.Absolute{
+				check.MustAbs("/mnt-root/nix/.ro-store"),
+				check.MustAbs("/mnt-root/nix/.ro-store0"),
 			},
 			noPrefix: true,
-		}, []kexpect{
-			{"evalSymlinks", expectArgs{"/mnt-root/nix/.ro-store"}, "/mnt-root/nix/.ro-store", nil},
-			{"evalSymlinks", expectArgs{"/mnt-root/nix/.ro-store0"}, "/mnt-root/nix/.ro-store0", nil},
-		}, nil, []kexpect{
-			{"mkdirAll", expectArgs{"/nix/store", os.FileMode(0755)}, nil, nil},
-			{"mount", expectArgs{"overlay", "/nix/store", "overlay", uintptr(0), "" +
+		}, []stub.Call{
+			call("evalSymlinks", stub.ExpectArgs{"/mnt-root/nix/.ro-store"}, "/mnt-root/nix/.ro-store", nil),
+			call("evalSymlinks", stub.ExpectArgs{"/mnt-root/nix/.ro-store0"}, "/mnt-root/nix/.ro-store0", nil),
+		}, nil, []stub.Call{
+			call("mkdirAll", stub.ExpectArgs{"/nix/store", os.FileMode(0755)}, nil, nil),
+			call("mount", stub.ExpectArgs{"overlay", "/nix/store", "overlay", uintptr(0), "" +
 				"lowerdir=" +
 				"/host/mnt-root/nix/.ro-store:" +
 				"/host/mnt-root/nix/.ro-store0," +
-				"userxattr"}, nil, nil},
+				"userxattr"}, nil, nil),
 		}, nil},

 		{"success ro", &Params{ParentPerm: 0755}, &MountOverlayOp{
-			Target: MustAbs("/nix/store"),
-			Lower: []*Absolute{
-				MustAbs("/mnt-root/nix/.ro-store"),
-				MustAbs("/mnt-root/nix/.ro-store0"),
+			Target: check.MustAbs("/nix/store"),
+			Lower: []*check.Absolute{
+				check.MustAbs("/mnt-root/nix/.ro-store"),
+				check.MustAbs("/mnt-root/nix/.ro-store0"),
 			},
-		}, []kexpect{
-			{"evalSymlinks", expectArgs{"/mnt-root/nix/.ro-store"}, "/mnt-root/nix/.ro-store", nil},
-			{"evalSymlinks", expectArgs{"/mnt-root/nix/.ro-store0"}, "/mnt-root/nix/.ro-store0", nil},
-		}, nil, []kexpect{
-			{"mkdirAll", expectArgs{"/sysroot/nix/store", os.FileMode(0755)}, nil, nil},
-			{"mount", expectArgs{"overlay", "/sysroot/nix/store", "overlay", uintptr(0), "" +
+		}, []stub.Call{
+			call("evalSymlinks", stub.ExpectArgs{"/mnt-root/nix/.ro-store"}, "/mnt-root/nix/.ro-store", nil),
+			call("evalSymlinks", stub.ExpectArgs{"/mnt-root/nix/.ro-store0"}, "/mnt-root/nix/.ro-store0", nil),
+		}, nil, []stub.Call{
+			call("mkdirAll", stub.ExpectArgs{"/sysroot/nix/store", os.FileMode(0755)}, nil, nil),
+			call("mount", stub.ExpectArgs{"overlay", "/sysroot/nix/store", "overlay", uintptr(0), "" +
 				"lowerdir=" +
 				"/host/mnt-root/nix/.ro-store:" +
 				"/host/mnt-root/nix/.ro-store0," +
-				"userxattr"}, nil, nil},
+				"userxattr"}, nil, nil),
 		}, nil},

 		{"nil lower", &Params{ParentPerm: 0700}, &MountOverlayOp{
-			Target: MustAbs("/nix/store"),
-			Upper:  MustAbs("/mnt-root/nix/.rw-store/upper"),
-			Work:   MustAbs("/mnt-root/nix/.rw-store/work"),
-		}, []kexpect{
-			{"evalSymlinks", expectArgs{"/mnt-root/nix/.rw-store/upper"}, "/mnt-root/nix/.rw-store/.upper", nil},
-			{"evalSymlinks", expectArgs{"/mnt-root/nix/.rw-store/work"}, "/mnt-root/nix/.rw-store/.work", nil},
-		}, nil, []kexpect{
-			{"mkdirAll", expectArgs{"/sysroot/nix/store", os.FileMode(0700)}, nil, nil},
-		}, msg.WrapErr(fs.ErrInvalid, "overlay requires at least one lowerdir")},
+			Target: check.MustAbs("/nix/store"),
+			Upper:  check.MustAbs("/mnt-root/nix/.rw-store/upper"),
+			Work:   check.MustAbs("/mnt-root/nix/.rw-store/work"),
+		}, []stub.Call{
+			call("evalSymlinks", stub.ExpectArgs{"/mnt-root/nix/.rw-store/upper"}, "/mnt-root/nix/.rw-store/.upper", nil),
+			call("evalSymlinks", stub.ExpectArgs{"/mnt-root/nix/.rw-store/work"}, "/mnt-root/nix/.rw-store/.work", nil),
+		}, nil, []stub.Call{
+			call("mkdirAll", stub.ExpectArgs{"/sysroot/nix/store", os.FileMode(0700)}, nil, nil),
+		}, &OverlayArgumentError{OverlayEmptyLower, zeroString}},

 		{"evalSymlinks upper", &Params{ParentPerm: 0700}, &MountOverlayOp{
-			Target: MustAbs("/nix/store"),
-			Lower:  []*Absolute{MustAbs("/mnt-root/nix/.ro-store")},
-			Upper:  MustAbs("/mnt-root/nix/.rw-store/upper"),
-			Work:   MustAbs("/mnt-root/nix/.rw-store/work"),
-		}, []kexpect{
-			{"evalSymlinks", expectArgs{"/mnt-root/nix/.rw-store/upper"}, "/mnt-root/nix/.rw-store/.upper", errUnique},
-		}, wrapErrSelf(errUnique), nil, nil},
+			Target: check.MustAbs("/nix/store"),
+			Lower:  []*check.Absolute{check.MustAbs("/mnt-root/nix/.ro-store")},
+			Upper:  check.MustAbs("/mnt-root/nix/.rw-store/upper"),
+			Work:   check.MustAbs("/mnt-root/nix/.rw-store/work"),
+		}, []stub.Call{
+			call("evalSymlinks", stub.ExpectArgs{"/mnt-root/nix/.rw-store/upper"}, "/mnt-root/nix/.rw-store/.upper", stub.UniqueError(4)),
+		}, stub.UniqueError(4), nil, nil},

 		{"evalSymlinks work", &Params{ParentPerm: 0700}, &MountOverlayOp{
-			Target: MustAbs("/nix/store"),
-			Lower:  []*Absolute{MustAbs("/mnt-root/nix/.ro-store")},
-			Upper:  MustAbs("/mnt-root/nix/.rw-store/upper"),
-			Work:   MustAbs("/mnt-root/nix/.rw-store/work"),
-		}, []kexpect{
-			{"evalSymlinks", expectArgs{"/mnt-root/nix/.rw-store/upper"}, "/mnt-root/nix/.rw-store/.upper", nil},
-			{"evalSymlinks", expectArgs{"/mnt-root/nix/.rw-store/work"}, "/mnt-root/nix/.rw-store/.work", errUnique},
-		}, wrapErrSelf(errUnique), nil, nil},
+			Target: check.MustAbs("/nix/store"),
+			Lower:  []*check.Absolute{check.MustAbs("/mnt-root/nix/.ro-store")},
+			Upper:  check.MustAbs("/mnt-root/nix/.rw-store/upper"),
+			Work:   check.MustAbs("/mnt-root/nix/.rw-store/work"),
+		}, []stub.Call{
+			call("evalSymlinks", stub.ExpectArgs{"/mnt-root/nix/.rw-store/upper"}, "/mnt-root/nix/.rw-store/.upper", nil),
+			call("evalSymlinks", stub.ExpectArgs{"/mnt-root/nix/.rw-store/work"}, "/mnt-root/nix/.rw-store/.work", stub.UniqueError(3)),
+		}, stub.UniqueError(3), nil, nil},

 		{"evalSymlinks lower", &Params{ParentPerm: 0700}, &MountOverlayOp{
-			Target: MustAbs("/nix/store"),
-			Lower:  []*Absolute{MustAbs("/mnt-root/nix/.ro-store")},
-			Upper:  MustAbs("/mnt-root/nix/.rw-store/upper"),
-			Work:   MustAbs("/mnt-root/nix/.rw-store/work"),
-		}, []kexpect{
-			{"evalSymlinks", expectArgs{"/mnt-root/nix/.rw-store/upper"}, "/mnt-root/nix/.rw-store/.upper", nil},
-			{"evalSymlinks", expectArgs{"/mnt-root/nix/.rw-store/work"}, "/mnt-root/nix/.rw-store/.work", nil},
-			{"evalSymlinks", expectArgs{"/mnt-root/nix/.ro-store"}, "/mnt-root/nix/ro-store", errUnique},
-		}, wrapErrSelf(errUnique), nil, nil},
+			Target: check.MustAbs("/nix/store"),
+			Lower:  []*check.Absolute{check.MustAbs("/mnt-root/nix/.ro-store")},
+			Upper:  check.MustAbs("/mnt-root/nix/.rw-store/upper"),
+			Work:   check.MustAbs("/mnt-root/nix/.rw-store/work"),
+		}, []stub.Call{
+			call("evalSymlinks", stub.ExpectArgs{"/mnt-root/nix/.rw-store/upper"}, "/mnt-root/nix/.rw-store/.upper", nil),
+			call("evalSymlinks", stub.ExpectArgs{"/mnt-root/nix/.rw-store/work"}, "/mnt-root/nix/.rw-store/.work", nil),
+			call("evalSymlinks", stub.ExpectArgs{"/mnt-root/nix/.ro-store"}, "/mnt-root/nix/ro-store", stub.UniqueError(2)),
+		}, stub.UniqueError(2), nil, nil},

 		{"mkdirAll", &Params{ParentPerm: 0700}, &MountOverlayOp{
-			Target: MustAbs("/nix/store"),
-			Lower:  []*Absolute{MustAbs("/mnt-root/nix/.ro-store")},
-			Upper:  MustAbs("/mnt-root/nix/.rw-store/upper"),
-			Work:   MustAbs("/mnt-root/nix/.rw-store/work"),
-		}, []kexpect{
-			{"evalSymlinks", expectArgs{"/mnt-root/nix/.rw-store/upper"}, "/mnt-root/nix/.rw-store/.upper", nil},
-			{"evalSymlinks", expectArgs{"/mnt-root/nix/.rw-store/work"}, "/mnt-root/nix/.rw-store/.work", nil},
-			{"evalSymlinks", expectArgs{"/mnt-root/nix/.ro-store"}, "/mnt-root/nix/ro-store", nil},
-		}, nil, []kexpect{
-			{"mkdirAll", expectArgs{"/sysroot/nix/store", os.FileMode(0700)}, nil, errUnique},
-		}, wrapErrSelf(errUnique)},
+			Target: check.MustAbs("/nix/store"),
+			Lower:  []*check.Absolute{check.MustAbs("/mnt-root/nix/.ro-store")},
+			Upper:  check.MustAbs("/mnt-root/nix/.rw-store/upper"),
+			Work:   check.MustAbs("/mnt-root/nix/.rw-store/work"),
+		}, []stub.Call{
+			call("evalSymlinks", stub.ExpectArgs{"/mnt-root/nix/.rw-store/upper"}, "/mnt-root/nix/.rw-store/.upper", nil),
+			call("evalSymlinks", stub.ExpectArgs{"/mnt-root/nix/.rw-store/work"}, "/mnt-root/nix/.rw-store/.work", nil),
+			call("evalSymlinks", stub.ExpectArgs{"/mnt-root/nix/.ro-store"}, "/mnt-root/nix/ro-store", nil),
+		}, nil, []stub.Call{
+			call("mkdirAll", stub.ExpectArgs{"/sysroot/nix/store", os.FileMode(0700)}, nil, stub.UniqueError(1)),
+		}, stub.UniqueError(1)},

 		{"mount", &Params{ParentPerm: 0700}, &MountOverlayOp{
-			Target: MustAbs("/nix/store"),
-			Lower:  []*Absolute{MustAbs("/mnt-root/nix/.ro-store")},
-			Upper:  MustAbs("/mnt-root/nix/.rw-store/upper"),
-			Work:   MustAbs("/mnt-root/nix/.rw-store/work"),
-		}, []kexpect{
-			{"evalSymlinks", expectArgs{"/mnt-root/nix/.rw-store/upper"}, "/mnt-root/nix/.rw-store/.upper", nil},
-			{"evalSymlinks", expectArgs{"/mnt-root/nix/.rw-store/work"}, "/mnt-root/nix/.rw-store/.work", nil},
-			{"evalSymlinks", expectArgs{"/mnt-root/nix/.ro-store"}, "/mnt-root/nix/ro-store", nil},
-		}, nil, []kexpect{
-			{"mkdirAll", expectArgs{"/sysroot/nix/store", os.FileMode(0700)}, nil, nil},
-			{"mount", expectArgs{"overlay", "/sysroot/nix/store", "overlay", uintptr(0), "upperdir=/host/mnt-root/nix/.rw-store/.upper,workdir=/host/mnt-root/nix/.rw-store/.work,lowerdir=/host/mnt-root/nix/ro-store,userxattr"}, nil, errUnique},
-		}, wrapErrSuffix(errUnique, `cannot mount overlay on "/nix/store":`)},
+			Target: check.MustAbs("/nix/store"),
+			Lower:  []*check.Absolute{check.MustAbs("/mnt-root/nix/.ro-store")},
+			Upper:  check.MustAbs("/mnt-root/nix/.rw-store/upper"),
+			Work:   check.MustAbs("/mnt-root/nix/.rw-store/work"),
+		}, []stub.Call{
+			call("evalSymlinks", stub.ExpectArgs{"/mnt-root/nix/.rw-store/upper"}, "/mnt-root/nix/.rw-store/.upper", nil),
+			call("evalSymlinks", stub.ExpectArgs{"/mnt-root/nix/.rw-store/work"}, "/mnt-root/nix/.rw-store/.work", nil),
+			call("evalSymlinks", stub.ExpectArgs{"/mnt-root/nix/.ro-store"}, "/mnt-root/nix/ro-store", nil),
+		}, nil, []stub.Call{
+			call("mkdirAll", stub.ExpectArgs{"/sysroot/nix/store", os.FileMode(0700)}, nil, nil),
+			call("mount", stub.ExpectArgs{"overlay", "/sysroot/nix/store", "overlay", uintptr(0), "upperdir=/host/mnt-root/nix/.rw-store/.upper,workdir=/host/mnt-root/nix/.rw-store/.work,lowerdir=/host/mnt-root/nix/ro-store,userxattr"}, nil, stub.UniqueError(0)),
+		}, stub.UniqueError(0)},

 		{"success single layer", &Params{ParentPerm: 0700}, &MountOverlayOp{
-			Target: MustAbs("/nix/store"),
-			Lower:  []*Absolute{MustAbs("/mnt-root/nix/.ro-store")},
-			Upper:  MustAbs("/mnt-root/nix/.rw-store/upper"),
-			Work:   MustAbs("/mnt-root/nix/.rw-store/work"),
-		}, []kexpect{
-			{"evalSymlinks", expectArgs{"/mnt-root/nix/.rw-store/upper"}, "/mnt-root/nix/.rw-store/.upper", nil},
-			{"evalSymlinks", expectArgs{"/mnt-root/nix/.rw-store/work"}, "/mnt-root/nix/.rw-store/.work", nil},
-			{"evalSymlinks", expectArgs{"/mnt-root/nix/.ro-store"}, "/mnt-root/nix/ro-store", nil},
-		}, nil, []kexpect{
-			{"mkdirAll", expectArgs{"/sysroot/nix/store", os.FileMode(0700)}, nil, nil},
-			{"mount", expectArgs{"overlay", "/sysroot/nix/store", "overlay", uintptr(0), "" +
+			Target: check.MustAbs("/nix/store"),
+			Lower:  []*check.Absolute{check.MustAbs("/mnt-root/nix/.ro-store")},
+			Upper:  check.MustAbs("/mnt-root/nix/.rw-store/upper"),
+			Work:   check.MustAbs("/mnt-root/nix/.rw-store/work"),
+		}, []stub.Call{
+			call("evalSymlinks", stub.ExpectArgs{"/mnt-root/nix/.rw-store/upper"}, "/mnt-root/nix/.rw-store/.upper", nil),
+			call("evalSymlinks", stub.ExpectArgs{"/mnt-root/nix/.rw-store/work"}, "/mnt-root/nix/.rw-store/.work", nil),
+			call("evalSymlinks", stub.ExpectArgs{"/mnt-root/nix/.ro-store"}, "/mnt-root/nix/ro-store", nil),
+		}, nil, []stub.Call{
+			call("mkdirAll", stub.ExpectArgs{"/sysroot/nix/store", os.FileMode(0700)}, nil, nil),
+			call("mount", stub.ExpectArgs{"overlay", "/sysroot/nix/store", "overlay", uintptr(0), "" +
 				"upperdir=/host/mnt-root/nix/.rw-store/.upper," +
 				"workdir=/host/mnt-root/nix/.rw-store/.work," +
 				"lowerdir=/host/mnt-root/nix/ro-store," +
-				"userxattr"}, nil, nil},
+				"userxattr"}, nil, nil),
 		}, nil},

 		{"success", &Params{ParentPerm: 0700}, &MountOverlayOp{
-			Target: MustAbs("/nix/store"),
-			Lower: []*Absolute{
-				MustAbs("/mnt-root/nix/.ro-store"),
-				MustAbs("/mnt-root/nix/.ro-store0"),
-				MustAbs("/mnt-root/nix/.ro-store1"),
-				MustAbs("/mnt-root/nix/.ro-store2"),
-				MustAbs("/mnt-root/nix/.ro-store3"),
+			Target: check.MustAbs("/nix/store"),
+			Lower: []*check.Absolute{
+				check.MustAbs("/mnt-root/nix/.ro-store"),
+				check.MustAbs("/mnt-root/nix/.ro-store0"),
+				check.MustAbs("/mnt-root/nix/.ro-store1"),
+				check.MustAbs("/mnt-root/nix/.ro-store2"),
+				check.MustAbs("/mnt-root/nix/.ro-store3"),
 			},
-			Upper: MustAbs("/mnt-root/nix/.rw-store/upper"),
-			Work:  MustAbs("/mnt-root/nix/.rw-store/work"),
-		}, []kexpect{
-			{"evalSymlinks", expectArgs{"/mnt-root/nix/.rw-store/upper"}, "/mnt-root/nix/.rw-store/.upper", nil},
-			{"evalSymlinks", expectArgs{"/mnt-root/nix/.rw-store/work"}, "/mnt-root/nix/.rw-store/.work", nil},
-			{"evalSymlinks", expectArgs{"/mnt-root/nix/.ro-store"}, "/mnt-root/nix/ro-store", nil},
-			{"evalSymlinks", expectArgs{"/mnt-root/nix/.ro-store0"}, "/mnt-root/nix/ro-store0", nil},
-			{"evalSymlinks", expectArgs{"/mnt-root/nix/.ro-store1"}, "/mnt-root/nix/ro-store1", nil},
-			{"evalSymlinks", expectArgs{"/mnt-root/nix/.ro-store2"}, "/mnt-root/nix/ro-store2", nil},
-			{"evalSymlinks", expectArgs{"/mnt-root/nix/.ro-store3"}, "/mnt-root/nix/ro-store3", nil},
-		}, nil, []kexpect{
-			{"mkdirAll", expectArgs{"/sysroot/nix/store", os.FileMode(0700)}, nil, nil},
-			{"mount", expectArgs{"overlay", "/sysroot/nix/store", "overlay", uintptr(0), "" +
+			Upper: check.MustAbs("/mnt-root/nix/.rw-store/upper"),
+			Work:  check.MustAbs("/mnt-root/nix/.rw-store/work"),
+		}, []stub.Call{
+			call("evalSymlinks", stub.ExpectArgs{"/mnt-root/nix/.rw-store/upper"}, "/mnt-root/nix/.rw-store/.upper", nil),
+			call("evalSymlinks", stub.ExpectArgs{"/mnt-root/nix/.rw-store/work"}, "/mnt-root/nix/.rw-store/.work", nil),
+			call("evalSymlinks", stub.ExpectArgs{"/mnt-root/nix/.ro-store"}, "/mnt-root/nix/ro-store", nil),
+			call("evalSymlinks", stub.ExpectArgs{"/mnt-root/nix/.ro-store0"}, "/mnt-root/nix/ro-store0", nil),
+			call("evalSymlinks", stub.ExpectArgs{"/mnt-root/nix/.ro-store1"}, "/mnt-root/nix/ro-store1", nil),
+			call("evalSymlinks", stub.ExpectArgs{"/mnt-root/nix/.ro-store2"}, "/mnt-root/nix/ro-store2", nil),
+			call("evalSymlinks", stub.ExpectArgs{"/mnt-root/nix/.ro-store3"}, "/mnt-root/nix/ro-store3", nil),
+		}, nil, []stub.Call{
+			call("mkdirAll", stub.ExpectArgs{"/sysroot/nix/store", os.FileMode(0700)}, nil, nil),
+			call("mount", stub.ExpectArgs{"overlay", "/sysroot/nix/store", "overlay", uintptr(0), "" +
 				"upperdir=/host/mnt-root/nix/.rw-store/.upper," +
 				"workdir=/host/mnt-root/nix/.rw-store/.work," +
 				"lowerdir=" +
@@ -236,15 +270,18 @@ func TestMountOverlayOp(t *testing.T) {
 				"/host/mnt-root/nix/ro-store1:" +
 				"/host/mnt-root/nix/ro-store2:" +
 				"/host/mnt-root/nix/ro-store3," +
-				"userxattr"}, nil, nil},
+				"userxattr"}, nil, nil),
 		}, nil},
 	})

 	t.Run("unreachable", func(t *testing.T) {
+		t.Parallel()
+
 		t.Run("nil Upper non-nil Work not ephemeral", func(t *testing.T) {
-			wantErr := msg.WrapErr(fs.ErrClosed, "impossible overlay state reached")
+			t.Parallel()
+			wantErr := OpStateError("overlay")
 			if err := (&MountOverlayOp{
-				Work: MustAbs("/"),
+				Work: check.MustAbs("/"),
 			}).early(nil, nil); !errors.Is(err, wantErr) {
 				t.Errorf("apply: error = %v, want %v", err, wantErr)
 			}
@@ -254,39 +291,39 @@ func TestMountOverlayOp(t *testing.T) {
 	checkOpsValid(t, []opValidTestCase{
 		{"nil", (*MountOverlayOp)(nil), false},
 		{"zero", new(MountOverlayOp), false},
-		{"nil lower", &MountOverlayOp{Target: MustAbs("/"), Lower: []*Absolute{nil}}, false},
-		{"ro", &MountOverlayOp{Target: MustAbs("/"), Lower: []*Absolute{MustAbs("/")}}, true},
-		{"ro work", &MountOverlayOp{Target: MustAbs("/"), Work: MustAbs("/tmp/")}, false},
-		{"rw", &MountOverlayOp{Target: MustAbs("/"), Lower: []*Absolute{MustAbs("/")}, Upper: MustAbs("/"), Work: MustAbs("/")}, true},
+		{"nil lower", &MountOverlayOp{Target: check.MustAbs("/"), Lower: []*check.Absolute{nil}}, false},
+		{"ro", &MountOverlayOp{Target: check.MustAbs("/"), Lower: []*check.Absolute{check.MustAbs("/")}}, true},
+		{"ro work", &MountOverlayOp{Target: check.MustAbs("/"), Work: check.MustAbs("/tmp/")}, false},
+		{"rw", &MountOverlayOp{Target: check.MustAbs("/"), Lower: []*check.Absolute{check.MustAbs("/")}, Upper: check.MustAbs("/"), Work: check.MustAbs("/")}, true},
 	})

 	checkOpsBuilder(t, []opsBuilderTestCase{
 		{"full", new(Ops).Overlay(
-			MustAbs("/nix/store"),
-			MustAbs("/mnt-root/nix/.rw-store/upper"),
-			MustAbs("/mnt-root/nix/.rw-store/work"),
-			MustAbs("/mnt-root/nix/.ro-store"),
+			check.MustAbs("/nix/store"),
+			check.MustAbs("/mnt-root/nix/.rw-store/upper"),
+			check.MustAbs("/mnt-root/nix/.rw-store/work"),
+			check.MustAbs("/mnt-root/nix/.ro-store"),
 		), Ops{
 			&MountOverlayOp{
-				Target: MustAbs("/nix/store"),
-				Lower:  []*Absolute{MustAbs("/mnt-root/nix/.ro-store")},
-				Upper:  MustAbs("/mnt-root/nix/.rw-store/upper"),
-				Work:   MustAbs("/mnt-root/nix/.rw-store/work"),
+				Target: check.MustAbs("/nix/store"),
+				Lower:  []*check.Absolute{check.MustAbs("/mnt-root/nix/.ro-store")},
+				Upper:  check.MustAbs("/mnt-root/nix/.rw-store/upper"),
+				Work:   check.MustAbs("/mnt-root/nix/.rw-store/work"),
 			},
 		}},

-		{"ephemeral", new(Ops).OverlayEphemeral(MustAbs("/nix/store"), MustAbs("/mnt-root/nix/.ro-store")), Ops{
+		{"ephemeral", new(Ops).OverlayEphemeral(check.MustAbs("/nix/store"), check.MustAbs("/mnt-root/nix/.ro-store")), Ops{
 			&MountOverlayOp{
-				Target: MustAbs("/nix/store"),
-				Lower:  []*Absolute{MustAbs("/mnt-root/nix/.ro-store")},
-				Upper:  MustAbs("/"),
+				Target: check.MustAbs("/nix/store"),
+				Lower:  []*check.Absolute{check.MustAbs("/mnt-root/nix/.ro-store")},
+				Upper:  check.MustAbs("/"),
 			},
 		}},

-		{"readonly", new(Ops).OverlayReadonly(MustAbs("/nix/store"), MustAbs("/mnt-root/nix/.ro-store")), Ops{
+		{"readonly", new(Ops).OverlayReadonly(check.MustAbs("/nix/store"), check.MustAbs("/mnt-root/nix/.ro-store")), Ops{
 			&MountOverlayOp{
-				Target: MustAbs("/nix/store"),
-				Lower:  []*Absolute{MustAbs("/mnt-root/nix/.ro-store")},
+				Target: check.MustAbs("/nix/store"),
+				Lower:  []*check.Absolute{check.MustAbs("/mnt-root/nix/.ro-store")},
 			},
 		}},
 	})
@@ -295,74 +332,74 @@ func TestMountOverlayOp(t *testing.T) {
 		{"zero", new(MountOverlayOp), new(MountOverlayOp), false},

 		{"differs target", &MountOverlayOp{
-			Target: MustAbs("/nix/store/differs"),
-			Lower:  []*Absolute{MustAbs("/mnt-root/nix/.ro-store")},
-			Upper:  MustAbs("/mnt-root/nix/.rw-store/upper"),
-			Work:   MustAbs("/mnt-root/nix/.rw-store/work"),
+			Target: check.MustAbs("/nix/store/differs"),
+			Lower:  []*check.Absolute{check.MustAbs("/mnt-root/nix/.ro-store")},
+			Upper:  check.MustAbs("/mnt-root/nix/.rw-store/upper"),
+			Work:   check.MustAbs("/mnt-root/nix/.rw-store/work"),
 		}, &MountOverlayOp{
-			Target: MustAbs("/nix/store"),
-			Lower:  []*Absolute{MustAbs("/mnt-root/nix/.ro-store")},
-			Upper:  MustAbs("/mnt-root/nix/.rw-store/upper"),
-			Work:   MustAbs("/mnt-root/nix/.rw-store/work")}, false},
+			Target: check.MustAbs("/nix/store"),
+			Lower:  []*check.Absolute{check.MustAbs("/mnt-root/nix/.ro-store")},
+			Upper:  check.MustAbs("/mnt-root/nix/.rw-store/upper"),
+			Work:   check.MustAbs("/mnt-root/nix/.rw-store/work")}, false},

 		{"differs lower", &MountOverlayOp{
-			Target: MustAbs("/nix/store"),
-			Lower:  []*Absolute{MustAbs("/mnt-root/nix/.ro-store/differs")},
-			Upper:  MustAbs("/mnt-root/nix/.rw-store/upper"),
-			Work:   MustAbs("/mnt-root/nix/.rw-store/work"),
+			Target: check.MustAbs("/nix/store"),
+			Lower:  []*check.Absolute{check.MustAbs("/mnt-root/nix/.ro-store/differs")},
+			Upper:  check.MustAbs("/mnt-root/nix/.rw-store/upper"),
+			Work:   check.MustAbs("/mnt-root/nix/.rw-store/work"),
 		}, &MountOverlayOp{
-			Target: MustAbs("/nix/store"),
-			Lower:  []*Absolute{MustAbs("/mnt-root/nix/.ro-store")},
-			Upper:  MustAbs("/mnt-root/nix/.rw-store/upper"),
-			Work:   MustAbs("/mnt-root/nix/.rw-store/work")}, false},
+			Target: check.MustAbs("/nix/store"),
+			Lower:  []*check.Absolute{check.MustAbs("/mnt-root/nix/.ro-store")},
+			Upper:  check.MustAbs("/mnt-root/nix/.rw-store/upper"),
+			Work:   check.MustAbs("/mnt-root/nix/.rw-store/work")}, false},

 		{"differs upper", &MountOverlayOp{
-			Target: MustAbs("/nix/store"),
-			Lower:  []*Absolute{MustAbs("/mnt-root/nix/.ro-store")},
-			Upper:  MustAbs("/mnt-root/nix/.rw-store/upper/differs"),
-			Work:   MustAbs("/mnt-root/nix/.rw-store/work"),
+			Target: check.MustAbs("/nix/store"),
+			Lower:  []*check.Absolute{check.MustAbs("/mnt-root/nix/.ro-store")},
+			Upper:  check.MustAbs("/mnt-root/nix/.rw-store/upper/differs"),
+			Work:   check.MustAbs("/mnt-root/nix/.rw-store/work"),
 		}, &MountOverlayOp{
-			Target: MustAbs("/nix/store"),
-			Lower:  []*Absolute{MustAbs("/mnt-root/nix/.ro-store")},
-			Upper:  MustAbs("/mnt-root/nix/.rw-store/upper"),
-			Work:   MustAbs("/mnt-root/nix/.rw-store/work")}, false},
+			Target: check.MustAbs("/nix/store"),
+			Lower:  []*check.Absolute{check.MustAbs("/mnt-root/nix/.ro-store")},
+			Upper:  check.MustAbs("/mnt-root/nix/.rw-store/upper"),
+			Work:   check.MustAbs("/mnt-root/nix/.rw-store/work")}, false},

 		{"differs work", &MountOverlayOp{
-			Target: MustAbs("/nix/store"),
-			Lower:  []*Absolute{MustAbs("/mnt-root/nix/.ro-store")},
-			Upper:  MustAbs("/mnt-root/nix/.rw-store/upper"),
-			Work:   MustAbs("/mnt-root/nix/.rw-store/work/differs"),
+			Target: check.MustAbs("/nix/store"),
+			Lower:  []*check.Absolute{check.MustAbs("/mnt-root/nix/.ro-store")},
+			Upper:  check.MustAbs("/mnt-root/nix/.rw-store/upper"),
+			Work:   check.MustAbs("/mnt-root/nix/.rw-store/work/differs"),
 		}, &MountOverlayOp{
-			Target: MustAbs("/nix/store"),
-			Lower:  []*Absolute{MustAbs("/mnt-root/nix/.ro-store")},
-			Upper:  MustAbs("/mnt-root/nix/.rw-store/upper"),
-			Work:   MustAbs("/mnt-root/nix/.rw-store/work")}, false},
+			Target: check.MustAbs("/nix/store"),
+			Lower:  []*check.Absolute{check.MustAbs("/mnt-root/nix/.ro-store")},
+			Upper:  check.MustAbs("/mnt-root/nix/.rw-store/upper"),
+			Work:   check.MustAbs("/mnt-root/nix/.rw-store/work")}, false},

 		{"equals ro", &MountOverlayOp{
-			Target: MustAbs("/nix/store"),
-			Lower:  []*Absolute{MustAbs("/mnt-root/nix/.ro-store")},
+			Target: check.MustAbs("/nix/store"),
+			Lower:  []*check.Absolute{check.MustAbs("/mnt-root/nix/.ro-store")},
 		}, &MountOverlayOp{
-			Target: MustAbs("/nix/store"),
-			Lower:  []*Absolute{MustAbs("/mnt-root/nix/.ro-store")}}, true},
+			Target: check.MustAbs("/nix/store"),
+			Lower:  []*check.Absolute{check.MustAbs("/mnt-root/nix/.ro-store")}}, true},

 		{"equals", &MountOverlayOp{
-			Target: MustAbs("/nix/store"),
-			Lower:  []*Absolute{MustAbs("/mnt-root/nix/.ro-store")},
-			Upper:  MustAbs("/mnt-root/nix/.rw-store/upper"),
-			Work:   MustAbs("/mnt-root/nix/.rw-store/work"),
+			Target: check.MustAbs("/nix/store"),
+			Lower:  []*check.Absolute{check.MustAbs("/mnt-root/nix/.ro-store")},
+			Upper:  check.MustAbs("/mnt-root/nix/.rw-store/upper"),
+			Work:   check.MustAbs("/mnt-root/nix/.rw-store/work"),
 		}, &MountOverlayOp{
-			Target: MustAbs("/nix/store"),
-			Lower:  []*Absolute{MustAbs("/mnt-root/nix/.ro-store")},
-			Upper:  MustAbs("/mnt-root/nix/.rw-store/upper"),
-			Work:   MustAbs("/mnt-root/nix/.rw-store/work")}, true},
+			Target: check.MustAbs("/nix/store"),
+			Lower:  []*check.Absolute{check.MustAbs("/mnt-root/nix/.ro-store")},
+			Upper:  check.MustAbs("/mnt-root/nix/.rw-store/upper"),
+			Work:   check.MustAbs("/mnt-root/nix/.rw-store/work")}, true},
 	})

 	checkOpMeta(t, []opMetaTestCase{
 		{"nix", &MountOverlayOp{
-			Target: MustAbs("/nix/store"),
-			Lower:  []*Absolute{MustAbs("/mnt-root/nix/.ro-store")},
-			Upper:  MustAbs("/mnt-root/nix/.rw-store/upper"),
-			Work:   MustAbs("/mnt-root/nix/.rw-store/work"),
+			Target: check.MustAbs("/nix/store"),
+			Lower:  []*check.Absolute{check.MustAbs("/mnt-root/nix/.ro-store")},
+			Upper:  check.MustAbs("/mnt-root/nix/.rw-store/upper"),
+			Work:   check.MustAbs("/mnt-root/nix/.rw-store/work"),
 		}, "mounting", `overlay on "/nix/store" with 1 layers`},
 	})
 }
--- a/container/initplace.go
+++ b/container/initplace.go
@@ -4,6 +4,9 @@ import (
 	"encoding/gob"
 	"fmt"
 	"syscall"
+
+	"hakurei.app/container/check"
+	"hakurei.app/container/fhs"
 )

 const (
@@ -14,23 +17,14 @@ const (
 func init() { gob.Register(new(TmpfileOp)) }

 // Place appends an [Op] that places a file in container path [TmpfileOp.Path] containing [TmpfileOp.Data].
-func (f *Ops) Place(name *Absolute, data []byte) *Ops {
+func (f *Ops) Place(name *check.Absolute, data []byte) *Ops {
 	*f = append(*f, &TmpfileOp{name, data})
 	return f
 }

-// PlaceP is like Place but writes the address of [TmpfileOp.Data] to the pointer dataP points to.
-func (f *Ops) PlaceP(name *Absolute, dataP **[]byte) *Ops {
-	t := &TmpfileOp{Path: name}
-	*dataP = &t.Data
-
-	*f = append(*f, t)
-	return f
-}
-
 // TmpfileOp places a file on container Path containing Data.
 type TmpfileOp struct {
-	Path *Absolute
+	Path *check.Absolute
 	Data []byte
 }

@@ -38,14 +32,12 @@ func (t *TmpfileOp) Valid() bool                                { return t != ni
 func (t *TmpfileOp) early(*setupState, syscallDispatcher) error { return nil }
 func (t *TmpfileOp) apply(state *setupState, k syscallDispatcher) error {
 	var tmpPath string
-	if f, err := k.createTemp(FHSRoot, intermediatePatternTmpfile); err != nil {
-		return wrapErrSelf(err)
+	if f, err := k.createTemp(fhs.Root, intermediatePatternTmpfile); err != nil {
+		return err
 	} else if _, err = f.Write(t.Data); err != nil {
-		return wrapErrSuffix(err,
-			"cannot write to intermediate file:")
+		return err
 	} else if err = f.Close(); err != nil {
-		return wrapErrSuffix(err,
-			"cannot close intermediate file:")
+		return err
 	} else {
 		tmpPath = f.Name()
 	}
@@ -54,14 +46,14 @@ func (t *TmpfileOp) apply(state *setupState, k syscallDispatcher) error {
 	if err := k.ensureFile(target, 0444, state.ParentPerm); err != nil {
 		return err
 	} else if err = k.bindMount(
+		state,
 		tmpPath,
 		target,
 		syscall.MS_RDONLY|syscall.MS_NODEV,
-		false,
 	); err != nil {
 		return err
 	} else if err = k.remove(tmpPath); err != nil {
-		return wrapErrSelf(err)
+		return err
 	}
 	return nil
 }
@@ -72,7 +64,7 @@ func (t *TmpfileOp) Is(op Op) bool {
 		t.Path.Is(vt.Path) &&
 		string(t.Data) == string(vt.Data)
 }
-func (*TmpfileOp) prefix() string { return "placing" }
+func (*TmpfileOp) prefix() (string, bool) { return "placing", true }
 func (t *TmpfileOp) String() string {
 	return fmt.Sprintf("tmpfile %q (%d bytes)", t.Path, len(t.Data))
 }
--- a/container/initplace_test.go
+++ b/container/initplace_test.go
@@ -3,72 +3,76 @@ package container
 import (
 	"os"
 	"testing"
+
+	"hakurei.app/container/check"
+	"hakurei.app/container/stub"
 )

 func TestTmpfileOp(t *testing.T) {
 	const sampleDataString = `chronos:x:65534:65534:Hakurei:/var/empty:/bin/zsh`
 	var (
-		samplePath = MustAbs("/etc/passwd")
+		samplePath = check.MustAbs("/etc/passwd")
 		sampleData = []byte(sampleDataString)
 	)
+	t.Parallel()

 	checkOpBehaviour(t, []opBehaviourTestCase{
 		{"createTemp", &Params{ParentPerm: 0700}, &TmpfileOp{
 			Path: samplePath,
 			Data: sampleData,
-		}, nil, nil, []kexpect{
-			{"createTemp", expectArgs{"/", "tmp.*"}, newCheckedFile(t, "tmp.32768", sampleDataString, nil), errUnique},
-		}, wrapErrSelf(errUnique)},
+		}, nil, nil, []stub.Call{
+			call("createTemp", stub.ExpectArgs{"/", "tmp.*"}, newCheckedFile(t, "tmp.32768", sampleDataString, nil), stub.UniqueError(5)),
+		}, stub.UniqueError(5)},

 		{"Write", &Params{ParentPerm: 0700}, &TmpfileOp{
 			Path: samplePath,
 			Data: sampleData,
-		}, nil, nil, []kexpect{
-			{"createTemp", expectArgs{"/", "tmp.*"}, writeErrOsFile{errUnique}, nil},
-		}, wrapErrSuffix(errUnique, "cannot write to intermediate file:")},
+		}, nil, nil, []stub.Call{
+			call("createTemp", stub.ExpectArgs{"/", "tmp.*"}, writeErrOsFile{stub.UniqueError(4)}, nil),
+		}, stub.UniqueError(4)},

 		{"Close", &Params{ParentPerm: 0700}, &TmpfileOp{
 			Path: samplePath,
 			Data: sampleData,
-		}, nil, nil, []kexpect{
-			{"createTemp", expectArgs{"/", "tmp.*"}, newCheckedFile(t, "tmp.32768", sampleDataString, errUnique), nil},
-		}, wrapErrSuffix(errUnique, "cannot close intermediate file:")},
+		}, nil, nil, []stub.Call{
+			call("createTemp", stub.ExpectArgs{"/", "tmp.*"}, newCheckedFile(t, "tmp.32768", sampleDataString, stub.UniqueError(3)), nil),
+		}, stub.UniqueError(3)},

 		{"ensureFile", &Params{ParentPerm: 0700}, &TmpfileOp{
 			Path: samplePath,
 			Data: sampleData,
-		}, nil, nil, []kexpect{
-			{"createTemp", expectArgs{"/", "tmp.*"}, newCheckedFile(t, "tmp.32768", sampleDataString, nil), nil},
-			{"ensureFile", expectArgs{"/sysroot/etc/passwd", os.FileMode(0444), os.FileMode(0700)}, nil, errUnique},
-		}, errUnique},
+		}, nil, nil, []stub.Call{
+			call("createTemp", stub.ExpectArgs{"/", "tmp.*"}, newCheckedFile(t, "tmp.32768", sampleDataString, nil), nil),
+			call("ensureFile", stub.ExpectArgs{"/sysroot/etc/passwd", os.FileMode(0444), os.FileMode(0700)}, nil, stub.UniqueError(2)),
+		}, stub.UniqueError(2)},

 		{"bindMount", &Params{ParentPerm: 0700}, &TmpfileOp{
 			Path: samplePath,
 			Data: sampleData,
-		}, nil, nil, []kexpect{
-			{"createTemp", expectArgs{"/", "tmp.*"}, newCheckedFile(t, "tmp.32768", sampleDataString, nil), nil},
-			{"ensureFile", expectArgs{"/sysroot/etc/passwd", os.FileMode(0444), os.FileMode(0700)}, nil, nil},
-			{"bindMount", expectArgs{"tmp.32768", "/sysroot/etc/passwd", uintptr(0x5), false}, nil, errUnique},
-		}, errUnique},
+		}, nil, nil, []stub.Call{
+			call("createTemp", stub.ExpectArgs{"/", "tmp.*"}, newCheckedFile(t, "tmp.32768", sampleDataString, nil), nil),
+			call("ensureFile", stub.ExpectArgs{"/sysroot/etc/passwd", os.FileMode(0444), os.FileMode(0700)}, nil, nil),
+			call("bindMount", stub.ExpectArgs{"tmp.32768", "/sysroot/etc/passwd", uintptr(0x5), false}, nil, stub.UniqueError(1)),
+		}, stub.UniqueError(1)},

 		{"remove", &Params{ParentPerm: 0700}, &TmpfileOp{
 			Path: samplePath,
 			Data: sampleData,
-		}, nil, nil, []kexpect{
-			{"createTemp", expectArgs{"/", "tmp.*"}, newCheckedFile(t, "tmp.32768", sampleDataString, nil), nil},
-			{"ensureFile", expectArgs{"/sysroot/etc/passwd", os.FileMode(0444), os.FileMode(0700)}, nil, nil},
-			{"bindMount", expectArgs{"tmp.32768", "/sysroot/etc/passwd", uintptr(0x5), false}, nil, nil},
-			{"remove", expectArgs{"tmp.32768"}, nil, errUnique},
-		}, wrapErrSelf(errUnique)},
+		}, nil, nil, []stub.Call{
+			call("createTemp", stub.ExpectArgs{"/", "tmp.*"}, newCheckedFile(t, "tmp.32768", sampleDataString, nil), nil),
+			call("ensureFile", stub.ExpectArgs{"/sysroot/etc/passwd", os.FileMode(0444), os.FileMode(0700)}, nil, nil),
+			call("bindMount", stub.ExpectArgs{"tmp.32768", "/sysroot/etc/passwd", uintptr(0x5), false}, nil, nil),
+			call("remove", stub.ExpectArgs{"tmp.32768"}, nil, stub.UniqueError(0)),
+		}, stub.UniqueError(0)},

 		{"success", &Params{ParentPerm: 0700}, &TmpfileOp{
 			Path: samplePath,
 			Data: sampleData,
-		}, nil, nil, []kexpect{
-			{"createTemp", expectArgs{"/", "tmp.*"}, newCheckedFile(t, "tmp.32768", sampleDataString, nil), nil},
-			{"ensureFile", expectArgs{"/sysroot/etc/passwd", os.FileMode(0444), os.FileMode(0700)}, nil, nil},
-			{"bindMount", expectArgs{"tmp.32768", "/sysroot/etc/passwd", uintptr(0x5), false}, nil, nil},
-			{"remove", expectArgs{"tmp.32768"}, nil, nil},
+		}, nil, nil, []stub.Call{
+			call("createTemp", stub.ExpectArgs{"/", "tmp.*"}, newCheckedFile(t, "tmp.32768", sampleDataString, nil), nil),
+			call("ensureFile", stub.ExpectArgs{"/sysroot/etc/passwd", os.FileMode(0444), os.FileMode(0700)}, nil, nil),
+			call("bindMount", stub.ExpectArgs{"tmp.32768", "/sysroot/etc/passwd", uintptr(0x5), false}, nil, nil),
+			call("remove", stub.ExpectArgs{"tmp.32768"}, nil, nil),
 		}, nil},
 	})

@@ -79,18 +83,8 @@ func TestTmpfileOp(t *testing.T) {
 	})

 	checkOpsBuilder(t, []opsBuilderTestCase{
-		{"noref", new(Ops).Place(samplePath, sampleData), Ops{
-			&TmpfileOp{
-				Path: samplePath,
-				Data: sampleData,
-			},
-		}},
-
-		{"ref", new(Ops).PlaceP(samplePath, new(*[]byte)), Ops{
-			&TmpfileOp{
-				Path: samplePath,
-				Data: []byte{},
-			},
+		{"full", new(Ops).Place(samplePath, sampleData), Ops{
+			&TmpfileOp{Path: samplePath, Data: sampleData},
 		}},
 	})

@@ -98,7 +92,7 @@ func TestTmpfileOp(t *testing.T) {
 		{"zero", new(TmpfileOp), new(TmpfileOp), false},

 		{"differs path", &TmpfileOp{
-			Path: MustAbs("/etc/group"),
+			Path: check.MustAbs("/etc/group"),
 			Data: sampleData,
 		}, &TmpfileOp{
 			Path: samplePath,
--- a/container/initproc.go
+++ b/container/initproc.go
@@ -4,28 +4,29 @@ import (
 	"encoding/gob"
 	"fmt"
 	. "syscall"
+
+	"hakurei.app/container/check"
 )

 func init() { gob.Register(new(MountProcOp)) }

 // Proc appends an [Op] that mounts a private instance of proc.
-func (f *Ops) Proc(target *Absolute) *Ops {
+func (f *Ops) Proc(target *check.Absolute) *Ops {
 	*f = append(*f, &MountProcOp{target})
 	return f
 }

 // MountProcOp mounts a new instance of [FstypeProc] on container path Target.
-type MountProcOp struct{ Target *Absolute }
+type MountProcOp struct{ Target *check.Absolute }

 func (p *MountProcOp) Valid() bool                                { return p != nil && p.Target != nil }
 func (p *MountProcOp) early(*setupState, syscallDispatcher) error { return nil }
 func (p *MountProcOp) apply(state *setupState, k syscallDispatcher) error {
 	target := toSysroot(p.Target.String())
 	if err := k.mkdirAll(target, state.ParentPerm); err != nil {
-		return wrapErrSelf(err)
+		return err
 	}
-	return wrapErrSuffix(k.mount(SourceProc, target, FstypeProc, MS_NOSUID|MS_NOEXEC|MS_NODEV, zeroString),
-		fmt.Sprintf("cannot mount proc on %q:", p.Target.String()))
+	return k.mount(SourceProc, target, FstypeProc, MS_NOSUID|MS_NOEXEC|MS_NODEV, zeroString)
 }

 func (p *MountProcOp) Is(op Op) bool {
@@ -33,5 +34,5 @@ func (p *MountProcOp) Is(op Op) bool {
 	return ok && p.Valid() && vp.Valid() &&
 		p.Target.Is(vp.Target)
 }
-func (*MountProcOp) prefix() string   { return "mounting" }
-func (p *MountProcOp) String() string { return fmt.Sprintf("proc on %q", p.Target) }
+func (*MountProcOp) prefix() (string, bool) { return "mounting", true }
+func (p *MountProcOp) String() string       { return fmt.Sprintf("proc on %q", p.Target) }
--- a/container/initproc_test.go
+++ b/container/initproc_test.go
@@ -3,35 +3,40 @@ package container
 import (
 	"os"
 	"testing"
+
+	"hakurei.app/container/check"
+	"hakurei.app/container/stub"
 )

 func TestMountProcOp(t *testing.T) {
+	t.Parallel()
+
 	checkOpBehaviour(t, []opBehaviourTestCase{
 		{"mkdir", &Params{ParentPerm: 0755},
 			&MountProcOp{
-				Target: MustAbs("/proc/"),
-			}, nil, nil, []kexpect{
-				{"mkdirAll", expectArgs{"/sysroot/proc", os.FileMode(0755)}, nil, errUnique},
-			}, wrapErrSelf(errUnique)},
+				Target: check.MustAbs("/proc/"),
+			}, nil, nil, []stub.Call{
+				call("mkdirAll", stub.ExpectArgs{"/sysroot/proc", os.FileMode(0755)}, nil, stub.UniqueError(0)),
+			}, stub.UniqueError(0)},

 		{"success", &Params{ParentPerm: 0700},
 			&MountProcOp{
-				Target: MustAbs("/proc/"),
-			}, nil, nil, []kexpect{
-				{"mkdirAll", expectArgs{"/sysroot/proc", os.FileMode(0700)}, nil, nil},
-				{"mount", expectArgs{"proc", "/sysroot/proc", "proc", uintptr(0xe), ""}, nil, nil},
+				Target: check.MustAbs("/proc/"),
+			}, nil, nil, []stub.Call{
+				call("mkdirAll", stub.ExpectArgs{"/sysroot/proc", os.FileMode(0700)}, nil, nil),
+				call("mount", stub.ExpectArgs{"proc", "/sysroot/proc", "proc", uintptr(0xe), ""}, nil, nil),
 			}, nil},
 	})

 	checkOpsValid(t, []opValidTestCase{
 		{"nil", (*MountProcOp)(nil), false},
 		{"zero", new(MountProcOp), false},
-		{"valid", &MountProcOp{Target: MustAbs("/proc/")}, true},
+		{"valid", &MountProcOp{Target: check.MustAbs("/proc/")}, true},
 	})

 	checkOpsBuilder(t, []opsBuilderTestCase{
-		{"proc", new(Ops).Proc(MustAbs("/proc/")), Ops{
-			&MountProcOp{Target: MustAbs("/proc/")},
+		{"proc", new(Ops).Proc(check.MustAbs("/proc/")), Ops{
+			&MountProcOp{Target: check.MustAbs("/proc/")},
 		}},
 	})

@@ -39,20 +44,20 @@ func TestMountProcOp(t *testing.T) {
 		{"zero", new(MountProcOp), new(MountProcOp), false},

 		{"target differs", &MountProcOp{
-			Target: MustAbs("/proc/nonexistent"),
+			Target: check.MustAbs("/proc/nonexistent"),
 		}, &MountProcOp{
-			Target: MustAbs("/proc/"),
+			Target: check.MustAbs("/proc/"),
 		}, false},

 		{"equals", &MountProcOp{
-			Target: MustAbs("/proc/"),
+			Target: check.MustAbs("/proc/"),
 		}, &MountProcOp{
-			Target: MustAbs("/proc/"),
+			Target: check.MustAbs("/proc/"),
 		}, true},
 	})

 	checkOpMeta(t, []opMetaTestCase{
-		{"proc", &MountProcOp{Target: MustAbs("/proc/")},
+		{"proc", &MountProcOp{Target: check.MustAbs("/proc/")},
 			"mounting", `proc on "/proc/"`},
 	})
 }
--- a/container/initremount.go
+++ b/container/initremount.go
@@ -3,27 +3,28 @@ package container
 import (
 	"encoding/gob"
 	"fmt"
+
+	"hakurei.app/container/check"
 )

 func init() { gob.Register(new(RemountOp)) }

 // Remount appends an [Op] that applies [RemountOp.Flags] on container path [RemountOp.Target].
-func (f *Ops) Remount(target *Absolute, flags uintptr) *Ops {
+func (f *Ops) Remount(target *check.Absolute, flags uintptr) *Ops {
 	*f = append(*f, &RemountOp{target, flags})
 	return f
 }

 // RemountOp remounts Target with Flags.
 type RemountOp struct {
-	Target *Absolute
+	Target *check.Absolute
 	Flags  uintptr
 }

 func (r *RemountOp) Valid() bool                              { return r != nil && r.Target != nil }
 func (*RemountOp) early(*setupState, syscallDispatcher) error { return nil }
-func (r *RemountOp) apply(_ *setupState, k syscallDispatcher) error {
-	return wrapErrSuffix(k.remount(toSysroot(r.Target.String()), r.Flags),
-		fmt.Sprintf("cannot remount %q:", r.Target))
+func (r *RemountOp) apply(state *setupState, k syscallDispatcher) error {
+	return k.remount(state, toSysroot(r.Target.String()), r.Flags)
 }

 func (r *RemountOp) Is(op Op) bool {
@@ -32,5 +33,5 @@ func (r *RemountOp) Is(op Op) bool {
 		r.Target.Is(vr.Target) &&
 		r.Flags == vr.Flags
 }
-func (*RemountOp) prefix() string   { return "remounting" }
-func (r *RemountOp) String() string { return fmt.Sprintf("%q flags %#x", r.Target, r.Flags) }
+func (*RemountOp) prefix() (string, bool) { return "remounting", true }
+func (r *RemountOp) String() string       { return fmt.Sprintf("%q flags %#x", r.Target, r.Flags) }
--- a/container/initremount_test.go
+++ b/container/initremount_test.go
@@ -3,28 +3,33 @@ package container
 import (
 	"syscall"
 	"testing"
+
+	"hakurei.app/container/check"
+	"hakurei.app/container/stub"
 )

 func TestRemountOp(t *testing.T) {
+	t.Parallel()
+
 	checkOpBehaviour(t, []opBehaviourTestCase{
 		{"success", new(Params), &RemountOp{
-			Target: MustAbs("/"),
+			Target: check.MustAbs("/"),
 			Flags:  syscall.MS_RDONLY,
-		}, nil, nil, []kexpect{
-			{"remount", expectArgs{"/sysroot", uintptr(1)}, nil, nil},
+		}, nil, nil, []stub.Call{
+			call("remount", stub.ExpectArgs{"/sysroot", uintptr(1)}, nil, nil),
 		}, nil},
 	})

 	checkOpsValid(t, []opValidTestCase{
 		{"nil", (*RemountOp)(nil), false},
 		{"zero", new(RemountOp), false},
-		{"valid", &RemountOp{Target: MustAbs("/"), Flags: syscall.MS_RDONLY}, true},
+		{"valid", &RemountOp{Target: check.MustAbs("/"), Flags: syscall.MS_RDONLY}, true},
 	})

 	checkOpsBuilder(t, []opsBuilderTestCase{
-		{"root", new(Ops).Remount(MustAbs("/"), syscall.MS_RDONLY), Ops{
+		{"root", new(Ops).Remount(check.MustAbs("/"), syscall.MS_RDONLY), Ops{
 			&RemountOp{
-				Target: MustAbs("/"),
+				Target: check.MustAbs("/"),
 				Flags:  syscall.MS_RDONLY,
 			},
 		}},
@@ -34,33 +39,33 @@ func TestRemountOp(t *testing.T) {
 		{"zero", new(RemountOp), new(RemountOp), false},

 		{"target differs", &RemountOp{
-			Target: MustAbs("/dev/"),
+			Target: check.MustAbs("/dev/"),
 			Flags:  syscall.MS_RDONLY,
 		}, &RemountOp{
-			Target: MustAbs("/"),
+			Target: check.MustAbs("/"),
 			Flags:  syscall.MS_RDONLY,
 		}, false},

 		{"flags differs", &RemountOp{
-			Target: MustAbs("/"),
+			Target: check.MustAbs("/"),
 			Flags:  syscall.MS_RDONLY | syscall.MS_NODEV,
 		}, &RemountOp{
-			Target: MustAbs("/"),
+			Target: check.MustAbs("/"),
 			Flags:  syscall.MS_RDONLY,
 		}, false},

 		{"equals", &RemountOp{
-			Target: MustAbs("/"),
+			Target: check.MustAbs("/"),
 			Flags:  syscall.MS_RDONLY,
 		}, &RemountOp{
-			Target: MustAbs("/"),
+			Target: check.MustAbs("/"),
 			Flags:  syscall.MS_RDONLY,
 		}, true},
 	})

 	checkOpMeta(t, []opMetaTestCase{
 		{"root", &RemountOp{
-			Target: MustAbs("/"),
+			Target: check.MustAbs("/"),
 			Flags:  syscall.MS_RDONLY,
 		}, "remounting", `"/" flags 0x1`},
 	})
--- a/container/initsymlink.go
+++ b/container/initsymlink.go
@@ -3,21 +3,22 @@ package container
 import (
 	"encoding/gob"
 	"fmt"
-	"io/fs"
 	"path"
+
+	"hakurei.app/container/check"
 )

 func init() { gob.Register(new(SymlinkOp)) }

 // Link appends an [Op] that creates a symlink in the container filesystem.
-func (f *Ops) Link(target *Absolute, linkName string, dereference bool) *Ops {
+func (f *Ops) Link(target *check.Absolute, linkName string, dereference bool) *Ops {
 	*f = append(*f, &SymlinkOp{target, linkName, dereference})
 	return f
 }

 // SymlinkOp optionally dereferences LinkName and creates a symlink at container path Target.
 type SymlinkOp struct {
-	Target *Absolute
+	Target *check.Absolute
 	// LinkName is an arbitrary uninterpreted pathname.
 	LinkName string

@@ -29,11 +30,11 @@ func (l *SymlinkOp) Valid() bool { return l != nil && l.Target != nil && l.LinkN

 func (l *SymlinkOp) early(_ *setupState, k syscallDispatcher) error {
 	if l.Dereference {
-		if !isAbs(l.LinkName) {
-			return msg.WrapErr(fs.ErrInvalid, fmt.Sprintf("path %q is not absolute", l.LinkName))
+		if !path.IsAbs(l.LinkName) {
+			return &check.AbsoluteError{Pathname: l.LinkName}
 		}
 		if name, err := k.readlink(l.LinkName); err != nil {
-			return wrapErrSelf(err)
+			return err
 		} else {
 			l.LinkName = name
 		}
@@ -44,9 +45,9 @@ func (l *SymlinkOp) early(_ *setupState, k syscallDispatcher) error {
 func (l *SymlinkOp) apply(state *setupState, k syscallDispatcher) error {
 	target := toSysroot(l.Target.String())
 	if err := k.mkdirAll(path.Dir(target), state.ParentPerm); err != nil {
-		return wrapErrSelf(err)
+		return err
 	}
-	return wrapErrSelf(k.symlink(l.LinkName, target))
+	return k.symlink(l.LinkName, target)
 }

 func (l *SymlinkOp) Is(op Op) bool {
@@ -56,7 +57,7 @@ func (l *SymlinkOp) Is(op Op) bool {
 		l.LinkName == vl.LinkName &&
 		l.Dereference == vl.Dereference
 }
-func (*SymlinkOp) prefix() string { return "creating" }
+func (*SymlinkOp) prefix() (string, bool) { return "creating", true }
 func (l *SymlinkOp) String() string {
 	return fmt.Sprintf("symlink on %q linkname %q", l.Target, l.LinkName)
 }
--- a/container/initsymlink_test.go
+++ b/container/initsymlink_test.go
@@ -1,51 +1,55 @@
 package container

 import (
-	"io/fs"
 	"os"
 	"testing"
+
+	"hakurei.app/container/check"
+	"hakurei.app/container/stub"
 )

 func TestSymlinkOp(t *testing.T) {
+	t.Parallel()
+
 	checkOpBehaviour(t, []opBehaviourTestCase{
 		{"mkdir", &Params{ParentPerm: 0700}, &SymlinkOp{
-			Target:   MustAbs("/etc/nixos"),
+			Target:   check.MustAbs("/etc/nixos"),
 			LinkName: "/etc/static/nixos",
-		}, nil, nil, []kexpect{
-			{"mkdirAll", expectArgs{"/sysroot/etc", os.FileMode(0700)}, nil, errUnique},
-		}, wrapErrSelf(errUnique)},
+		}, nil, nil, []stub.Call{
+			call("mkdirAll", stub.ExpectArgs{"/sysroot/etc", os.FileMode(0700)}, nil, stub.UniqueError(1)),
+		}, stub.UniqueError(1)},

 		{"abs", &Params{ParentPerm: 0755}, &SymlinkOp{
-			Target:      MustAbs("/etc/mtab"),
+			Target:      check.MustAbs("/etc/mtab"),
 			LinkName:    "etc/mtab",
 			Dereference: true,
-		}, nil, msg.WrapErr(fs.ErrInvalid, `path "etc/mtab" is not absolute`), nil, nil},
+		}, nil, &check.AbsoluteError{Pathname: "etc/mtab"}, nil, nil},

 		{"readlink", &Params{ParentPerm: 0755}, &SymlinkOp{
-			Target:      MustAbs("/etc/mtab"),
+			Target:      check.MustAbs("/etc/mtab"),
 			LinkName:    "/etc/mtab",
 			Dereference: true,
-		}, []kexpect{
-			{"readlink", expectArgs{"/etc/mtab"}, "/proc/mounts", errUnique},
-		}, wrapErrSelf(errUnique), nil, nil},
+		}, []stub.Call{
+			call("readlink", stub.ExpectArgs{"/etc/mtab"}, "/proc/mounts", stub.UniqueError(0)),
+		}, stub.UniqueError(0), nil, nil},

 		{"success noderef", &Params{ParentPerm: 0700}, &SymlinkOp{
-			Target:   MustAbs("/etc/nixos"),
+			Target:   check.MustAbs("/etc/nixos"),
 			LinkName: "/etc/static/nixos",
-		}, nil, nil, []kexpect{
-			{"mkdirAll", expectArgs{"/sysroot/etc", os.FileMode(0700)}, nil, nil},
-			{"symlink", expectArgs{"/etc/static/nixos", "/sysroot/etc/nixos"}, nil, nil},
+		}, nil, nil, []stub.Call{
+			call("mkdirAll", stub.ExpectArgs{"/sysroot/etc", os.FileMode(0700)}, nil, nil),
+			call("symlink", stub.ExpectArgs{"/etc/static/nixos", "/sysroot/etc/nixos"}, nil, nil),
 		}, nil},

 		{"success", &Params{ParentPerm: 0755}, &SymlinkOp{
-			Target:      MustAbs("/etc/mtab"),
+			Target:      check.MustAbs("/etc/mtab"),
 			LinkName:    "/etc/mtab",
 			Dereference: true,
-		}, []kexpect{
-			{"readlink", expectArgs{"/etc/mtab"}, "/proc/mounts", nil},
-		}, nil, []kexpect{
-			{"mkdirAll", expectArgs{"/sysroot/etc", os.FileMode(0755)}, nil, nil},
-			{"symlink", expectArgs{"/proc/mounts", "/sysroot/etc/mtab"}, nil, nil},
+		}, []stub.Call{
+			call("readlink", stub.ExpectArgs{"/etc/mtab"}, "/proc/mounts", nil),
+		}, nil, []stub.Call{
+			call("mkdirAll", stub.ExpectArgs{"/sysroot/etc", os.FileMode(0755)}, nil, nil),
+			call("symlink", stub.ExpectArgs{"/proc/mounts", "/sysroot/etc/mtab"}, nil, nil),
 		}, nil},
 	})

@@ -53,18 +57,18 @@ func TestSymlinkOp(t *testing.T) {
 		{"nil", (*SymlinkOp)(nil), false},
 		{"zero", new(SymlinkOp), false},
 		{"nil target", &SymlinkOp{LinkName: "/run/current-system"}, false},
-		{"zero linkname", &SymlinkOp{Target: MustAbs("/run/current-system")}, false},
-		{"valid", &SymlinkOp{Target: MustAbs("/run/current-system"), LinkName: "/run/current-system", Dereference: true}, true},
+		{"zero linkname", &SymlinkOp{Target: check.MustAbs("/run/current-system")}, false},
+		{"valid", &SymlinkOp{Target: check.MustAbs("/run/current-system"), LinkName: "/run/current-system", Dereference: true}, true},
 	})

 	checkOpsBuilder(t, []opsBuilderTestCase{
 		{"current-system", new(Ops).Link(
-			MustAbs("/run/current-system"),
+			check.MustAbs("/run/current-system"),
 			"/run/current-system",
 			true,
 		), Ops{
 			&SymlinkOp{
-				Target:      MustAbs("/run/current-system"),
+				Target:      check.MustAbs("/run/current-system"),
 				LinkName:    "/run/current-system",
 				Dereference: true,
 			},
@@ -75,40 +79,40 @@ func TestSymlinkOp(t *testing.T) {
 		{"zero", new(SymlinkOp), new(SymlinkOp), false},

 		{"target differs", &SymlinkOp{
-			Target:      MustAbs("/run/current-system/differs"),
+			Target:      check.MustAbs("/run/current-system/differs"),
 			LinkName:    "/run/current-system",
 			Dereference: true,
 		}, &SymlinkOp{
-			Target:      MustAbs("/run/current-system"),
+			Target:      check.MustAbs("/run/current-system"),
 			LinkName:    "/run/current-system",
 			Dereference: true,
 		}, false},

 		{"linkname differs", &SymlinkOp{
-			Target:      MustAbs("/run/current-system"),
+			Target:      check.MustAbs("/run/current-system"),
 			LinkName:    "/run/current-system/differs",
 			Dereference: true,
 		}, &SymlinkOp{
-			Target:      MustAbs("/run/current-system"),
+			Target:      check.MustAbs("/run/current-system"),
 			LinkName:    "/run/current-system",
 			Dereference: true,
 		}, false},

 		{"dereference differs", &SymlinkOp{
-			Target:   MustAbs("/run/current-system"),
+			Target:   check.MustAbs("/run/current-system"),
 			LinkName: "/run/current-system",
 		}, &SymlinkOp{
-			Target:      MustAbs("/run/current-system"),
+			Target:      check.MustAbs("/run/current-system"),
 			LinkName:    "/run/current-system",
 			Dereference: true,
 		}, false},

 		{"equals", &SymlinkOp{
-			Target:      MustAbs("/run/current-system"),
+			Target:      check.MustAbs("/run/current-system"),
 			LinkName:    "/run/current-system",
 			Dereference: true,
 		}, &SymlinkOp{
-			Target:      MustAbs("/run/current-system"),
+			Target:      check.MustAbs("/run/current-system"),
 			LinkName:    "/run/current-system",
 			Dereference: true,
 		}, true},
@@ -116,7 +120,7 @@ func TestSymlinkOp(t *testing.T) {

 	checkOpMeta(t, []opMetaTestCase{
 		{"current-system", &SymlinkOp{
-			Target:      MustAbs("/run/current-system"),
+			Target:      check.MustAbs("/run/current-system"),
 			LinkName:    "/run/current-system",
 			Dereference: true,
 		}, "creating", `symlink on "/run/current-system" linkname "/run/current-system"`},
--- a/container/inittmpfs.go
+++ b/container/inittmpfs.go
@@ -3,22 +3,30 @@ package container
 import (
 	"encoding/gob"
 	"fmt"
-	"io/fs"
 	"math"
 	"os"
+	"strconv"
 	. "syscall"
+
+	"hakurei.app/container/check"
 )

 func init() { gob.Register(new(MountTmpfsOp)) }

+type TmpfsSizeError int
+
+func (e TmpfsSizeError) Error() string {
+	return "tmpfs size " + strconv.Itoa(int(e)) + " out of bounds"
+}
+
 // Tmpfs appends an [Op] that mounts tmpfs on container path [MountTmpfsOp.Path].
-func (f *Ops) Tmpfs(target *Absolute, size int, perm os.FileMode) *Ops {
+func (f *Ops) Tmpfs(target *check.Absolute, size int, perm os.FileMode) *Ops {
 	*f = append(*f, &MountTmpfsOp{SourceTmpfsEphemeral, target, MS_NOSUID | MS_NODEV, size, perm})
 	return f
 }

 // Readonly appends an [Op] that mounts read-only tmpfs on container path [MountTmpfsOp.Path].
-func (f *Ops) Readonly(target *Absolute, perm os.FileMode) *Ops {
+func (f *Ops) Readonly(target *check.Absolute, perm os.FileMode) *Ops {
 	*f = append(*f, &MountTmpfsOp{SourceTmpfsReadonly, target, MS_RDONLY | MS_NOSUID | MS_NODEV, 0, perm})
 	return f
 }
@@ -26,7 +34,7 @@ func (f *Ops) Readonly(target *Absolute, perm os.FileMode) *Ops {
 // MountTmpfsOp mounts [FstypeTmpfs] on container Path.
 type MountTmpfsOp struct {
 	FSName string
-	Path   *Absolute
+	Path   *check.Absolute
 	Flags  uintptr
 	Size   int
 	Perm   os.FileMode
@@ -36,7 +44,7 @@ func (t *MountTmpfsOp) Valid() bool                                { return t !=
 func (t *MountTmpfsOp) early(*setupState, syscallDispatcher) error { return nil }
 func (t *MountTmpfsOp) apply(_ *setupState, k syscallDispatcher) error {
 	if t.Size < 0 || t.Size > math.MaxUint>>1 {
-		return msg.WrapErr(fs.ErrInvalid, fmt.Sprintf("size %d out of bounds", t.Size))
+		return TmpfsSizeError(t.Size)
 	}
 	return k.mountTmpfs(t.FSName, toSysroot(t.Path.String()), t.Flags, t.Size, t.Perm)
 }
@@ -50,5 +58,5 @@ func (t *MountTmpfsOp) Is(op Op) bool {
 		t.Size == vt.Size &&
 		t.Perm == vt.Perm
 }
-func (*MountTmpfsOp) prefix() string   { return "mounting" }
-func (t *MountTmpfsOp) String() string { return fmt.Sprintf("tmpfs on %q size %d", t.Path, t.Size) }
+func (*MountTmpfsOp) prefix() (string, bool) { return "mounting", true }
+func (t *MountTmpfsOp) String() string       { return fmt.Sprintf("tmpfs on %q size %d", t.Path, t.Size) }
--- a/container/inittmpfs_test.go
+++ b/container/inittmpfs_test.go
@@ -1,31 +1,44 @@
 package container

 import (
-	"io/fs"
 	"os"
 	"syscall"
 	"testing"
+
+	"hakurei.app/container/check"
+	"hakurei.app/container/stub"
 )

 func TestMountTmpfsOp(t *testing.T) {
+	t.Parallel()
+
+	t.Run("size error", func(t *testing.T) {
+		t.Parallel()
+		tmpfsSizeError := TmpfsSizeError(-1)
+		want := "tmpfs size -1 out of bounds"
+		if got := tmpfsSizeError.Error(); got != want {
+			t.Errorf("Error: %q, want %q", got, want)
+		}
+	})
+
 	checkOpBehaviour(t, []opBehaviourTestCase{
 		{"size oob", new(Params), &MountTmpfsOp{
 			Size: -1,
-		}, nil, nil, nil, msg.WrapErr(fs.ErrInvalid, "size -1 out of bounds")},
+		}, nil, nil, nil, TmpfsSizeError(-1)},

 		{"success", new(Params), &MountTmpfsOp{
 			FSName: "ephemeral",
-			Path:   MustAbs("/run/user/1000/"),
+			Path:   check.MustAbs("/run/user/1000/"),
 			Size:   1 << 10,
 			Perm:   0700,
-		}, nil, nil, []kexpect{
-			{"mountTmpfs", expectArgs{
+		}, nil, nil, []stub.Call{
+			call("mountTmpfs", stub.ExpectArgs{
 				"ephemeral",              // fsname
 				"/sysroot/run/user/1000", // target
 				uintptr(0),               // flags
 				0x400,                    // size
 				os.FileMode(0700),        // perm
-			}, nil, nil},
+			}, nil, nil),
 		}, nil},
 	})

@@ -33,19 +46,19 @@ func TestMountTmpfsOp(t *testing.T) {
 		{"nil", (*MountTmpfsOp)(nil), false},
 		{"zero", new(MountTmpfsOp), false},
 		{"nil path", &MountTmpfsOp{FSName: "tmpfs"}, false},
-		{"zero fsname", &MountTmpfsOp{Path: MustAbs("/tmp/")}, false},
-		{"valid", &MountTmpfsOp{FSName: "tmpfs", Path: MustAbs("/tmp/")}, true},
+		{"zero fsname", &MountTmpfsOp{Path: check.MustAbs("/tmp/")}, false},
+		{"valid", &MountTmpfsOp{FSName: "tmpfs", Path: check.MustAbs("/tmp/")}, true},
 	})

 	checkOpsBuilder(t, []opsBuilderTestCase{
 		{"runtime", new(Ops).Tmpfs(
-			MustAbs("/run/user"),
+			check.MustAbs("/run/user"),
 			1<<10,
 			0755,
 		), Ops{
 			&MountTmpfsOp{
 				FSName: "ephemeral",
-				Path:   MustAbs("/run/user"),
+				Path:   check.MustAbs("/run/user"),
 				Flags:  syscall.MS_NOSUID | syscall.MS_NODEV,
 				Size:   1 << 10,
 				Perm:   0755,
@@ -53,12 +66,12 @@ func TestMountTmpfsOp(t *testing.T) {
 		}},

 		{"nscd", new(Ops).Readonly(
-			MustAbs("/var/run/nscd"),
+			check.MustAbs("/var/run/nscd"),
 			0755,
 		), Ops{
 			&MountTmpfsOp{
 				FSName: "readonly",
-				Path:   MustAbs("/var/run/nscd"),
+				Path:   check.MustAbs("/var/run/nscd"),
 				Flags:  syscall.MS_NOSUID | syscall.MS_NODEV | syscall.MS_RDONLY,
 				Perm:   0755,
 			},
@@ -70,13 +83,13 @@ func TestMountTmpfsOp(t *testing.T) {

 		{"fsname differs", &MountTmpfsOp{
 			FSName: "readonly",
-			Path:   MustAbs("/run/user"),
+			Path:   check.MustAbs("/run/user"),
 			Flags:  syscall.MS_NOSUID | syscall.MS_NODEV,
 			Size:   1 << 10,
 			Perm:   0755,
 		}, &MountTmpfsOp{
 			FSName: "ephemeral",
-			Path:   MustAbs("/run/user"),
+			Path:   check.MustAbs("/run/user"),
 			Flags:  syscall.MS_NOSUID | syscall.MS_NODEV,
 			Size:   1 << 10,
 			Perm:   0755,
@@ -84,13 +97,13 @@ func TestMountTmpfsOp(t *testing.T) {

 		{"path differs", &MountTmpfsOp{
 			FSName: "ephemeral",
-			Path:   MustAbs("/run/user/differs"),
+			Path:   check.MustAbs("/run/user/differs"),
 			Flags:  syscall.MS_NOSUID | syscall.MS_NODEV,
 			Size:   1 << 10,
 			Perm:   0755,
 		}, &MountTmpfsOp{
 			FSName: "ephemeral",
-			Path:   MustAbs("/run/user"),
+			Path:   check.MustAbs("/run/user"),
 			Flags:  syscall.MS_NOSUID | syscall.MS_NODEV,
 			Size:   1 << 10,
 			Perm:   0755,
@@ -98,13 +111,13 @@ func TestMountTmpfsOp(t *testing.T) {

 		{"flags differs", &MountTmpfsOp{
 			FSName: "ephemeral",
-			Path:   MustAbs("/run/user"),
+			Path:   check.MustAbs("/run/user"),
 			Flags:  syscall.MS_NOSUID | syscall.MS_NODEV | syscall.MS_RDONLY,
 			Size:   1 << 10,
 			Perm:   0755,
 		}, &MountTmpfsOp{
 			FSName: "ephemeral",
-			Path:   MustAbs("/run/user"),
+			Path:   check.MustAbs("/run/user"),
 			Flags:  syscall.MS_NOSUID | syscall.MS_NODEV,
 			Size:   1 << 10,
 			Perm:   0755,
@@ -112,13 +125,13 @@ func TestMountTmpfsOp(t *testing.T) {

 		{"size differs", &MountTmpfsOp{
 			FSName: "ephemeral",
-			Path:   MustAbs("/run/user"),
+			Path:   check.MustAbs("/run/user"),
 			Flags:  syscall.MS_NOSUID | syscall.MS_NODEV,
 			Size:   1,
 			Perm:   0755,
 		}, &MountTmpfsOp{
 			FSName: "ephemeral",
-			Path:   MustAbs("/run/user"),
+			Path:   check.MustAbs("/run/user"),
 			Flags:  syscall.MS_NOSUID | syscall.MS_NODEV,
 			Size:   1 << 10,
 			Perm:   0755,
@@ -126,13 +139,13 @@ func TestMountTmpfsOp(t *testing.T) {

 		{"perm differs", &MountTmpfsOp{
 			FSName: "ephemeral",
-			Path:   MustAbs("/run/user"),
+			Path:   check.MustAbs("/run/user"),
 			Flags:  syscall.MS_NOSUID | syscall.MS_NODEV,
 			Size:   1 << 10,
 			Perm:   0700,
 		}, &MountTmpfsOp{
 			FSName: "ephemeral",
-			Path:   MustAbs("/run/user"),
+			Path:   check.MustAbs("/run/user"),
 			Flags:  syscall.MS_NOSUID | syscall.MS_NODEV,
 			Size:   1 << 10,
 			Perm:   0755,
@@ -140,13 +153,13 @@ func TestMountTmpfsOp(t *testing.T) {

 		{"equals", &MountTmpfsOp{
 			FSName: "ephemeral",
-			Path:   MustAbs("/run/user"),
+			Path:   check.MustAbs("/run/user"),
 			Flags:  syscall.MS_NOSUID | syscall.MS_NODEV,
 			Size:   1 << 10,
 			Perm:   0755,
 		}, &MountTmpfsOp{
 			FSName: "ephemeral",
-			Path:   MustAbs("/run/user"),
+			Path:   check.MustAbs("/run/user"),
 			Flags:  syscall.MS_NOSUID | syscall.MS_NODEV,
 			Size:   1 << 10,
 			Perm:   0755,
@@ -156,7 +169,7 @@ func TestMountTmpfsOp(t *testing.T) {
 	checkOpMeta(t, []opMetaTestCase{
 		{"runtime", &MountTmpfsOp{
 			FSName: "ephemeral",
-			Path:   MustAbs("/run/user"),
+			Path:   check.MustAbs("/run/user"),
 			Flags:  syscall.MS_NOSUID | syscall.MS_NODEV,
 			Size:   1 << 10,
 			Perm:   0755,
--- a/container/landlock_test.go
+++ b/container/landlock_test.go
@@ -8,6 +8,8 @@ import (
 )

 func TestLandlockString(t *testing.T) {
+	t.Parallel()
+
 	testCases := []struct {
 		name        string
 		rulesetAttr *container.RulesetAttr
@@ -46,6 +48,7 @@ func TestLandlockString(t *testing.T) {
 	}
 	for _, tc := range testCases {
 		t.Run(tc.name, func(t *testing.T) {
+			t.Parallel()
 			if got := tc.rulesetAttr.String(); got != tc.want {
 				t.Errorf("String: %s, want %s", got, tc.want)
 			}
@@ -54,6 +57,7 @@ func TestLandlockString(t *testing.T) {
 }

 func TestLandlockAttrSize(t *testing.T) {
+	t.Parallel()
 	want := 24
 	if got := unsafe.Sizeof(container.RulesetAttr{}); got != uintptr(want) {
 		t.Errorf("Sizeof: %d, want %d", got, want)
--- a/container/mount.go
+++ b/container/mount.go
@@ -4,10 +4,10 @@ import (
 	"errors"
 	"fmt"
 	"os"
-	"strings"
 	. "syscall"

 	"hakurei.app/container/vfs"
+	"hakurei.app/message"
 )

 /*
@@ -43,6 +43,8 @@ const (
 	// Note that any source value is allowed when fstype is [FstypeOverlay].
 	SourceOverlay = "overlay"

+	// SourceTmpfs is used when mounting tmpfs.
+	SourceTmpfs = "tmpfs"
 	// SourceTmpfsRootfs is used when mounting the tmpfs instance backing the intermediate root.
 	SourceTmpfsRootfs = "rootfs"
 	// SourceTmpfsDevtmpfs is used when mounting tmpfs representing a subset of host devtmpfs.
@@ -57,7 +59,6 @@ const (
 	FstypeNULL = zeroString
 	// FstypeProc represents the proc pseudo-filesystem.
 	// A fully visible instance of proc must be available in the mount namespace for proc to be mounted.
-	// This filesystem type is usually mounted on [FHSProc].
 	FstypeProc = "proc"
 	// FstypeDevpts represents the devpts pseudo-filesystem.
 	// This type of filesystem is usually mounted on /dev/pts.
@@ -84,44 +85,29 @@ const (
 	// OptionOverlayUserxattr represents the userxattr option of the overlay pseudo-filesystem.
 	// Use the "user.overlay." xattr namespace instead of "trusted.overlay.".
 	OptionOverlayUserxattr = "userxattr"
-
-	// SpecialOverlayEscape is the escape string for overlay mount options.
-	SpecialOverlayEscape = `\`
-	// SpecialOverlayOption is the separator string between overlay mount options.
-	SpecialOverlayOption = ","
-	// SpecialOverlayPath is the separator string between overlay paths.
-	SpecialOverlayPath = ":"
 )

 // bindMount mounts source on target and recursively applies flags if MS_REC is set.
-func (p *procPaths) bindMount(source, target string, flags uintptr, eq bool) error {
+func (p *procPaths) bindMount(msg message.Msg, source, target string, flags uintptr) error {
 	// syscallDispatcher.bindMount and procPaths.remount must not be called from this function

-	if eq {
-		p.k.verbosef("resolved %q flags %#x", target, flags)
-	} else {
-		p.k.verbosef("resolved %q on %q flags %#x", source, target, flags)
-	}
-
 	if err := p.k.mount(source, target, FstypeNULL, MS_SILENT|MS_BIND|flags&MS_REC, zeroString); err != nil {
-		return wrapErrSuffix(err,
-			fmt.Sprintf("cannot mount %q on %q:", source, target))
+		return err
 	}
-
-	return p.k.remount(target, flags)
+	return p.k.remount(msg, target, flags)
 }

 // remount applies flags on target, recursively if MS_REC is set.
-func (p *procPaths) remount(target string, flags uintptr) error {
+func (p *procPaths) remount(msg message.Msg, target string, flags uintptr) error {
 	// syscallDispatcher methods bindMount, remount must not be called from this function

 	var targetFinal string
 	if v, err := p.k.evalSymlinks(target); err != nil {
-		return wrapErrSelf(err)
+		return err
 	} else {
 		targetFinal = v
 		if targetFinal != target {
-			p.k.verbosef("target resolves to %q", targetFinal)
+			msg.Verbosef("target resolves to %q", targetFinal)
 		}
 	}

@@ -133,14 +119,12 @@ func (p *procPaths) remount(target string, flags uintptr) error {
 			destFd, err = p.k.open(targetFinal, O_PATH|O_CLOEXEC, 0)
 			return
 		}); err != nil {
-			return wrapErrSuffix(err,
-				fmt.Sprintf("cannot open %q:", targetFinal))
+			return &os.PathError{Op: "open", Path: targetFinal, Err: err}
 		}
 		if v, err := p.k.readlink(p.fd(destFd)); err != nil {
-			return wrapErrSelf(err)
+			return err
 		} else if err = p.k.close(destFd); err != nil {
-			return wrapErrSuffix(err,
-				fmt.Sprintf("cannot close %q:", targetFinal))
+			return &os.PathError{Op: "close", Path: targetFinal, Err: err}
 		} else {
 			targetKFinal = v
 		}
@@ -150,17 +134,11 @@ func (p *procPaths) remount(target string, flags uintptr) error {
 	return p.mountinfo(func(d *vfs.MountInfoDecoder) error {
 		n, err := d.Unfold(targetKFinal)
 		if err != nil {
-			if errors.Is(err, ESTALE) {
-				return msg.WrapErr(err,
-					fmt.Sprintf("mount point %q never appeared in mountinfo", targetKFinal))
-			}
-			return wrapErrSuffix(err,
-				"cannot unfold mount hierarchy:")
+			return err
 		}

-		if err = remountWithFlags(p.k, n, mf); err != nil {
-			return wrapErrSuffix(err,
-				fmt.Sprintf("cannot remount %q:", n.Clean))
+		if err = remountWithFlags(p.k, msg, n, mf); err != nil {
+			return err
 		}
 		if flags&MS_REC == 0 {
 			return nil
@@ -172,11 +150,8 @@ func (p *procPaths) remount(target string, flags uintptr) error {
 				continue
 			}

-			err = remountWithFlags(p.k, cur, mf)
-
-			if err != nil && !errors.Is(err, EACCES) {
-				return wrapErrSuffix(err,
-					fmt.Sprintf("cannot propagate flags to %q:", cur.Clean))
+			if err = remountWithFlags(p.k, msg, cur, mf); err != nil && !errors.Is(err, EACCES) {
+				return err
 			}
 		}

@@ -185,12 +160,12 @@ func (p *procPaths) remount(target string, flags uintptr) error {
 }

 // remountWithFlags remounts mount point described by [vfs.MountInfoNode].
-func remountWithFlags(k syscallDispatcher, n *vfs.MountInfoNode, mf uintptr) error {
+func remountWithFlags(k syscallDispatcher, msg message.Msg, n *vfs.MountInfoNode, mf uintptr) error {
 	// syscallDispatcher methods bindMount, remount must not be called from this function

 	kf, unmatched := n.Flags()
 	if len(unmatched) != 0 {
-		k.verbosef("unmatched vfs options: %q", unmatched)
+		msg.Verbosef("unmatched vfs options: %q", unmatched)
 	}

 	if kf&mf != mf {
@@ -205,15 +180,13 @@ func mountTmpfs(k syscallDispatcher, fsname, target string, flags uintptr, size
 	// syscallDispatcher.mountTmpfs must not be called from this function

 	if err := k.mkdirAll(target, parentPerm(perm)); err != nil {
-		return wrapErrSelf(err)
+		return err
 	}
 	opt := fmt.Sprintf("mode=%#o", perm)
 	if size > 0 {
 		opt += fmt.Sprintf(",size=%d", size)
 	}
-	return wrapErrSuffix(
-		k.mount(fsname, target, FstypeTmpfs, flags, opt),
-		fmt.Sprintf("cannot mount tmpfs on %q:", target))
+	return k.mount(fsname, target, FstypeTmpfs, flags, opt)
 }

 func parentPerm(perm os.FileMode) os.FileMode {
@@ -226,20 +199,3 @@ func parentPerm(perm os.FileMode) os.FileMode {
 	}
 	return os.FileMode(pperm)
 }
-
-// EscapeOverlayDataSegment escapes a string for formatting into the data argument of an overlay mount call.
-func EscapeOverlayDataSegment(s string) string {
-	if s == zeroString {
-		return zeroString
-	}
-
-	if f := strings.SplitN(s, "\x00", 2); len(f) > 0 {
-		s = f[0]
-	}
-
-	return strings.NewReplacer(
-		SpecialOverlayEscape, SpecialOverlayEscape+SpecialOverlayEscape,
-		SpecialOverlayOption, SpecialOverlayEscape+SpecialOverlayOption,
-		SpecialOverlayPath, SpecialOverlayEscape+SpecialOverlayPath,
-	).Replace(s)
-}
--- a/container/mount_test.go
+++ b/container/mount_test.go
@@ -5,37 +5,39 @@ import (
 	"syscall"
 	"testing"

+	"hakurei.app/container/stub"
 	"hakurei.app/container/vfs"
 )

 func TestBindMount(t *testing.T) {
-	checkSimple(t, "bindMount", []simpleTestCase{
-		{"mount", func(k syscallDispatcher) error {
-			return newProcPaths(k, hostPath).bindMount("/host/nix", "/sysroot/nix", syscall.MS_RDONLY, true)
-		}, [][]kexpect{{
-			{"verbosef", expectArgs{"resolved %q flags %#x", []any{"/sysroot/nix", uintptr(1)}}, nil, nil},
-			{"mount", expectArgs{"/host/nix", "/sysroot/nix", "", uintptr(0x9000), ""}, nil, errUnique},
-		}}, wrapErrSuffix(errUnique, `cannot mount "/host/nix" on "/sysroot/nix":`)},
+	t.Parallel()

-		{"success ne", func(k syscallDispatcher) error {
-			return newProcPaths(k, hostPath).bindMount("/host/nix", "/sysroot/.host-nix", syscall.MS_RDONLY, false)
-		}, [][]kexpect{{
-			{"verbosef", expectArgs{"resolved %q on %q flags %#x", []any{"/host/nix", "/sysroot/.host-nix", uintptr(1)}}, nil, nil},
-			{"mount", expectArgs{"/host/nix", "/sysroot/.host-nix", "", uintptr(0x9000), ""}, nil, nil},
-			{"remount", expectArgs{"/sysroot/.host-nix", uintptr(1)}, nil, nil},
+	checkSimple(t, "bindMount", []simpleTestCase{
+		{"mount", func(k *kstub) error {
+			return newProcPaths(k, hostPath).bindMount(nil, "/host/nix", "/sysroot/nix", syscall.MS_RDONLY)
+		}, stub.Expect{Calls: []stub.Call{
+			call("mount", stub.ExpectArgs{"/host/nix", "/sysroot/nix", "", uintptr(0x9000), ""}, nil, stub.UniqueError(0xbad)),
+		}}, stub.UniqueError(0xbad)},
+
+		{"success ne", func(k *kstub) error {
+			return newProcPaths(k, hostPath).bindMount(k, "/host/nix", "/sysroot/.host-nix", syscall.MS_RDONLY)
+		}, stub.Expect{Calls: []stub.Call{
+			call("mount", stub.ExpectArgs{"/host/nix", "/sysroot/.host-nix", "", uintptr(0x9000), ""}, nil, nil),
+			call("remount", stub.ExpectArgs{"/sysroot/.host-nix", uintptr(1)}, nil, nil),
 		}}, nil},

-		{"success", func(k syscallDispatcher) error {
-			return newProcPaths(k, hostPath).bindMount("/host/nix", "/sysroot/nix", syscall.MS_RDONLY, true)
-		}, [][]kexpect{{
-			{"verbosef", expectArgs{"resolved %q flags %#x", []any{"/sysroot/nix", uintptr(1)}}, nil, nil},
-			{"mount", expectArgs{"/host/nix", "/sysroot/nix", "", uintptr(0x9000), ""}, nil, nil},
-			{"remount", expectArgs{"/sysroot/nix", uintptr(1)}, nil, nil},
+		{"success", func(k *kstub) error {
+			return newProcPaths(k, hostPath).bindMount(k, "/host/nix", "/sysroot/nix", syscall.MS_RDONLY)
+		}, stub.Expect{Calls: []stub.Call{
+			call("mount", stub.ExpectArgs{"/host/nix", "/sysroot/nix", "", uintptr(0x9000), ""}, nil, nil),
+			call("remount", stub.ExpectArgs{"/sysroot/nix", uintptr(1)}, nil, nil),
 		}}, nil},
 	})
 }

 func TestRemount(t *testing.T) {
+	t.Parallel()
+
 	const sampleMountinfoNix = `254 407 253:0 / /host rw,relatime master:1 - ext4 /dev/disk/by-label/nixos rw
 255 254 0:28 / /host/mnt/.ro-cwd ro,noatime master:2 - 9p cwd ro,access=client,msize=16384,trans=virtio
 256 254 0:29 / /host/nix/.ro-store rw,relatime master:3 - 9p nix-store rw,cache=f,access=client,msize=16384,trans=virtio
@@ -79,189 +81,195 @@ func TestRemount(t *testing.T) {
 416 415 0:30 / /sysroot/nix/store ro,relatime master:5 - overlay overlay rw,lowerdir=/mnt-root/nix/.ro-store,upperdir=/mnt-root/nix/.rw-store/upper,workdir=/mnt-root/nix/.rw-store/work`

 	checkSimple(t, "remount", []simpleTestCase{
-		{"evalSymlinks", func(k syscallDispatcher) error {
-			return newProcPaths(k, hostPath).remount("/sysroot/nix", syscall.MS_REC|syscall.MS_RDONLY|syscall.MS_NODEV)
-		}, [][]kexpect{{
-			{"evalSymlinks", expectArgs{"/sysroot/nix"}, "/sysroot/nix", errUnique},
-		}}, wrapErrSelf(errUnique)},
+		{"evalSymlinks", func(k *kstub) error {
+			return newProcPaths(k, hostPath).remount(nil, "/sysroot/nix", syscall.MS_REC|syscall.MS_RDONLY|syscall.MS_NODEV)
+		}, stub.Expect{Calls: []stub.Call{
+			call("evalSymlinks", stub.ExpectArgs{"/sysroot/nix"}, "/sysroot/nix", stub.UniqueError(6)),
+		}}, stub.UniqueError(6)},

-		{"open", func(k syscallDispatcher) error {
-			return newProcPaths(k, hostPath).remount("/sysroot/nix", syscall.MS_REC|syscall.MS_RDONLY|syscall.MS_NODEV)
-		}, [][]kexpect{{
-			{"evalSymlinks", expectArgs{"/sysroot/nix"}, "/sysroot/nix", nil},
-			{"open", expectArgs{"/sysroot/nix", 0x280000, uint32(0)}, 0xdeadbeef, errUnique},
-		}}, wrapErrSuffix(errUnique, `cannot open "/sysroot/nix":`)},
+		{"open", func(k *kstub) error {
+			return newProcPaths(k, hostPath).remount(nil, "/sysroot/nix", syscall.MS_REC|syscall.MS_RDONLY|syscall.MS_NODEV)
+		}, stub.Expect{Calls: []stub.Call{
+			call("evalSymlinks", stub.ExpectArgs{"/sysroot/nix"}, "/sysroot/nix", nil),
+			call("open", stub.ExpectArgs{"/sysroot/nix", 0x280000, uint32(0)}, 0xdeadbeef, stub.UniqueError(5)),
+		}}, &os.PathError{Op: "open", Path: "/sysroot/nix", Err: stub.UniqueError(5)}},

-		{"readlink", func(k syscallDispatcher) error {
-			return newProcPaths(k, hostPath).remount("/sysroot/nix", syscall.MS_REC|syscall.MS_RDONLY|syscall.MS_NODEV)
-		}, [][]kexpect{{
-			{"evalSymlinks", expectArgs{"/sysroot/nix"}, "/sysroot/nix", nil},
-			{"open", expectArgs{"/sysroot/nix", 0x280000, uint32(0)}, 0xdeadbeef, nil},
-			{"readlink", expectArgs{"/host/proc/self/fd/3735928559"}, "/sysroot/nix", errUnique},
-		}}, wrapErrSelf(errUnique)},
+		{"readlink", func(k *kstub) error {
+			return newProcPaths(k, hostPath).remount(nil, "/sysroot/nix", syscall.MS_REC|syscall.MS_RDONLY|syscall.MS_NODEV)
+		}, stub.Expect{Calls: []stub.Call{
+			call("evalSymlinks", stub.ExpectArgs{"/sysroot/nix"}, "/sysroot/nix", nil),
+			call("open", stub.ExpectArgs{"/sysroot/nix", 0x280000, uint32(0)}, 0xdeadbeef, nil),
+			call("readlink", stub.ExpectArgs{"/host/proc/self/fd/3735928559"}, "/sysroot/nix", stub.UniqueError(4)),
+		}}, stub.UniqueError(4)},

-		{"close", func(k syscallDispatcher) error {
-			return newProcPaths(k, hostPath).remount("/sysroot/nix", syscall.MS_REC|syscall.MS_RDONLY|syscall.MS_NODEV)
-		}, [][]kexpect{{
-			{"evalSymlinks", expectArgs{"/sysroot/nix"}, "/sysroot/nix", nil},
-			{"open", expectArgs{"/sysroot/nix", 0x280000, uint32(0)}, 0xdeadbeef, nil},
-			{"readlink", expectArgs{"/host/proc/self/fd/3735928559"}, "/sysroot/nix", nil},
-			{"close", expectArgs{0xdeadbeef}, nil, errUnique},
-		}}, wrapErrSuffix(errUnique, `cannot close "/sysroot/nix":`)},
+		{"close", func(k *kstub) error {
+			return newProcPaths(k, hostPath).remount(nil, "/sysroot/nix", syscall.MS_REC|syscall.MS_RDONLY|syscall.MS_NODEV)
+		}, stub.Expect{Calls: []stub.Call{
+			call("evalSymlinks", stub.ExpectArgs{"/sysroot/nix"}, "/sysroot/nix", nil),
+			call("open", stub.ExpectArgs{"/sysroot/nix", 0x280000, uint32(0)}, 0xdeadbeef, nil),
+			call("readlink", stub.ExpectArgs{"/host/proc/self/fd/3735928559"}, "/sysroot/nix", nil),
+			call("close", stub.ExpectArgs{0xdeadbeef}, nil, stub.UniqueError(3)),
+		}}, &os.PathError{Op: "close", Path: "/sysroot/nix", Err: stub.UniqueError(3)}},

-		{"mountinfo stale", func(k syscallDispatcher) error {
-			return newProcPaths(k, hostPath).remount("/sysroot/nix", syscall.MS_REC|syscall.MS_RDONLY|syscall.MS_NODEV)
-		}, [][]kexpect{{
-			{"evalSymlinks", expectArgs{"/sysroot/nix"}, "/sysroot/.hakurei", nil},
-			{"verbosef", expectArgs{"target resolves to %q", []any{"/sysroot/.hakurei"}}, nil, nil},
-			{"open", expectArgs{"/sysroot/.hakurei", 0x280000, uint32(0)}, 0xdeadbeef, nil},
-			{"readlink", expectArgs{"/host/proc/self/fd/3735928559"}, "/sysroot/.hakurei", nil},
-			{"close", expectArgs{0xdeadbeef}, nil, nil},
-			{"openNew", expectArgs{"/host/proc/self/mountinfo"}, newConstFile(sampleMountinfoNix), nil},
-		}}, msg.WrapErr(syscall.ESTALE, `mount point "/sysroot/.hakurei" never appeared in mountinfo`)},
+		{"mountinfo no match", func(k *kstub) error {
+			return newProcPaths(k, hostPath).remount(k, "/sysroot/nix", syscall.MS_REC|syscall.MS_RDONLY|syscall.MS_NODEV)
+		}, stub.Expect{Calls: []stub.Call{
+			call("evalSymlinks", stub.ExpectArgs{"/sysroot/nix"}, "/sysroot/.hakurei", nil),
+			call("verbosef", stub.ExpectArgs{"target resolves to %q", []any{"/sysroot/.hakurei"}}, nil, nil),
+			call("open", stub.ExpectArgs{"/sysroot/.hakurei", 0x280000, uint32(0)}, 0xdeadbeef, nil),
+			call("readlink", stub.ExpectArgs{"/host/proc/self/fd/3735928559"}, "/sysroot/.hakurei", nil),
+			call("close", stub.ExpectArgs{0xdeadbeef}, nil, nil),
+			call("openNew", stub.ExpectArgs{"/host/proc/self/mountinfo"}, newConstFile(sampleMountinfoNix), nil),
+		}}, &vfs.DecoderError{Op: "unfold", Line: -1, Err: vfs.UnfoldTargetError("/sysroot/.hakurei")}},

-		{"mountinfo", func(k syscallDispatcher) error {
-			return newProcPaths(k, hostPath).remount("/sysroot/nix", syscall.MS_REC|syscall.MS_RDONLY|syscall.MS_NODEV)
-		}, [][]kexpect{{
-			{"evalSymlinks", expectArgs{"/sysroot/nix"}, "/sysroot/nix", nil},
-			{"open", expectArgs{"/sysroot/nix", 0x280000, uint32(0)}, 0xdeadbeef, nil},
-			{"readlink", expectArgs{"/host/proc/self/fd/3735928559"}, "/sysroot/nix", nil},
-			{"close", expectArgs{0xdeadbeef}, nil, nil},
-			{"openNew", expectArgs{"/host/proc/self/mountinfo"}, newConstFile("\x00"), nil},
-		}}, wrapErrSuffix(vfs.ErrMountInfoFields, `cannot parse mountinfo:`)},
+		{"mountinfo", func(k *kstub) error {
+			return newProcPaths(k, hostPath).remount(nil, "/sysroot/nix", syscall.MS_REC|syscall.MS_RDONLY|syscall.MS_NODEV)
+		}, stub.Expect{Calls: []stub.Call{
+			call("evalSymlinks", stub.ExpectArgs{"/sysroot/nix"}, "/sysroot/nix", nil),
+			call("open", stub.ExpectArgs{"/sysroot/nix", 0x280000, uint32(0)}, 0xdeadbeef, nil),
+			call("readlink", stub.ExpectArgs{"/host/proc/self/fd/3735928559"}, "/sysroot/nix", nil),
+			call("close", stub.ExpectArgs{0xdeadbeef}, nil, nil),
+			call("openNew", stub.ExpectArgs{"/host/proc/self/mountinfo"}, newConstFile("\x00"), nil),
+		}}, &vfs.DecoderError{Op: "parse", Line: 0, Err: vfs.ErrMountInfoFields}},

-		{"mount", func(k syscallDispatcher) error {
-			return newProcPaths(k, hostPath).remount("/sysroot/nix", syscall.MS_REC|syscall.MS_RDONLY|syscall.MS_NODEV)
-		}, [][]kexpect{{
-			{"evalSymlinks", expectArgs{"/sysroot/nix"}, "/sysroot/nix", nil},
-			{"open", expectArgs{"/sysroot/nix", 0x280000, uint32(0)}, 0xdeadbeef, nil},
-			{"readlink", expectArgs{"/host/proc/self/fd/3735928559"}, "/sysroot/nix", nil},
-			{"close", expectArgs{0xdeadbeef}, nil, nil},
-			{"openNew", expectArgs{"/host/proc/self/mountinfo"}, newConstFile(sampleMountinfoNix), nil},
-			{"mount", expectArgs{"none", "/sysroot/nix", "", uintptr(0x209027), ""}, nil, errUnique},
-		}}, wrapErrSuffix(errUnique, `cannot remount "/sysroot/nix":`)},
+		{"mount", func(k *kstub) error {
+			return newProcPaths(k, hostPath).remount(nil, "/sysroot/nix", syscall.MS_REC|syscall.MS_RDONLY|syscall.MS_NODEV)
+		}, stub.Expect{Calls: []stub.Call{
+			call("evalSymlinks", stub.ExpectArgs{"/sysroot/nix"}, "/sysroot/nix", nil),
+			call("open", stub.ExpectArgs{"/sysroot/nix", 0x280000, uint32(0)}, 0xdeadbeef, nil),
+			call("readlink", stub.ExpectArgs{"/host/proc/self/fd/3735928559"}, "/sysroot/nix", nil),
+			call("close", stub.ExpectArgs{0xdeadbeef}, nil, nil),
+			call("openNew", stub.ExpectArgs{"/host/proc/self/mountinfo"}, newConstFile(sampleMountinfoNix), nil),
+			call("mount", stub.ExpectArgs{"none", "/sysroot/nix", "", uintptr(0x209027), ""}, nil, stub.UniqueError(2)),
+		}}, stub.UniqueError(2)},

-		{"mount propagate", func(k syscallDispatcher) error {
-			return newProcPaths(k, hostPath).remount("/sysroot/nix", syscall.MS_REC|syscall.MS_RDONLY|syscall.MS_NODEV)
-		}, [][]kexpect{{
-			{"evalSymlinks", expectArgs{"/sysroot/nix"}, "/sysroot/nix", nil},
-			{"open", expectArgs{"/sysroot/nix", 0x280000, uint32(0)}, 0xdeadbeef, nil},
-			{"readlink", expectArgs{"/host/proc/self/fd/3735928559"}, "/sysroot/nix", nil},
-			{"close", expectArgs{0xdeadbeef}, nil, nil},
-			{"openNew", expectArgs{"/host/proc/self/mountinfo"}, newConstFile(sampleMountinfoNix), nil},
-			{"mount", expectArgs{"none", "/sysroot/nix", "", uintptr(0x209027), ""}, nil, nil},
-			{"mount", expectArgs{"none", "/sysroot/nix/.ro-store", "", uintptr(0x209027), ""}, nil, errUnique},
-		}}, wrapErrSuffix(errUnique, `cannot propagate flags to "/sysroot/nix/.ro-store":`)},
+		{"mount propagate", func(k *kstub) error {
+			return newProcPaths(k, hostPath).remount(nil, "/sysroot/nix", syscall.MS_REC|syscall.MS_RDONLY|syscall.MS_NODEV)
+		}, stub.Expect{Calls: []stub.Call{
+			call("evalSymlinks", stub.ExpectArgs{"/sysroot/nix"}, "/sysroot/nix", nil),
+			call("open", stub.ExpectArgs{"/sysroot/nix", 0x280000, uint32(0)}, 0xdeadbeef, nil),
+			call("readlink", stub.ExpectArgs{"/host/proc/self/fd/3735928559"}, "/sysroot/nix", nil),
+			call("close", stub.ExpectArgs{0xdeadbeef}, nil, nil),
+			call("openNew", stub.ExpectArgs{"/host/proc/self/mountinfo"}, newConstFile(sampleMountinfoNix), nil),
+			call("mount", stub.ExpectArgs{"none", "/sysroot/nix", "", uintptr(0x209027), ""}, nil, nil),
+			call("mount", stub.ExpectArgs{"none", "/sysroot/nix/.ro-store", "", uintptr(0x209027), ""}, nil, stub.UniqueError(1)),
+		}}, stub.UniqueError(1)},

-		{"success toplevel", func(k syscallDispatcher) error {
-			return newProcPaths(k, hostPath).remount("/sysroot/bin", syscall.MS_REC|syscall.MS_RDONLY|syscall.MS_NODEV)
-		}, [][]kexpect{{
-			{"evalSymlinks", expectArgs{"/sysroot/bin"}, "/sysroot/bin", nil},
-			{"open", expectArgs{"/sysroot/bin", 0x280000, uint32(0)}, 0xbabe, nil},
-			{"readlink", expectArgs{"/host/proc/self/fd/47806"}, "/sysroot/bin", nil},
-			{"close", expectArgs{0xbabe}, nil, nil},
-			{"openNew", expectArgs{"/host/proc/self/mountinfo"}, newConstFile(sampleMountinfoNix), nil},
-			{"mount", expectArgs{"none", "/sysroot/bin", "", uintptr(0x209027), ""}, nil, nil},
+		{"success toplevel", func(k *kstub) error {
+			return newProcPaths(k, hostPath).remount(nil, "/sysroot/bin", syscall.MS_REC|syscall.MS_RDONLY|syscall.MS_NODEV)
+		}, stub.Expect{Calls: []stub.Call{
+			call("evalSymlinks", stub.ExpectArgs{"/sysroot/bin"}, "/sysroot/bin", nil),
+			call("open", stub.ExpectArgs{"/sysroot/bin", 0x280000, uint32(0)}, 0xbabe, nil),
+			call("readlink", stub.ExpectArgs{"/host/proc/self/fd/47806"}, "/sysroot/bin", nil),
+			call("close", stub.ExpectArgs{0xbabe}, nil, nil),
+			call("openNew", stub.ExpectArgs{"/host/proc/self/mountinfo"}, newConstFile(sampleMountinfoNix), nil),
+			call("mount", stub.ExpectArgs{"none", "/sysroot/bin", "", uintptr(0x209027), ""}, nil, nil),
 		}}, nil},

-		{"success EACCES", func(k syscallDispatcher) error {
-			return newProcPaths(k, hostPath).remount("/sysroot/nix", syscall.MS_REC|syscall.MS_RDONLY|syscall.MS_NODEV)
-		}, [][]kexpect{{
-			{"evalSymlinks", expectArgs{"/sysroot/nix"}, "/sysroot/nix", nil},
-			{"open", expectArgs{"/sysroot/nix", 0x280000, uint32(0)}, 0xdeadbeef, nil},
-			{"readlink", expectArgs{"/host/proc/self/fd/3735928559"}, "/sysroot/nix", nil},
-			{"close", expectArgs{0xdeadbeef}, nil, nil},
-			{"openNew", expectArgs{"/host/proc/self/mountinfo"}, newConstFile(sampleMountinfoNix), nil},
-			{"mount", expectArgs{"none", "/sysroot/nix", "", uintptr(0x209027), ""}, nil, nil},
-			{"mount", expectArgs{"none", "/sysroot/nix/.ro-store", "", uintptr(0x209027), ""}, nil, syscall.EACCES},
-			{"mount", expectArgs{"none", "/sysroot/nix/store", "", uintptr(0x209027), ""}, nil, nil},
+		{"success EACCES", func(k *kstub) error {
+			return newProcPaths(k, hostPath).remount(nil, "/sysroot/nix", syscall.MS_REC|syscall.MS_RDONLY|syscall.MS_NODEV)
+		}, stub.Expect{Calls: []stub.Call{
+			call("evalSymlinks", stub.ExpectArgs{"/sysroot/nix"}, "/sysroot/nix", nil),
+			call("open", stub.ExpectArgs{"/sysroot/nix", 0x280000, uint32(0)}, 0xdeadbeef, nil),
+			call("readlink", stub.ExpectArgs{"/host/proc/self/fd/3735928559"}, "/sysroot/nix", nil),
+			call("close", stub.ExpectArgs{0xdeadbeef}, nil, nil),
+			call("openNew", stub.ExpectArgs{"/host/proc/self/mountinfo"}, newConstFile(sampleMountinfoNix), nil),
+			call("mount", stub.ExpectArgs{"none", "/sysroot/nix", "", uintptr(0x209027), ""}, nil, nil),
+			call("mount", stub.ExpectArgs{"none", "/sysroot/nix/.ro-store", "", uintptr(0x209027), ""}, nil, syscall.EACCES),
+			call("mount", stub.ExpectArgs{"none", "/sysroot/nix/store", "", uintptr(0x209027), ""}, nil, nil),
 		}}, nil},

-		{"success no propagate", func(k syscallDispatcher) error {
-			return newProcPaths(k, hostPath).remount("/sysroot/nix", syscall.MS_RDONLY|syscall.MS_NODEV)
-		}, [][]kexpect{{
-			{"evalSymlinks", expectArgs{"/sysroot/nix"}, "/sysroot/nix", nil},
-			{"open", expectArgs{"/sysroot/nix", 0x280000, uint32(0)}, 0xdeadbeef, nil},
-			{"readlink", expectArgs{"/host/proc/self/fd/3735928559"}, "/sysroot/nix", nil},
-			{"close", expectArgs{0xdeadbeef}, nil, nil},
-			{"openNew", expectArgs{"/host/proc/self/mountinfo"}, newConstFile(sampleMountinfoNix), nil},
-			{"mount", expectArgs{"none", "/sysroot/nix", "", uintptr(0x209027), ""}, nil, nil},
+		{"success no propagate", func(k *kstub) error {
+			return newProcPaths(k, hostPath).remount(nil, "/sysroot/nix", syscall.MS_RDONLY|syscall.MS_NODEV)
+		}, stub.Expect{Calls: []stub.Call{
+			call("evalSymlinks", stub.ExpectArgs{"/sysroot/nix"}, "/sysroot/nix", nil),
+			call("open", stub.ExpectArgs{"/sysroot/nix", 0x280000, uint32(0)}, 0xdeadbeef, nil),
+			call("readlink", stub.ExpectArgs{"/host/proc/self/fd/3735928559"}, "/sysroot/nix", nil),
+			call("close", stub.ExpectArgs{0xdeadbeef}, nil, nil),
+			call("openNew", stub.ExpectArgs{"/host/proc/self/mountinfo"}, newConstFile(sampleMountinfoNix), nil),
+			call("mount", stub.ExpectArgs{"none", "/sysroot/nix", "", uintptr(0x209027), ""}, nil, nil),
 		}}, nil},

-		{"success case sensitive", func(k syscallDispatcher) error {
-			return newProcPaths(k, hostPath).remount("/sysroot/nix", syscall.MS_REC|syscall.MS_RDONLY|syscall.MS_NODEV)
-		}, [][]kexpect{{
-			{"evalSymlinks", expectArgs{"/sysroot/nix"}, "/sysroot/nix", nil},
-			{"open", expectArgs{"/sysroot/nix", 0x280000, uint32(0)}, 0xdeadbeef, nil},
-			{"readlink", expectArgs{"/host/proc/self/fd/3735928559"}, "/sysroot/nix", nil},
-			{"close", expectArgs{0xdeadbeef}, nil, nil},
-			{"openNew", expectArgs{"/host/proc/self/mountinfo"}, newConstFile(sampleMountinfoNix), nil},
-			{"mount", expectArgs{"none", "/sysroot/nix", "", uintptr(0x209027), ""}, nil, nil},
-			{"mount", expectArgs{"none", "/sysroot/nix/.ro-store", "", uintptr(0x209027), ""}, nil, nil},
-			{"mount", expectArgs{"none", "/sysroot/nix/store", "", uintptr(0x209027), ""}, nil, nil},
+		{"success case sensitive", func(k *kstub) error {
+			return newProcPaths(k, hostPath).remount(nil, "/sysroot/nix", syscall.MS_REC|syscall.MS_RDONLY|syscall.MS_NODEV)
+		}, stub.Expect{Calls: []stub.Call{
+			call("evalSymlinks", stub.ExpectArgs{"/sysroot/nix"}, "/sysroot/nix", nil),
+			call("open", stub.ExpectArgs{"/sysroot/nix", 0x280000, uint32(0)}, 0xdeadbeef, nil),
+			call("readlink", stub.ExpectArgs{"/host/proc/self/fd/3735928559"}, "/sysroot/nix", nil),
+			call("close", stub.ExpectArgs{0xdeadbeef}, nil, nil),
+			call("openNew", stub.ExpectArgs{"/host/proc/self/mountinfo"}, newConstFile(sampleMountinfoNix), nil),
+			call("mount", stub.ExpectArgs{"none", "/sysroot/nix", "", uintptr(0x209027), ""}, nil, nil),
+			call("mount", stub.ExpectArgs{"none", "/sysroot/nix/.ro-store", "", uintptr(0x209027), ""}, nil, nil),
+			call("mount", stub.ExpectArgs{"none", "/sysroot/nix/store", "", uintptr(0x209027), ""}, nil, nil),
 		}}, nil},

-		{"success", func(k syscallDispatcher) error {
-			return newProcPaths(k, hostPath).remount("/sysroot/.nix", syscall.MS_REC|syscall.MS_RDONLY|syscall.MS_NODEV)
-		}, [][]kexpect{{
-			{"evalSymlinks", expectArgs{"/sysroot/.nix"}, "/sysroot/NIX", nil},
-			{"verbosef", expectArgs{"target resolves to %q", []any{"/sysroot/NIX"}}, nil, nil},
-			{"open", expectArgs{"/sysroot/NIX", 0x280000, uint32(0)}, 0xdeadbeef, nil},
-			{"readlink", expectArgs{"/host/proc/self/fd/3735928559"}, "/sysroot/nix", nil},
-			{"close", expectArgs{0xdeadbeef}, nil, nil},
-			{"openNew", expectArgs{"/host/proc/self/mountinfo"}, newConstFile(sampleMountinfoNix), nil},
-			{"mount", expectArgs{"none", "/sysroot/nix", "", uintptr(0x209027), ""}, nil, nil},
-			{"mount", expectArgs{"none", "/sysroot/nix/.ro-store", "", uintptr(0x209027), ""}, nil, nil},
-			{"mount", expectArgs{"none", "/sysroot/nix/store", "", uintptr(0x209027), ""}, nil, nil},
+		{"success", func(k *kstub) error {
+			return newProcPaths(k, hostPath).remount(k, "/sysroot/.nix", syscall.MS_REC|syscall.MS_RDONLY|syscall.MS_NODEV)
+		}, stub.Expect{Calls: []stub.Call{
+			call("evalSymlinks", stub.ExpectArgs{"/sysroot/.nix"}, "/sysroot/NIX", nil),
+			call("verbosef", stub.ExpectArgs{"target resolves to %q", []any{"/sysroot/NIX"}}, nil, nil),
+			call("open", stub.ExpectArgs{"/sysroot/NIX", 0x280000, uint32(0)}, 0xdeadbeef, nil),
+			call("readlink", stub.ExpectArgs{"/host/proc/self/fd/3735928559"}, "/sysroot/nix", nil),
+			call("close", stub.ExpectArgs{0xdeadbeef}, nil, nil),
+			call("openNew", stub.ExpectArgs{"/host/proc/self/mountinfo"}, newConstFile(sampleMountinfoNix), nil),
+			call("mount", stub.ExpectArgs{"none", "/sysroot/nix", "", uintptr(0x209027), ""}, nil, nil),
+			call("mount", stub.ExpectArgs{"none", "/sysroot/nix/.ro-store", "", uintptr(0x209027), ""}, nil, nil),
+			call("mount", stub.ExpectArgs{"none", "/sysroot/nix/store", "", uintptr(0x209027), ""}, nil, nil),
 		}}, nil},
 	})
 }

 func TestRemountWithFlags(t *testing.T) {
+	t.Parallel()
+
 	checkSimple(t, "remountWithFlags", []simpleTestCase{
-		{"noop unmatched", func(k syscallDispatcher) error {
-			return remountWithFlags(k, &vfs.MountInfoNode{MountInfoEntry: &vfs.MountInfoEntry{VfsOptstr: "rw,relatime,cat"}}, 0)
-		}, [][]kexpect{{
-			{"verbosef", expectArgs{"unmatched vfs options: %q", []any{[]string{"cat"}}}, nil, nil},
+		{"noop unmatched", func(k *kstub) error {
+			return remountWithFlags(k, k, &vfs.MountInfoNode{MountInfoEntry: &vfs.MountInfoEntry{VfsOptstr: "rw,relatime,cat"}}, 0)
+		}, stub.Expect{Calls: []stub.Call{
+			call("verbosef", stub.ExpectArgs{"unmatched vfs options: %q", []any{[]string{"cat"}}}, nil, nil),
 		}}, nil},

-		{"noop", func(k syscallDispatcher) error {
-			return remountWithFlags(k, &vfs.MountInfoNode{MountInfoEntry: &vfs.MountInfoEntry{VfsOptstr: "rw,relatime"}}, 0)
-		}, nil, nil},
+		{"noop", func(k *kstub) error {
+			return remountWithFlags(k, nil, &vfs.MountInfoNode{MountInfoEntry: &vfs.MountInfoEntry{VfsOptstr: "rw,relatime"}}, 0)
+		}, stub.Expect{}, nil},

-		{"success", func(k syscallDispatcher) error {
-			return remountWithFlags(k, &vfs.MountInfoNode{MountInfoEntry: &vfs.MountInfoEntry{VfsOptstr: "rw,relatime"}}, syscall.MS_RDONLY)
-		}, [][]kexpect{{
-			{"mount", expectArgs{"none", "", "", uintptr(0x209021), ""}, nil, nil},
+		{"success", func(k *kstub) error {
+			return remountWithFlags(k, nil, &vfs.MountInfoNode{MountInfoEntry: &vfs.MountInfoEntry{VfsOptstr: "rw,relatime"}}, syscall.MS_RDONLY)
+		}, stub.Expect{Calls: []stub.Call{
+			call("mount", stub.ExpectArgs{"none", "", "", uintptr(0x209021), ""}, nil, nil),
 		}}, nil},
 	})
 }

 func TestMountTmpfs(t *testing.T) {
-	checkSimple(t, "mountTmpfs", []simpleTestCase{
-		{"mkdirAll", func(k syscallDispatcher) error {
-			return mountTmpfs(k, "ephemeral", "/sysroot/run/user/1000", 0, 1<<10, 0700)
-		}, [][]kexpect{{
-			{"mkdirAll", expectArgs{"/sysroot/run/user/1000", os.FileMode(0700)}, nil, errUnique},
-		}}, wrapErrSelf(errUnique)},
+	t.Parallel()

-		{"success no size", func(k syscallDispatcher) error {
+	checkSimple(t, "mountTmpfs", []simpleTestCase{
+		{"mkdirAll", func(k *kstub) error {
+			return mountTmpfs(k, "ephemeral", "/sysroot/run/user/1000", 0, 1<<10, 0700)
+		}, stub.Expect{Calls: []stub.Call{
+			call("mkdirAll", stub.ExpectArgs{"/sysroot/run/user/1000", os.FileMode(0700)}, nil, stub.UniqueError(0)),
+		}}, stub.UniqueError(0)},
+
+		{"success no size", func(k *kstub) error {
 			return mountTmpfs(k, "ephemeral", "/sysroot/run/user/1000", 0, 0, 0710)
-		}, [][]kexpect{{
-			{"mkdirAll", expectArgs{"/sysroot/run/user/1000", os.FileMode(0750)}, nil, nil},
-			{"mount", expectArgs{"ephemeral", "/sysroot/run/user/1000", "tmpfs", uintptr(0), "mode=0710"}, nil, nil},
+		}, stub.Expect{Calls: []stub.Call{
+			call("mkdirAll", stub.ExpectArgs{"/sysroot/run/user/1000", os.FileMode(0750)}, nil, nil),
+			call("mount", stub.ExpectArgs{"ephemeral", "/sysroot/run/user/1000", "tmpfs", uintptr(0), "mode=0710"}, nil, nil),
 		}}, nil},

-		{"success", func(k syscallDispatcher) error {
+		{"success", func(k *kstub) error {
 			return mountTmpfs(k, "ephemeral", "/sysroot/run/user/1000", 0, 1<<10, 0700)
-		}, [][]kexpect{{
-			{"mkdirAll", expectArgs{"/sysroot/run/user/1000", os.FileMode(0700)}, nil, nil},
-			{"mount", expectArgs{"ephemeral", "/sysroot/run/user/1000", "tmpfs", uintptr(0), "mode=0700,size=1024"}, nil, nil},
+		}, stub.Expect{Calls: []stub.Call{
+			call("mkdirAll", stub.ExpectArgs{"/sysroot/run/user/1000", os.FileMode(0700)}, nil, nil),
+			call("mount", stub.ExpectArgs{"ephemeral", "/sysroot/run/user/1000", "tmpfs", uintptr(0), "mode=0700,size=1024"}, nil, nil),
 		}}, nil},
 	})
 }

 func TestParentPerm(t *testing.T) {
+	t.Parallel()
+
 	testCases := []struct {
 		perm os.FileMode
 		want os.FileMode
@@ -277,29 +285,10 @@ func TestParentPerm(t *testing.T) {

 	for _, tc := range testCases {
 		t.Run(tc.perm.String(), func(t *testing.T) {
+			t.Parallel()
 			if got := parentPerm(tc.perm); got != tc.want {
 				t.Errorf("parentPerm: %#o, want %#o", got, tc.want)
 			}
 		})
 	}
 }
-
-func TestEscapeOverlayDataSegment(t *testing.T) {
-	testCases := []struct {
-		name string
-		s    string
-		want string
-	}{
-		{"zero", zeroString, zeroString},
-		{"multi", `\\\:,:,\\\`, `\\\\\\\:\,\:\,\\\\\\`},
-		{"bwrap", `/path :,\`, `/path \:\,\\`},
-	}
-
-	for _, tc := range testCases {
-		t.Run(tc.name, func(t *testing.T) {
-			if got := EscapeOverlayDataSegment(tc.s); got != tc.want {
-				t.Errorf("escapeOverlayDataSegment: %s, want %s", got, tc.want)
-			}
-		})
-	}
-}
--- a/container/msg.go
+++ b/container/msg.go
@@ -1,68 +0,0 @@
-package container
-
-import (
-	"errors"
-	"fmt"
-	"log"
-	"os"
-	"reflect"
-	"sync/atomic"
-	"testing"
-)
-
-type Msg interface {
-	IsVerbose() bool
-	Verbose(v ...any)
-	Verbosef(format string, v ...any)
-	WrapErr(err error, a ...any) error
-	PrintBaseErr(err error, fallback string)
-
-	Suspend()
-	Resume() bool
-
-	BeforeExit()
-}
-
-type DefaultMsg struct{ inactive atomic.Bool }
-
-func (msg *DefaultMsg) IsVerbose() bool { return true }
-func (msg *DefaultMsg) Verbose(v ...any) {
-	if !msg.inactive.Load() {
-		log.Println(v...)
-	}
-}
-func (msg *DefaultMsg) Verbosef(format string, v ...any) {
-	if !msg.inactive.Load() {
-		log.Printf(format, v...)
-	}
-}
-
-// checkedWrappedErr implements error with strict checks for wrapped values.
-type checkedWrappedErr struct {
-	err error
-	a   []any
-}
-
-func (c *checkedWrappedErr) Error() string { return fmt.Sprintf("%v, a = %s", c.err, c.a) }
-func (c *checkedWrappedErr) Is(err error) bool {
-	var concreteErr *checkedWrappedErr
-	if !errors.As(err, &concreteErr) {
-		return false
-	}
-	return reflect.DeepEqual(c, concreteErr)
-}
-
-func (msg *DefaultMsg) WrapErr(err error, a ...any) error {
-	// provide a mostly bulletproof path to bypass this behaviour in tests
-	if testing.Testing() && os.Getenv("GOPATH") != Nonexistent {
-		return &checkedWrappedErr{err, a}
-	}
-
-	log.Println(a...)
-	return err
-}
-func (msg *DefaultMsg) PrintBaseErr(err error, fallback string) { log.Println(fallback, err) }
-
-func (msg *DefaultMsg) Suspend()     { msg.inactive.Store(true) }
-func (msg *DefaultMsg) Resume() bool { return msg.inactive.CompareAndSwap(true, false) }
-func (msg *DefaultMsg) BeforeExit()  {}
--- a/container/msg_test.go
+++ b/container/msg_test.go
@@ -1,162 +0,0 @@
-package container_test
-
-import (
-	"errors"
-	"log"
-	"strings"
-	"sync/atomic"
-	"syscall"
-	"testing"
-
-	"hakurei.app/container"
-	"hakurei.app/internal/hlog"
-)
-
-func TestDefaultMsg(t *testing.T) {
-	// bypass WrapErr testing behaviour
-	t.Setenv("GOPATH", container.Nonexistent)
-
-	{
-		w := log.Writer()
-		f := log.Flags()
-		t.Cleanup(func() { log.SetOutput(w); log.SetFlags(f) })
-	}
-	msg := new(container.DefaultMsg)
-
-	t.Run("is verbose", func(t *testing.T) {
-		if !msg.IsVerbose() {
-			t.Error("IsVerbose unexpected outcome")
-		}
-	})
-
-	t.Run("verbose", func(t *testing.T) {
-		log.SetOutput(panicWriter{})
-		msg.Suspend()
-		msg.Verbose()
-		msg.Verbosef("\x00")
-		msg.Resume()
-
-		buf := new(strings.Builder)
-		log.SetOutput(buf)
-		log.SetFlags(0)
-		msg.Verbose()
-		msg.Verbosef("\x00")
-
-		want := "\n\x00\n"
-		if buf.String() != want {
-			t.Errorf("Verbose: %q, want %q", buf.String(), want)
-		}
-	})
-
-	t.Run("wrapErr", func(t *testing.T) {
-		buf := new(strings.Builder)
-		log.SetOutput(buf)
-		log.SetFlags(0)
-		if err := msg.WrapErr(syscall.EBADE, "\x00", "\x00"); err != syscall.EBADE {
-			t.Errorf("WrapErr: %v", err)
-		}
-		msg.PrintBaseErr(syscall.ENOTRECOVERABLE, "cannot cuddle cat:")
-
-		want := "\x00 \x00\ncannot cuddle cat: state not recoverable\n"
-		if buf.String() != want {
-			t.Errorf("WrapErr: %q, want %q", buf.String(), want)
-		}
-	})
-
-	t.Run("inactive", func(t *testing.T) {
-		{
-			inactive := msg.Resume()
-			if inactive {
-				t.Cleanup(func() { msg.Suspend() })
-			}
-		}
-
-		if msg.Resume() {
-			t.Error("Resume unexpected outcome")
-		}
-
-		msg.Suspend()
-		if !msg.Resume() {
-			t.Error("Resume unexpected outcome")
-		}
-	})
-
-	// the function is a noop
-	t.Run("beforeExit", func(t *testing.T) { msg.BeforeExit() })
-
-	t.Run("checkedWrappedErr", func(t *testing.T) {
-		// temporarily re-enable testing behaviour
-		t.Setenv("GOPATH", "")
-		wrappedErr := msg.WrapErr(syscall.ENOTRECOVERABLE, "cannot cuddle cat:", syscall.ENOTRECOVERABLE)
-
-		t.Run("string", func(t *testing.T) {
-			want := "state not recoverable, a = [cannot cuddle cat: state not recoverable]"
-			if got := wrappedErr.Error(); got != want {
-				t.Errorf("Error: %q, want %q", got, want)
-			}
-		})
-
-		t.Run("bad concrete type", func(t *testing.T) {
-			if errors.Is(wrappedErr, syscall.ENOTRECOVERABLE) {
-				t.Error("incorrect type assertion")
-			}
-		})
-	})
-}
-
-type panicWriter struct{}
-
-func (panicWriter) Write([]byte) (int, error) { panic("unreachable") }
-
-func saveRestoreOutput(t *testing.T) {
-	out := container.GetOutput()
-	t.Cleanup(func() { container.SetOutput(out) })
-}
-
-func replaceOutput(t *testing.T) {
-	saveRestoreOutput(t)
-	container.SetOutput(&testOutput{t: t})
-}
-
-type testOutput struct {
-	t         *testing.T
-	suspended atomic.Bool
-}
-
-func (out *testOutput) IsVerbose() bool { return testing.Verbose() }
-
-func (out *testOutput) Verbose(v ...any) {
-	if !out.IsVerbose() {
-		return
-	}
-	out.t.Log(v...)
-}
-
-func (out *testOutput) Verbosef(format string, v ...any) {
-	if !out.IsVerbose() {
-		return
-	}
-	out.t.Logf(format, v...)
-}
-
-func (out *testOutput) WrapErr(err error, a ...any) error       { return hlog.WrapErr(err, a...) }
-func (out *testOutput) PrintBaseErr(err error, fallback string) { hlog.PrintBaseError(err, fallback) }
-
-func (out *testOutput) Suspend() {
-	if out.suspended.CompareAndSwap(false, true) {
-		out.Verbose("suspend called")
-		return
-	}
-	out.Verbose("suspend called on suspended output")
-}
-
-func (out *testOutput) Resume() bool {
-	if out.suspended.CompareAndSwap(true, false) {
-		out.Verbose("resume called")
-		return true
-	}
-	out.Verbose("resume called on unsuspended output")
-	return false
-}
-
-func (out *testOutput) BeforeExit() { out.Verbose("beforeExit called") }
--- a/container/output.go
+++ b/container/output.go
@@ -1,26 +0,0 @@
-package container
-
-var msg Msg = new(DefaultMsg)
-
-func GetOutput() Msg { return msg }
-func SetOutput(v Msg) {
-	if v == nil {
-		msg = new(DefaultMsg)
-	} else {
-		msg = v
-	}
-}
-
-func wrapErrSuffix(err error, a ...any) error {
-	if err == nil {
-		return nil
-	}
-	return msg.WrapErr(err, append(a, err)...)
-}
-
-func wrapErrSelf(err error) error {
-	if err == nil {
-		return nil
-	}
-	return msg.WrapErr(err, err.Error())
-}
--- a/container/output_test.go
+++ b/container/output_test.go
@@ -1,110 +0,0 @@
-package container
-
-import (
-	"reflect"
-	"syscall"
-	"testing"
-)
-
-func TestGetSetOutput(t *testing.T) {
-	{
-		out := GetOutput()
-		t.Cleanup(func() { SetOutput(out) })
-	}
-
-	t.Run("default", func(t *testing.T) {
-		SetOutput(new(stubOutput))
-		if v, ok := GetOutput().(*DefaultMsg); ok {
-			t.Fatalf("SetOutput: got unexpected output %#v", v)
-		}
-		SetOutput(nil)
-		if _, ok := GetOutput().(*DefaultMsg); !ok {
-			t.Fatalf("SetOutput: got unexpected output %#v", GetOutput())
-		}
-	})
-
-	t.Run("stub", func(t *testing.T) {
-		SetOutput(new(stubOutput))
-		if _, ok := GetOutput().(*stubOutput); !ok {
-			t.Fatalf("SetOutput: got unexpected output %#v", GetOutput())
-		}
-	})
-}
-
-func TestWrapErr(t *testing.T) {
-	{
-		out := GetOutput()
-		t.Cleanup(func() { SetOutput(out) })
-	}
-
-	var wrapFp *func(error, ...any) error
-	s := new(stubOutput)
-	SetOutput(s)
-	wrapFp = &s.wrapF
-
-	testCases := []struct {
-		name    string
-		f       func(t *testing.T)
-		wantErr error
-		wantA   []any
-	}{
-		{"suffix nil", func(t *testing.T) {
-			if err := wrapErrSuffix(nil, "\x00"); err != nil {
-				t.Errorf("wrapErrSuffix: %v", err)
-			}
-		}, nil, nil},
-		{"suffix val", func(t *testing.T) {
-			if err := wrapErrSuffix(syscall.ENOTRECOVERABLE, "\x00\x00"); err != syscall.ENOTRECOVERABLE {
-				t.Errorf("wrapErrSuffix: %v", err)
-			}
-		}, syscall.ENOTRECOVERABLE, []any{"\x00\x00", syscall.ENOTRECOVERABLE}},
-		{"self nil", func(t *testing.T) {
-			if err := wrapErrSelf(nil); err != nil {
-				t.Errorf("wrapErrSelf: %v", err)
-			}
-		}, nil, nil},
-		{"self val", func(t *testing.T) {
-			if err := wrapErrSelf(syscall.ENOTRECOVERABLE); err != syscall.ENOTRECOVERABLE {
-				t.Errorf("wrapErrSelf: %v", err)
-			}
-		}, syscall.ENOTRECOVERABLE, []any{"state not recoverable"}},
-	}
-
-	for _, tc := range testCases {
-		t.Run(tc.name, func(t *testing.T) {
-			var (
-				gotErr error
-				gotA   []any
-			)
-			*wrapFp = func(err error, a ...any) error { gotErr = err; gotA = a; return err }
-
-			tc.f(t)
-			if gotErr != tc.wantErr {
-				t.Errorf("WrapErr: err = %v, want %v", gotErr, tc.wantErr)
-			}
-
-			if !reflect.DeepEqual(gotA, tc.wantA) {
-				t.Errorf("WrapErr: a = %v, want %v", gotA, tc.wantA)
-			}
-		})
-	}
-}
-
-type stubOutput struct {
-	wrapF func(error, ...any) error
-}
-
-func (*stubOutput) IsVerbose() bool            { panic("unreachable") }
-func (*stubOutput) Verbose(...any)             { panic("unreachable") }
-func (*stubOutput) Verbosef(string, ...any)    { panic("unreachable") }
-func (*stubOutput) PrintBaseErr(error, string) { panic("unreachable") }
-func (*stubOutput) Suspend()                   { panic("unreachable") }
-func (*stubOutput) Resume() bool               { panic("unreachable") }
-func (*stubOutput) BeforeExit()                { panic("unreachable") }
-
-func (s *stubOutput) WrapErr(err error, v ...any) error {
-	if s.wrapF == nil {
-		panic("unreachable")
-	}
-	return s.wrapF(err, v...)
-}
--- a/container/params.go
+++ b/container/params.go
@@ -8,11 +8,6 @@ import (
 	"syscall"
 )

-var (
-	ErrNotSet   = errors.New("environment variable not set")
-	ErrFdFormat = errors.New("bad file descriptor representation")
-)
-
 // Setup appends the read end of a pipe for setup params transmission and returns its fd.
 func Setup(extraFiles *[]*os.File) (int, *gob.Encoder, error) {
 	if r, w, err := os.Pipe(); err != nil {
@@ -24,19 +19,23 @@ func Setup(extraFiles *[]*os.File) (int, *gob.Encoder, error) {
 	}
 }

+var (
+	ErrReceiveEnv = errors.New("environment variable not set")
+)
+
 // Receive retrieves setup fd from the environment and receives params.
 func Receive(key string, e any, fdp *uintptr) (func() error, error) {
 	var setup *os.File

 	if s, ok := os.LookupEnv(key); !ok {
-		return nil, ErrNotSet
+		return nil, ErrReceiveEnv
 	} else {
 		if fd, err := strconv.Atoi(s); err != nil {
-			return nil, ErrFdFormat
+			return nil, optionalErrorUnwrap(err)
 		} else {
 			setup = os.NewFile(uintptr(fd), "setup")
 			if setup == nil {
-				return nil, syscall.EBADF
+				return nil, syscall.EDOM
 			}
 			if fdp != nil {
 				*fdp = setup.Fd()
--- a/container/params_test.go
+++ b/container/params_test.go
@@ -29,8 +29,8 @@ func TestSetupReceive(t *testing.T) {
 			})
 		}

-		if _, err := container.Receive(key, nil, nil); !errors.Is(err, container.ErrNotSet) {
-			t.Errorf("Receive: error = %v, want %v", err, container.ErrNotSet)
+		if _, err := container.Receive(key, nil, nil); !errors.Is(err, container.ErrReceiveEnv) {
+			t.Errorf("Receive: error = %v, want %v", err, container.ErrReceiveEnv)
 		}
 	})

@@ -38,8 +38,8 @@ func TestSetupReceive(t *testing.T) {
 		const key = "TEST_ENV_FORMAT"
 		t.Setenv(key, "")

-		if _, err := container.Receive(key, nil, nil); !errors.Is(err, container.ErrFdFormat) {
-			t.Errorf("Receive: error = %v, want %v", err, container.ErrFdFormat)
+		if _, err := container.Receive(key, nil, nil); !errors.Is(err, strconv.ErrSyntax) {
+			t.Errorf("Receive: error = %v, want %v", err, strconv.ErrSyntax)
 		}
 	})

@@ -47,8 +47,8 @@ func TestSetupReceive(t *testing.T) {
 		const key = "TEST_ENV_RANGE"
 		t.Setenv(key, "-1")

-		if _, err := container.Receive(key, nil, nil); !errors.Is(err, syscall.EBADF) {
-			t.Errorf("Receive: error = %v, want %v", err, syscall.EBADF)
+		if _, err := container.Receive(key, nil, nil); !errors.Is(err, syscall.EDOM) {
+			t.Errorf("Receive: error = %v, want %v", err, syscall.EDOM)
 		}
 	})

--- a/container/path.go
+++ b/container/path.go
@@ -2,7 +2,6 @@ package container

 import (
 	"errors"
-	"fmt"
 	"io/fs"
 	"os"
 	"path"
@@ -10,84 +9,18 @@ import (
 	"strings"
 	"syscall"

+	"hakurei.app/container/fhs"
 	"hakurei.app/container/vfs"
 )

-/* constants in this file bypass abs check, be extremely careful when changing them! */
-
-const (
-	// FHSRoot points to the file system root.
-	FHSRoot = "/"
-	// FHSEtc points to the directory for system-specific configuration.
-	FHSEtc = "/etc/"
-	// FHSTmp points to the place for small temporary files.
-	FHSTmp = "/tmp/"
-
-	// FHSRun points to a "tmpfs" file system for system packages to place runtime data, socket files, and similar.
-	FHSRun = "/run/"
-	// FHSRunUser points to a directory containing per-user runtime directories,
-	// each usually individually mounted "tmpfs" instances.
-	FHSRunUser = FHSRun + "user/"
-
-	// FHSUsr points to vendor-supplied operating system resources.
-	FHSUsr = "/usr/"
-	// FHSUsrBin points to binaries and executables for user commands that shall appear in the $PATH search path.
-	FHSUsrBin = FHSUsr + "bin/"
-
-	// FHSVar points to persistent, variable system data. Writable during normal system operation.
-	FHSVar = "/var/"
-	// FHSVarLib points to persistent system data.
-	FHSVarLib = FHSVar + "lib/"
-	// FHSVarEmpty points to a nonstandard directory that is usually empty.
-	FHSVarEmpty = FHSVar + "empty/"
-
-	// FHSDev points to the root directory for device nodes.
-	FHSDev = "/dev/"
-	// FHSProc points to a virtual kernel file system exposing the process list and other functionality.
-	FHSProc = "/proc/"
-	// FHSProcSys points to a hierarchy below /proc/ that exposes a number of kernel tunables.
-	FHSProcSys = FHSProc + "sys/"
-	// FHSSys points to a virtual kernel file system exposing discovered devices and other functionality.
-	FHSSys = "/sys/"
-)
-
-var (
-	// AbsFHSRoot is [FHSRoot] as [Absolute].
-	AbsFHSRoot = &Absolute{FHSRoot}
-	// AbsFHSEtc is [FHSEtc] as [Absolute].
-	AbsFHSEtc = &Absolute{FHSEtc}
-	// AbsFHSTmp is [FHSTmp] as [Absolute].
-	AbsFHSTmp = &Absolute{FHSTmp}
-
-	// AbsFHSRun is [FHSRun] as [Absolute].
-	AbsFHSRun = &Absolute{FHSRun}
-	// AbsFHSRunUser is [FHSRunUser] as [Absolute].
-	AbsFHSRunUser = &Absolute{FHSRunUser}
-
-	// AbsFHSUsrBin is [FHSUsrBin] as [Absolute].
-	AbsFHSUsrBin = &Absolute{FHSUsrBin}
-
-	// AbsFHSVar is [FHSVar] as [Absolute].
-	AbsFHSVar = &Absolute{FHSVar}
-	// AbsFHSVarLib is [FHSVarLib] as [Absolute].
-	AbsFHSVarLib = &Absolute{FHSVarLib}
-
-	// AbsFHSDev is [FHSDev] as [Absolute].
-	AbsFHSDev = &Absolute{FHSDev}
-	// AbsFHSProc is [FHSProc] as [Absolute].
-	AbsFHSProc = &Absolute{FHSProc}
-	// AbsFHSSys is [FHSSys] as [Absolute].
-	AbsFHSSys = &Absolute{FHSSys}
-)
-
 const (
 	// Nonexistent is a path that cannot exist.
 	// /proc is chosen because a system with covered /proc is unsupported by this package.
-	Nonexistent = FHSProc + "nonexistent"
+	Nonexistent = fhs.Proc + "nonexistent"

-	hostPath    = FHSRoot + hostDir
+	hostPath    = fhs.Root + hostDir
 	hostDir     = "host"
-	sysrootPath = FHSRoot + sysrootDir
+	sysrootPath = fhs.Root + sysrootDir
 	sysrootDir  = "sysroot"
 )

@@ -103,30 +36,29 @@ func toHost(name string) string {

 func createFile(name string, perm, pperm os.FileMode, content []byte) error {
 	if err := os.MkdirAll(path.Dir(name), pperm); err != nil {
-		return wrapErrSelf(err)
+		return err
 	}
 	f, err := os.OpenFile(name, syscall.O_CREAT|syscall.O_EXCL|syscall.O_WRONLY, perm)
 	if err != nil {
-		return wrapErrSelf(err)
+		return err
 	}
 	if content != nil {
 		_, err = f.Write(content)
 	}
-	return errors.Join(f.Close(), wrapErrSelf(err))
+	return errors.Join(f.Close(), err)
 }

 func ensureFile(name string, perm, pperm os.FileMode) error {
 	fi, err := os.Stat(name)
 	if err != nil {
 		if !os.IsNotExist(err) {
-			return wrapErrSelf(err)
+			return err
 		}
 		return createFile(name, perm, pperm, nil)
 	}

 	if mode := fi.Mode(); mode&fs.ModeDir != 0 || mode&fs.ModeSymlink != 0 {
-		err = msg.WrapErr(syscall.EISDIR,
-			fmt.Sprintf("path %q is a directory", name))
+		err = &os.PathError{Op: "ensure", Path: name, Err: syscall.EISDIR}
 	}
 	return err
 }
@@ -147,15 +79,14 @@ func (p *procPaths) stdout() string   { return p.self + "/fd/1" }
 func (p *procPaths) fd(fd int) string { return p.self + "/fd/" + strconv.Itoa(fd) }
 func (p *procPaths) mountinfo(f func(d *vfs.MountInfoDecoder) error) error {
 	if r, err := p.k.openNew(p.self + "/mountinfo"); err != nil {
-		return wrapErrSelf(err)
+		return err
 	} else {
 		d := vfs.NewMountInfoDecoder(r)
 		err0 := f(d)
 		if err = r.Close(); err != nil {
-			return wrapErrSelf(err)
+			return err
 		} else if err = d.Err(); err != nil {
-			return wrapErrSuffix(err,
-				"cannot parse mountinfo:")
+			return err
 		}
 		return err0
 	}
--- a/container/path_test.go
+++ b/container/path_test.go
@@ -1,8 +1,6 @@
 package container

 import (
-	"errors"
-	"fmt"
 	"io"
 	"math"
 	"os"
@@ -12,6 +10,7 @@ import (
 	"testing"
 	"unsafe"

+	"hakurei.app/container/check"
 	"hakurei.app/container/vfs"
 )

@@ -51,25 +50,32 @@ func TestToHost(t *testing.T) {
 	}
 }

-// InternalToHostOvlEscape exports toHost passed to EscapeOverlayDataSegment.
-func InternalToHostOvlEscape(s string) string { return EscapeOverlayDataSegment(toHost(s)) }
+// InternalToHostOvlEscape exports toHost passed to [check.EscapeOverlayDataSegment].
+func InternalToHostOvlEscape(s string) string { return check.EscapeOverlayDataSegment(toHost(s)) }

 func TestCreateFile(t *testing.T) {
 	t.Run("nonexistent", func(t *testing.T) {
-		if err := createFile(path.Join(Nonexistent, ":3"), 0644, 0755, nil); !errors.Is(err, wrapErrSelf(&os.PathError{
-			Op:   "mkdir",
-			Path: "/proc/nonexistent",
-			Err:  syscall.ENOENT,
-		})) {
-			t.Errorf("createFile: error = %v", err)
-		}
-		if err := createFile(path.Join(Nonexistent), 0644, 0755, nil); !errors.Is(err, wrapErrSelf(&os.PathError{
-			Op:   "open",
-			Path: "/proc/nonexistent",
-			Err:  syscall.ENOENT,
-		})) {
-			t.Errorf("createFile: error = %v", err)
-		}
+		t.Run("mkdir", func(t *testing.T) {
+			wantErr := &os.PathError{
+				Op:   "mkdir",
+				Path: "/proc/nonexistent",
+				Err:  syscall.ENOENT,
+			}
+			if err := createFile(path.Join(Nonexistent, ":3"), 0644, 0755, nil); !reflect.DeepEqual(err, wantErr) {
+				t.Errorf("createFile: error = %#v, want %#v", err, wantErr)
+			}
+		})
+
+		t.Run("open", func(t *testing.T) {
+			wantErr := &os.PathError{
+				Op:   "open",
+				Path: "/proc/nonexistent",
+				Err:  syscall.ENOENT,
+			}
+			if err := createFile(path.Join(Nonexistent), 0644, 0755, nil); !reflect.DeepEqual(err, wantErr) {
+				t.Errorf("createFile: error = %#v, want %#v", err, wantErr)
+			}
+		})
 	})

 	t.Run("touch", func(t *testing.T) {
@@ -120,13 +126,13 @@ func TestEnsureFile(t *testing.T) {
 				t.Fatalf("Chmod: error = %v", err)
 			}

-			wantErr := wrapErrSelf(&os.PathError{
+			wantErr := &os.PathError{
 				Op:   "stat",
 				Path: pathname,
 				Err:  syscall.EACCES,
-			})
-			if err := ensureFile(pathname, 0644, 0755); !errors.Is(err, wantErr) {
-				t.Errorf("ensureFile: error = %v, want %v", err, wantErr)
+			}
+			if err := ensureFile(pathname, 0644, 0755); !reflect.DeepEqual(err, wantErr) {
+				t.Errorf("ensureFile: error = %#v, want %#v", err, wantErr)
 			}

 			if err := os.Chmod(tempDir, 0755); err != nil {
@@ -136,9 +142,9 @@ func TestEnsureFile(t *testing.T) {

 		t.Run("directory", func(t *testing.T) {
 			pathname := t.TempDir()
-			wantErr := msg.WrapErr(syscall.EISDIR, fmt.Sprintf("path %q is a directory", pathname))
-			if err := ensureFile(pathname, 0644, 0755); !errors.Is(err, wantErr) {
-				t.Errorf("ensureFile: error = %v, want %v", err, wantErr)
+			wantErr := &os.PathError{Op: "ensure", Path: pathname, Err: syscall.EISDIR}
+			if err := ensureFile(pathname, 0644, 0755); !reflect.DeepEqual(err, wantErr) {
+				t.Errorf("ensureFile: error = %#v, want %#v", err, wantErr)
 			}
 		})

@@ -177,12 +183,12 @@ func TestProcPaths(t *testing.T) {
 	t.Run("mountinfo", func(t *testing.T) {
 		t.Run("nonexistent", func(t *testing.T) {
 			nonexistentProc := newProcPaths(direct{}, t.TempDir())
-			wantErr := wrapErrSelf(&os.PathError{
+			wantErr := &os.PathError{
 				Op:   "open",
 				Path: nonexistentProc.self + "/mountinfo",
 				Err:  syscall.ENOENT,
-			})
-			if err := nonexistentProc.mountinfo(func(*vfs.MountInfoDecoder) error { return syscall.EINVAL }); !errors.Is(err, wantErr) {
+			}
+			if err := nonexistentProc.mountinfo(func(*vfs.MountInfoDecoder) error { return syscall.EINVAL }); !reflect.DeepEqual(err, wantErr) {
 				t.Errorf("mountinfo: error = %v, want %v", err, wantErr)
 			}
 		})
@@ -217,11 +223,11 @@ func TestProcPaths(t *testing.T) {

 			t.Run("closed", func(t *testing.T) {
 				p := newProcPaths(direct{}, tempDir)
-				wantErr := wrapErrSelf(&os.PathError{
+				wantErr := &os.PathError{
 					Op:   "close",
 					Path: p.self + "/mountinfo",
 					Err:  os.ErrClosed,
-				})
+				}
 				if err := p.mountinfo(func(d *vfs.MountInfoDecoder) error {
 					v := reflect.ValueOf(d).Elem().FieldByName("s").Elem().FieldByName("r")
 					v = reflect.NewAt(v.Type(), unsafe.Pointer(v.UnsafeAddr()))
@@ -231,8 +237,8 @@ func TestProcPaths(t *testing.T) {
 					} else {
 						return f.Close()
 					}
-				}); !errors.Is(err, wantErr) {
-					t.Errorf("mountinfo: error = %v, want %v", err, wantErr)
+				}); !reflect.DeepEqual(err, wantErr) {
+					t.Errorf("mountinfo: error = %#v, want %#v", err, wantErr)
 				}
 			})

@@ -242,8 +248,8 @@ func TestProcPaths(t *testing.T) {
 					t.Fatalf("WriteFile: error = %v", err)
 				}

-				wantErr := wrapErrSuffix(vfs.ErrMountInfoFields, "cannot parse mountinfo:")
-				if err := newProcPaths(direct{}, tempDir).mountinfo(func(d *vfs.MountInfoDecoder) error { return d.Decode(new(*vfs.MountInfo)) }); !errors.Is(err, wantErr) {
+				wantErr := &vfs.DecoderError{Op: "parse", Line: 0, Err: vfs.ErrMountInfoFields}
+				if err := newProcPaths(direct{}, tempDir).mountinfo(func(d *vfs.MountInfoDecoder) error { return d.Decode(new(*vfs.MountInfo)) }); !reflect.DeepEqual(err, wantErr) {
 					t.Fatalf("mountinfo: error = %v, want %v", err, wantErr)
 				}
 			})
--- a/container/seccomp/hash_amd64_test.go
+++ b/container/seccomp/hash_amd64_test.go
@@ -1,6 +1,9 @@
 package seccomp_test

-import . "hakurei.app/container/seccomp"
+import (
+	. "hakurei.app/container/bits"
+	. "hakurei.app/container/seccomp"
+)

 var bpfExpected = bpfLookup{
 	{AllowMultiarch | AllowCAN |
--- a/container/seccomp/hash_arm64_test.go
+++ b/container/seccomp/hash_arm64_test.go
@@ -1,6 +1,9 @@
 package seccomp_test

-import . "hakurei.app/container/seccomp"
+import (
+	. "hakurei.app/container/bits"
+	. "hakurei.app/container/seccomp"
+)

 var bpfExpected = bpfLookup{
 	{AllowMultiarch | AllowCAN |
--- a/container/seccomp/hash_test.go
+++ b/container/seccomp/hash_test.go
@@ -3,13 +3,14 @@ package seccomp_test
 import (
 	"encoding/hex"

+	"hakurei.app/container/bits"
 	"hakurei.app/container/seccomp"
 )

 type (
 	bpfPreset = struct {
 		seccomp.ExportFlag
-		seccomp.FilterPreset
+		bits.FilterPreset
 	}
 	bpfLookup map[bpfPreset][]byte
 )
--- a/container/seccomp/libseccomp.go
+++ b/container/seccomp/libseccomp.go
@@ -117,7 +117,7 @@ func Export(fd int, rules []NativeRule, flags ExportFlag) error {

 	var ret C.int

-	rulesPinner := new(runtime.Pinner)
+	var rulesPinner runtime.Pinner
 	for i := range rules {
 		rule := &rules[i]
 		rulesPinner.Pin(rule)
@@ -189,6 +189,5 @@ func syscallResolveName(s string) (trap int) {
 	v := C.CString(s)
 	trap = int(C.seccomp_syscall_resolve_name(v))
 	C.free(unsafe.Pointer(v))
-
 	return
 }
--- a/container/seccomp/libseccomp_test.go
+++ b/container/seccomp/libseccomp_test.go
@@ -8,10 +8,13 @@ import (
 	"syscall"
 	"testing"

+	. "hakurei.app/container/bits"
 	. "hakurei.app/container/seccomp"
 )

 func TestExport(t *testing.T) {
+	t.Parallel()
+
 	testCases := []struct {
 		name    string
 		flags   ExportFlag
@@ -31,14 +34,15 @@ func TestExport(t *testing.T) {
 		{"hakurei tty", 0, PresetExt | PresetDenyNS | PresetDenyDevel, false},
 	}

-	buf := make([]byte, 8)
 	for _, tc := range testCases {
 		t.Run(tc.name, func(t *testing.T) {
+			t.Parallel()
+
 			e := New(Preset(tc.presets, tc.flags), tc.flags)
 			want := bpfExpected[bpfPreset{tc.flags, tc.presets}]
 			digest := sha512.New()

-			if _, err := io.CopyBuffer(digest, e, buf); (err != nil) != tc.wantErr {
+			if _, err := io.Copy(digest, e); (err != nil) != tc.wantErr {
 				t.Errorf("Exporter: error = %v, wantErr %v", err, tc.wantErr)
 				return
 			}
@@ -46,7 +50,7 @@ func TestExport(t *testing.T) {
 				t.Errorf("Close: error = %v", err)
 			}
 			if got := digest.Sum(nil); !slices.Equal(got, want) {
-				t.Fatalf("Export() hash = %x, want %x",
+				t.Fatalf("Export: hash = %x, want %x",
 					got, want)
 				return
 			}
--- a/container/seccomp/presets.go
+++ b/container/seccomp/presets.go
@@ -4,46 +4,33 @@ package seccomp

 import (
 	. "syscall"
+
+	"hakurei.app/container/bits"
 )

-type FilterPreset int
-
-const (
-	// PresetExt are project-specific extensions.
-	PresetExt FilterPreset = 1 << iota
-	// PresetDenyNS denies namespace setup syscalls.
-	PresetDenyNS
-	// PresetDenyTTY denies faking input.
-	PresetDenyTTY
-	// PresetDenyDevel denies development-related syscalls.
-	PresetDenyDevel
-	// PresetLinux32 sets PER_LINUX32.
-	PresetLinux32
-)
-
-func Preset(presets FilterPreset, flags ExportFlag) (rules []NativeRule) {
+func Preset(presets bits.FilterPreset, flags ExportFlag) (rules []NativeRule) {
 	allowedPersonality := PER_LINUX
-	if presets&PresetLinux32 != 0 {
+	if presets&bits.PresetLinux32 != 0 {
 		allowedPersonality = PER_LINUX32
 	}
 	presetDevelFinal := presetDevel(ScmpDatum(allowedPersonality))

 	l := len(presetCommon)
-	if presets&PresetDenyNS != 0 {
+	if presets&bits.PresetDenyNS != 0 {
 		l += len(presetNamespace)
 	}
-	if presets&PresetDenyTTY != 0 {
+	if presets&bits.PresetDenyTTY != 0 {
 		l += len(presetTTY)
 	}
-	if presets&PresetDenyDevel != 0 {
+	if presets&bits.PresetDenyDevel != 0 {
 		l += len(presetDevelFinal)
 	}
 	if flags&AllowMultiarch == 0 {
 		l += len(presetEmu)
 	}
-	if presets&PresetExt != 0 {
+	if presets&bits.PresetExt != 0 {
 		l += len(presetCommonExt)
-		if presets&PresetDenyNS != 0 {
+		if presets&bits.PresetDenyNS != 0 {
 			l += len(presetNamespaceExt)
 		}
 		if flags&AllowMultiarch == 0 {
@@ -53,21 +40,21 @@ func Preset(presets FilterPreset, flags ExportFlag) (rules []NativeRule) {

 	rules = make([]NativeRule, 0, l)
 	rules = append(rules, presetCommon...)
-	if presets&PresetDenyNS != 0 {
+	if presets&bits.PresetDenyNS != 0 {
 		rules = append(rules, presetNamespace...)
 	}
-	if presets&PresetDenyTTY != 0 {
+	if presets&bits.PresetDenyTTY != 0 {
 		rules = append(rules, presetTTY...)
 	}
-	if presets&PresetDenyDevel != 0 {
+	if presets&bits.PresetDenyDevel != 0 {
 		rules = append(rules, presetDevelFinal...)
 	}
 	if flags&AllowMultiarch == 0 {
 		rules = append(rules, presetEmu...)
 	}
-	if presets&PresetExt != 0 {
+	if presets&bits.PresetExt != 0 {
 		rules = append(rules, presetCommonExt...)
-		if presets&PresetDenyNS != 0 {
+		if presets&bits.PresetDenyNS != 0 {
 			rules = append(rules, presetNamespaceExt...)
 		}
 		if flags&AllowMultiarch == 0 {
--- a/container/seccomp/proc.go
+++ b/container/seccomp/proc.go
@@ -8,10 +8,6 @@ import (
 	"hakurei.app/helper/proc"
 )

-const (
-	PresetStrict = PresetExt | PresetDenyNS | PresetDenyTTY | PresetDenyDevel
-)
-
 // New returns an inactive Encoder instance.
 func New(rules []NativeRule, flags ExportFlag) *Encoder { return &Encoder{newExporter(rules, flags)} }

@@ -25,9 +21,7 @@ Methods of Encoder are not safe for concurrent use.

 An Encoder must not be copied after first use.
 */
-type Encoder struct {
-	*exporter
-}
+type Encoder struct{ *exporter }

 func (e *Encoder) Read(p []byte) (n int, err error) {
 	if err = e.prepare(); err != nil {
--- a/container/seccomp/seccomp_test.go
+++ b/container/seccomp/seccomp_test.go
@@ -10,6 +10,8 @@ import (
 )

 func TestLibraryError(t *testing.T) {
+	t.Parallel()
+
 	testCases := []struct {
 		name    string
 		sample  *seccomp.LibraryError
@@ -41,6 +43,8 @@ func TestLibraryError(t *testing.T) {
 	}
 	for _, tc := range testCases {
 		t.Run(tc.name, func(t *testing.T) {
+			t.Parallel()
+
 			if errors.Is(tc.sample, tc.compare) != tc.wantIs {
 				t.Errorf("errors.Is(%#v, %#v) did not return %v",
 					tc.sample, tc.compare, tc.wantIs)
@@ -54,6 +58,8 @@ func TestLibraryError(t *testing.T) {
 	}

 	t.Run("invalid", func(t *testing.T) {
+		t.Parallel()
+
 		wantPanic := "invalid libseccomp error"
 		defer func() {
 			if r := recover(); r != wantPanic {
--- a/container/seccomp/syscall_test.go
+++ b/container/seccomp/syscall_test.go
@@ -5,15 +5,17 @@ import (
 )

 func TestSyscallResolveName(t *testing.T) {
+	t.Parallel()
+
 	for name, want := range Syscalls() {
 		t.Run(name, func(t *testing.T) {
+			t.Parallel()
+
 			if got := syscallResolveName(name); got != want {
-				t.Errorf("syscallResolveName(%q) = %d, want %d",
-					name, got, want)
+				t.Errorf("syscallResolveName(%q) = %d, want %d", name, got, want)
 			}
 			if got, ok := SyscallResolveName(name); !ok || got != want {
-				t.Errorf("SyscallResolveName(%q) = %d, want %d",
-					name, got, want)
+				t.Errorf("SyscallResolveName(%q) = %d, want %d", name, got, want)
 			}
 		})
 	}
--- a/container/stub/call.go
+++ b/container/stub/call.go
@@ -0,0 +1,37 @@
+package stub
+
+import (
+	"slices"
+)
+
+// ExpectArgs is an array primarily for storing expected function arguments.
+// Its actual use is defined by the implementation.
+type ExpectArgs = [5]any
+
+// An Expect stores expected calls of a goroutine.
+type Expect struct {
+	Calls []Call
+
+	// Tracks are handed out to descendant goroutines in order.
+	Tracks []Expect
+}
+
+// A Call holds expected arguments of a function call and its outcome.
+type Call struct {
+	// Name is the function Name of this call. Must be unique.
+	Name string
+	// Args are the expected arguments of this Call.
+	Args ExpectArgs
+	// Ret is the return value of this Call.
+	Ret any
+	// Err is the returned error of this Call.
+	Err error
+}
+
+// Error returns [Call.Err] if all arguments are true, or [ErrCheck] otherwise.
+func (k *Call) Error(ok ...bool) error {
+	if !slices.Contains(ok, false) {
+		return k.Err
+	}
+	return ErrCheck
+}
--- a/container/stub/call_test.go
+++ b/container/stub/call_test.go
@@ -0,0 +1,27 @@
+package stub_test
+
+import (
+	"reflect"
+	"testing"
+
+	"hakurei.app/container/stub"
+)
+
+func TestCallError(t *testing.T) {
+	t.Parallel()
+
+	t.Run("contains false", func(t *testing.T) {
+		t.Parallel()
+		if err := new(stub.Call).Error(true, false, true); !reflect.DeepEqual(err, stub.ErrCheck) {
+			t.Errorf("Error: %#v, want %#v", err, stub.ErrCheck)
+		}
+	})
+
+	t.Run("passthrough", func(t *testing.T) {
+		t.Parallel()
+		wantErr := stub.UniqueError(0xbabe)
+		if err := (&stub.Call{Err: wantErr}).Error(true); !reflect.DeepEqual(err, wantErr) {
+			t.Errorf("Error: %#v, want %#v", err, wantErr)
+		}
+	})
+}
--- a/container/stub/errors.go
+++ b/container/stub/errors.go
@@ -0,0 +1,25 @@
+package stub
+
+import (
+	"errors"
+	"strconv"
+)
+
+var (
+	ErrCheck = errors.New("one or more arguments did not match")
+)
+
+// UniqueError is an error that only equivalates to other [UniqueError] with the same magic value.
+type UniqueError uintptr
+
+func (e UniqueError) Error() string {
+	return "unique error " + strconv.Itoa(int(e)) + " injected by the test suite"
+}
+
+func (e UniqueError) Is(target error) bool {
+	var u UniqueError
+	if !errors.As(target, &u) {
+		return false
+	}
+	return e == u
+}
--- a/container/stub/errors_test.go
+++ b/container/stub/errors_test.go
@@ -0,0 +1,42 @@
+package stub_test
+
+import (
+	"errors"
+	"syscall"
+	"testing"
+
+	"hakurei.app/container/stub"
+)
+
+func TestUniqueError(t *testing.T) {
+	t.Parallel()
+
+	t.Run("format", func(t *testing.T) {
+		t.Parallel()
+		want := "unique error 2989 injected by the test suite"
+		if got := stub.UniqueError(0xbad).Error(); got != want {
+			t.Errorf("Error: %q, want %q", got, want)
+		}
+	})
+
+	t.Run("is", func(t *testing.T) {
+		t.Parallel()
+
+		t.Run("type", func(t *testing.T) {
+			t.Parallel()
+			if errors.Is(stub.UniqueError(0), syscall.ENOTRECOVERABLE) {
+				t.Error("Is: unexpected true")
+			}
+		})
+
+		t.Run("val", func(t *testing.T) {
+			t.Parallel()
+			if errors.Is(stub.UniqueError(0), stub.UniqueError(1)) {
+				t.Error("Is: unexpected true")
+			}
+			if !errors.Is(stub.UniqueError(0xbad), stub.UniqueError(0xbad)) {
+				t.Error("Is: unexpected false")
+			}
+		})
+	})
+}
--- a/container/stub/exit.go
+++ b/container/stub/exit.go
@@ -0,0 +1,44 @@
+package stub
+
+import "testing"
+
+// PanicExit is a magic panic value treated as a simulated exit.
+const PanicExit = 0xdeadbeef
+
+const (
+	panicFailNow = 0xcafe0000 + iota
+	panicFatal
+	panicFatalf
+)
+
+// HandleExit must be deferred before calling with the stub.
+func HandleExit(t testing.TB) {
+	switch r := recover(); r {
+	case PanicExit:
+		break
+
+	case panicFailNow:
+		t.FailNow()
+
+	case panicFatal, panicFatalf, nil:
+		break
+
+	default:
+		panic(r)
+	}
+}
+
+// handleExitNew handles exits from goroutines created by [Stub.New].
+func handleExitNew(t testing.TB) {
+	switch r := recover(); r {
+	case PanicExit, panicFatal, panicFatalf, nil:
+		break
+
+	case panicFailNow:
+		t.Fail()
+		break
+
+	default:
+		panic(r)
+	}
+}
--- a/container/stub/exit_test.go
+++ b/container/stub/exit_test.go
@@ -0,0 +1,109 @@
+package stub_test
+
+import (
+	"testing"
+	_ "unsafe"
+
+	"hakurei.app/container/stub"
+)
+
+//go:linkname handleExitNew hakurei.app/container/stub.handleExitNew
+func handleExitNew(_ testing.TB)
+
+// overrideTFailNow overrides the Fail and FailNow method.
+type overrideTFailNow struct {
+	*testing.T
+	failNow bool
+	fail    bool
+}
+
+func (o *overrideTFailNow) FailNow() {
+	if o.failNow {
+		o.Errorf("attempted to FailNow twice")
+	}
+	o.failNow = true
+}
+
+func (o *overrideTFailNow) Fail() {
+	if o.fail {
+		o.Errorf("attempted to Fail twice")
+	}
+	o.fail = true
+}
+
+func TestHandleExit(t *testing.T) {
+	t.Parallel()
+
+	t.Run("exit", func(t *testing.T) {
+		t.Parallel()
+		defer stub.HandleExit(t)
+		panic(stub.PanicExit)
+	})
+
+	t.Run("goexit", func(t *testing.T) {
+		t.Parallel()
+
+		t.Run("FailNow", func(t *testing.T) {
+			t.Parallel()
+
+			ot := &overrideTFailNow{T: t}
+			defer func() {
+				if !ot.failNow {
+					t.Errorf("FailNow was never called")
+				}
+			}()
+			defer stub.HandleExit(ot)
+			panic(0xcafe0000)
+		})
+
+		t.Run("Fail", func(t *testing.T) {
+			t.Parallel()
+
+			ot := &overrideTFailNow{T: t}
+			defer func() {
+				if !ot.fail {
+					t.Errorf("Fail was never called")
+				}
+			}()
+			defer handleExitNew(ot)
+			panic(0xcafe0000)
+		})
+	})
+
+	t.Run("nil", func(t *testing.T) {
+		t.Parallel()
+		defer stub.HandleExit(t)
+	})
+
+	t.Run("passthrough", func(t *testing.T) {
+		t.Parallel()
+
+		t.Run("toplevel", func(t *testing.T) {
+			t.Parallel()
+
+			defer func() {
+				want := 0xcafebabe
+				if r := recover(); r != want {
+					t.Errorf("recover: %v, want %v", r, want)
+				}
+
+			}()
+			defer stub.HandleExit(t)
+			panic(0xcafebabe)
+		})
+
+		t.Run("new", func(t *testing.T) {
+			t.Parallel()
+
+			defer func() {
+				want := 0xcafe
+				if r := recover(); r != want {
+					t.Errorf("recover: %v, want %v", r, want)
+				}
+
+			}()
+			defer handleExitNew(t)
+			panic(0xcafe)
+		})
+	})
+}
--- a/container/stub/stub.go
+++ b/container/stub/stub.go
@@ -0,0 +1,153 @@
+// Package stub provides function call level stubbing and validation
+// for library functions that are impossible to check otherwise.
+package stub
+
+import (
+	"reflect"
+	"sync"
+	"testing"
+)
+
+// this should prevent stub from being inadvertently imported outside tests
+var _ = func() {
+	if !testing.Testing() {
+		panic("stub imported while not in a test")
+	}
+}
+
+const (
+	// A CallSeparator denotes an injected separation between two groups of calls.
+	CallSeparator = "\x00"
+)
+
+// A Stub is a collection of tracks of expected calls.
+type Stub[K any] struct {
+	testing.TB
+
+	// makeK creates a new K for a descendant [Stub].
+	// This function may be called concurrently.
+	makeK func(s *Stub[K]) K
+
+	// want is a hierarchy of expected calls.
+	want Expect
+	// pos is the current position in [Expect.Calls].
+	pos int
+	// goroutine counts the number of goroutines created by this [Stub].
+	goroutine int
+	// sub stores the addresses of descendant [Stub] created by New.
+	sub []*Stub[K]
+	// wg waits for all descendants to complete.
+	wg *sync.WaitGroup
+}
+
+// New creates a root [Stub].
+func New[K any](tb testing.TB, makeK func(s *Stub[K]) K, want Expect) *Stub[K] {
+	return &Stub[K]{TB: tb, makeK: makeK, want: want, wg: new(sync.WaitGroup)}
+}
+
+func (s *Stub[K]) FailNow()          { s.Helper(); panic(panicFailNow) }
+func (s *Stub[K]) Fatal(args ...any) { s.Helper(); s.Error(args...); panic(panicFatal) }
+func (s *Stub[K]) Fatalf(format string, args ...any) {
+	s.Helper()
+	s.Errorf(format, args...)
+	panic(panicFatalf)
+}
+
+func (s *Stub[K]) SkipNow()             { s.Helper(); panic("invalid call to SkipNow") }
+func (s *Stub[K]) Skip(...any)          { s.Helper(); panic("invalid call to Skip") }
+func (s *Stub[K]) Skipf(string, ...any) { s.Helper(); panic("invalid call to Skipf") }
+
+// New calls f in a new goroutine
+func (s *Stub[K]) New(f func(k K)) {
+	s.Helper()
+
+	s.Expects("New")
+	if len(s.want.Tracks) <= s.goroutine {
+		s.Fatal("New: track overrun")
+	}
+	ds := &Stub[K]{TB: s.TB, makeK: s.makeK, want: s.want.Tracks[s.goroutine], wg: s.wg}
+	s.goroutine++
+	s.sub = append(s.sub, ds)
+	s.wg.Add(1)
+	go func() {
+		s.Helper()
+
+		defer s.wg.Done()
+		defer handleExitNew(s.TB)
+		f(s.makeK(ds))
+	}()
+}
+
+// Pos returns the current position of [Stub] in its [Expect.Calls]
+func (s *Stub[K]) Pos() int { return s.pos }
+
+// Len returns the length of [Expect.Calls].
+func (s *Stub[K]) Len() int { return len(s.want.Calls) }
+
+// VisitIncomplete calls f on an incomplete s and all its descendants.
+func (s *Stub[K]) VisitIncomplete(f func(s *Stub[K])) {
+	s.Helper()
+	s.wg.Wait()
+
+	if s.want.Calls != nil && len(s.want.Calls) != s.pos {
+		f(s)
+	}
+	for _, ds := range s.sub {
+		ds.VisitIncomplete(f)
+	}
+}
+
+// Expects checks the name of and returns the current [Call] and advances pos.
+func (s *Stub[K]) Expects(name string) (expect *Call) {
+	s.Helper()
+
+	if len(s.want.Calls) == s.pos {
+		s.Fatal("Expects: advancing beyond expected calls")
+	}
+	expect = &s.want.Calls[s.pos]
+	if name != expect.Name {
+		if expect.Name == CallSeparator {
+			s.Fatalf("Expects: func = %s, separator overrun", name)
+		}
+		if name == CallSeparator {
+			s.Fatalf("Expects: separator, want %s", expect.Name)
+		}
+		s.Fatalf("Expects: func = %s, want %s", name, expect.Name)
+	}
+	s.pos++
+	return
+}
+
+// CheckArg checks an argument comparable with the == operator. Avoid using this with pointers.
+func CheckArg[T comparable, K any](s *Stub[K], arg string, got T, n int) bool {
+	s.Helper()
+
+	pos := s.pos - 1
+	if pos < 0 || pos >= len(s.want.Calls) {
+		panic("invalid call to CheckArg")
+	}
+	expect := s.want.Calls[pos]
+	want, ok := expect.Args[n].(T)
+	if !ok || got != want {
+		s.Errorf("%s: %s = %#v, want %#v (%d)", expect.Name, arg, got, want, pos)
+		return false
+	}
+	return true
+}
+
+// CheckArgReflect checks an argument of any type.
+func CheckArgReflect[K any](s *Stub[K], arg string, got any, n int) bool {
+	s.Helper()
+
+	pos := s.pos - 1
+	if pos < 0 || pos >= len(s.want.Calls) {
+		panic("invalid call to CheckArgReflect")
+	}
+	expect := s.want.Calls[pos]
+	want := expect.Args[n]
+	if !reflect.DeepEqual(got, want) {
+		s.Errorf("%s: %s = %#v, want %#v (%d)", expect.Name, arg, got, want, pos)
+		return false
+	}
+	return true
+}
--- a/container/stub/stub_test.go
+++ b/container/stub/stub_test.go
@@ -0,0 +1,335 @@
+package stub
+
+import (
+	"reflect"
+	"sync/atomic"
+	"testing"
+)
+
+// stubHolder embeds [Stub].
+type stubHolder struct{ *Stub[stubHolder] }
+
+// overrideT allows some methods of [testing.T] to be overridden.
+type overrideT struct {
+	*testing.T
+
+	error  atomic.Pointer[func(args ...any)]
+	errorf atomic.Pointer[func(format string, args ...any)]
+}
+
+func (t *overrideT) Error(args ...any) {
+	fp := t.error.Load()
+	if fp == nil || *fp == nil {
+		t.T.Error(args...)
+		return
+	}
+	(*fp)(args...)
+}
+
+func (t *overrideT) Errorf(format string, args ...any) {
+	fp := t.errorf.Load()
+	if fp == nil || *fp == nil {
+		t.T.Errorf(format, args...)
+		return
+	}
+	(*fp)(format, args...)
+}
+
+func TestStub(t *testing.T) {
+	t.Parallel()
+
+	t.Run("goexit", func(t *testing.T) {
+		t.Parallel()
+
+		t.Run("FailNow", func(t *testing.T) {
+			t.Parallel()
+
+			defer func() {
+				if r := recover(); r != panicFailNow {
+					t.Errorf("recover: %v", r)
+				}
+			}()
+			stubHolder{&Stub[stubHolder]{TB: t}}.FailNow()
+		})
+
+		t.Run("SkipNow", func(t *testing.T) {
+			t.Parallel()
+
+			defer func() {
+				want := "invalid call to SkipNow"
+				if r := recover(); r != want {
+					t.Errorf("recover: %v, want %v", r, want)
+				}
+			}()
+			stubHolder{&Stub[stubHolder]{TB: t}}.SkipNow()
+		})
+
+		t.Run("Skip", func(t *testing.T) {
+			t.Parallel()
+
+			defer func() {
+				want := "invalid call to Skip"
+				if r := recover(); r != want {
+					t.Errorf("recover: %v, want %v", r, want)
+				}
+			}()
+			stubHolder{&Stub[stubHolder]{TB: t}}.Skip()
+		})
+
+		t.Run("Skipf", func(t *testing.T) {
+			t.Parallel()
+
+			defer func() {
+				want := "invalid call to Skipf"
+				if r := recover(); r != want {
+					t.Errorf("recover: %v, want %v", r, want)
+				}
+			}()
+			stubHolder{&Stub[stubHolder]{TB: t}}.Skipf("")
+		})
+	})
+
+	t.Run("new", func(t *testing.T) {
+		t.Parallel()
+
+		t.Run("success", func(t *testing.T) {
+			t.Parallel()
+
+			s := New(t, func(s *Stub[stubHolder]) stubHolder { return stubHolder{s} }, Expect{Calls: []Call{
+				{"New", ExpectArgs{}, nil, nil},
+			}, Tracks: []Expect{{Calls: []Call{
+				{"done", ExpectArgs{0xbabe}, nil, nil},
+			}}}})
+
+			s.New(func(k stubHolder) {
+				expect := k.Expects("done")
+				if expect.Name != "done" {
+					t.Errorf("New: Name = %s, want done", expect.Name)
+				}
+				if expect.Args != (ExpectArgs{0xbabe}) {
+					t.Errorf("New: Args = %#v", expect.Args)
+				}
+				if expect.Ret != nil {
+					t.Errorf("New: Ret = %#v", expect.Ret)
+				}
+				if expect.Err != nil {
+					t.Errorf("New: Err = %#v", expect.Err)
+				}
+			})
+
+			if pos := s.Pos(); pos != 1 {
+				t.Errorf("Pos: %d, want 1", pos)
+			}
+			if l := s.Len(); l != 1 {
+				t.Errorf("Len: %d, want 1", l)
+			}
+
+			s.VisitIncomplete(func(s *Stub[stubHolder]) { panic("unreachable") })
+		})
+
+		t.Run("overrun", func(t *testing.T) {
+			t.Parallel()
+
+			ot := &overrideT{T: t}
+			ot.error.Store(checkError(t, "New: track overrun"))
+			s := New(ot, func(s *Stub[stubHolder]) stubHolder { return stubHolder{s} }, Expect{Calls: []Call{
+				{"New", ExpectArgs{}, nil, nil},
+				{"panic", ExpectArgs{"unreachable"}, nil, nil},
+			}})
+			func() { defer HandleExit(t); s.New(func(k stubHolder) { panic("unreachable") }) }()
+
+			var visit int
+			s.VisitIncomplete(func(s *Stub[stubHolder]) {
+				visit++
+				if visit > 1 {
+					panic("unexpected visit count")
+				}
+
+				want := Call{"panic", ExpectArgs{"unreachable"}, nil, nil}
+				if got := s.want.Calls[s.pos]; !reflect.DeepEqual(got, want) {
+					t.Errorf("VisitIncomplete: %#v, want %#v", got, want)
+				}
+			})
+		})
+
+		t.Run("expects", func(t *testing.T) {
+			t.Parallel()
+
+			t.Run("overrun", func(t *testing.T) {
+				t.Parallel()
+
+				ot := &overrideT{T: t}
+				ot.error.Store(checkError(t, "Expects: advancing beyond expected calls"))
+				s := New(ot, func(s *Stub[stubHolder]) stubHolder { return stubHolder{s} }, Expect{})
+				func() { defer HandleExit(t); s.Expects("unreachable") }()
+			})
+
+			t.Run("separator", func(t *testing.T) {
+				t.Parallel()
+
+				t.Run("overrun", func(t *testing.T) {
+					t.Parallel()
+
+					ot := &overrideT{T: t}
+					ot.errorf.Store(checkErrorf(t, "Expects: func = %s, separator overrun", "meow"))
+					s := New(ot, func(s *Stub[stubHolder]) stubHolder { return stubHolder{s} }, Expect{Calls: []Call{
+						{CallSeparator, ExpectArgs{}, nil, nil},
+					}})
+					func() { defer HandleExit(t); s.Expects("meow") }()
+				})
+
+				t.Run("mismatch", func(t *testing.T) {
+					t.Parallel()
+
+					ot := &overrideT{T: t}
+					ot.errorf.Store(checkErrorf(t, "Expects: separator, want %s", "panic"))
+					s := New(ot, func(s *Stub[stubHolder]) stubHolder { return stubHolder{s} }, Expect{Calls: []Call{
+						{"panic", ExpectArgs{}, nil, nil},
+					}})
+					func() { defer HandleExit(t); s.Expects(CallSeparator) }()
+				})
+			})
+
+			t.Run("mismatch", func(t *testing.T) {
+				t.Parallel()
+
+				ot := &overrideT{T: t}
+				ot.errorf.Store(checkErrorf(t, "Expects: func = %s, want %s", "meow", "nya"))
+				s := New(ot, func(s *Stub[stubHolder]) stubHolder { return stubHolder{s} }, Expect{Calls: []Call{
+					{"nya", ExpectArgs{}, nil, nil},
+				}})
+				func() { defer HandleExit(t); s.Expects("meow") }()
+			})
+		})
+	})
+}
+
+func TestCheckArg(t *testing.T) {
+	t.Run("oob negative", func(t *testing.T) {
+		t.Parallel()
+
+		defer func() {
+			want := "invalid call to CheckArg"
+			if r := recover(); r != want {
+				t.Errorf("recover: %v, want %v", r, want)
+			}
+		}()
+		s := New(t, func(s *Stub[stubHolder]) stubHolder { return stubHolder{s} }, Expect{})
+		CheckArg(s, "unreachable", struct{}{}, 0)
+	})
+
+	ot := &overrideT{T: t}
+	s := New(ot, func(s *Stub[stubHolder]) stubHolder { return stubHolder{s} }, Expect{Calls: []Call{
+		{"panic", ExpectArgs{PanicExit}, nil, nil},
+		{"meow", ExpectArgs{-1}, nil, nil},
+	}})
+
+	t.Run("match", func(t *testing.T) {
+		s.Expects("panic")
+		if !CheckArg(s, "v", PanicExit, 0) {
+			t.Errorf("CheckArg: unexpected false")
+		}
+	})
+
+	t.Run("mismatch", func(t *testing.T) {
+		defer HandleExit(t)
+		s.Expects("meow")
+		ot.errorf.Store(checkErrorf(t, "%s: %s = %#v, want %#v (%d)", "meow", "time", 0, -1, 1))
+		if CheckArg(s, "time", 0, 0) {
+			t.Errorf("CheckArg: unexpected true")
+		}
+	})
+
+	t.Run("oob", func(t *testing.T) {
+		s.pos++
+		defer func() {
+			want := "invalid call to CheckArg"
+			if r := recover(); r != want {
+				t.Errorf("recover: %v, want %v", r, want)
+			}
+		}()
+		CheckArg(s, "unreachable", struct{}{}, 0)
+	})
+}
+
+func TestCheckArgReflect(t *testing.T) {
+	t.Parallel()
+
+	t.Run("oob lower", func(t *testing.T) {
+		t.Parallel()
+
+		defer func() {
+			want := "invalid call to CheckArgReflect"
+			if r := recover(); r != want {
+				t.Errorf("recover: %v, want %v", r, want)
+			}
+		}()
+		s := New(t, func(s *Stub[stubHolder]) stubHolder { return stubHolder{s} }, Expect{})
+		CheckArgReflect(s, "unreachable", struct{}{}, 0)
+	})
+
+	ot := &overrideT{T: t}
+	s := New(ot, func(s *Stub[stubHolder]) stubHolder { return stubHolder{s} }, Expect{Calls: []Call{
+		{"panic", ExpectArgs{PanicExit}, nil, nil},
+		{"meow", ExpectArgs{-1}, nil, nil},
+	}})
+	t.Run("match", func(t *testing.T) {
+		s.Expects("panic")
+		if !CheckArgReflect(s, "v", PanicExit, 0) {
+			t.Errorf("CheckArgReflect: unexpected false")
+		}
+	})
+	t.Run("mismatch", func(t *testing.T) {
+		defer HandleExit(t)
+		s.Expects("meow")
+		ot.errorf.Store(checkErrorf(t, "%s: %s = %#v, want %#v (%d)", "meow", "time", 0, -1, 1))
+		if CheckArgReflect(s, "time", 0, 0) {
+			t.Errorf("CheckArgReflect: unexpected true")
+		}
+	})
+	t.Run("oob", func(t *testing.T) {
+		s.pos++
+		defer func() {
+			want := "invalid call to CheckArgReflect"
+			if r := recover(); r != want {
+				t.Errorf("recover: %v, want %v", r, want)
+			}
+		}()
+		CheckArgReflect(s, "unreachable", struct{}{}, 0)
+	})
+}
+
+func checkError(t *testing.T, wantArgs ...any) *func(args ...any) {
+	var called bool
+	f := func(args ...any) {
+		if called {
+			panic("invalid call to error")
+		}
+		called = true
+
+		if !reflect.DeepEqual(args, wantArgs) {
+			t.Errorf("Error: %#v, want %#v", args, wantArgs)
+		}
+		panic(PanicExit)
+	}
+	return &f
+}
+
+func checkErrorf(t *testing.T, wantFormat string, wantArgs ...any) *func(format string, args ...any) {
+	var called bool
+	f := func(format string, args ...any) {
+		if called {
+			panic("invalid call to errorf")
+		}
+		called = true
+
+		if format != wantFormat {
+			t.Errorf("Errorf: format = %q, want %q", format, wantFormat)
+		}
+		if !reflect.DeepEqual(args, wantArgs) {
+			t.Errorf("Errorf: args = %#v, want %#v", args, wantArgs)
+		}
+		panic(PanicExit)
+	}
+	return &f
+}
--- a/container/sysctl.go
+++ b/container/sysctl.go
@@ -2,10 +2,12 @@ package container

 import (
 	"bytes"
-	"log"
 	"os"
 	"strconv"
 	"sync"
+
+	"hakurei.app/container/fhs"
+	"hakurei.app/message"
 )

 var (
@@ -17,31 +19,33 @@ var (
 )

 const (
-	kernelOverflowuidPath = FHSProcSys + "kernel/overflowuid"
-	kernelOverflowgidPath = FHSProcSys + "kernel/overflowgid"
-	kernelCapLastCapPath  = FHSProcSys + "kernel/cap_last_cap"
+	kernelOverflowuidPath = fhs.ProcSys + "kernel/overflowuid"
+	kernelOverflowgidPath = fhs.ProcSys + "kernel/overflowgid"
+	kernelCapLastCapPath  = fhs.ProcSys + "kernel/cap_last_cap"
 )

-func mustReadSysctl() {
-	if v, err := os.ReadFile(kernelOverflowuidPath); err != nil {
-		log.Fatalf("cannot read %q: %v", kernelOverflowuidPath, err)
-	} else if kernelOverflowuid, err = strconv.Atoi(string(bytes.TrimSpace(v))); err != nil {
-		log.Fatalf("cannot interpret %q: %v", kernelOverflowuidPath, err)
-	}
+func mustReadSysctl(msg message.Msg) {
+	sysctlOnce.Do(func() {
+		if v, err := os.ReadFile(kernelOverflowuidPath); err != nil {
+			msg.GetLogger().Fatalf("cannot read %q: %v", kernelOverflowuidPath, err)
+		} else if kernelOverflowuid, err = strconv.Atoi(string(bytes.TrimSpace(v))); err != nil {
+			msg.GetLogger().Fatalf("cannot interpret %q: %v", kernelOverflowuidPath, err)
+		}

-	if v, err := os.ReadFile(kernelOverflowgidPath); err != nil {
-		log.Fatalf("cannot read %q: %v", kernelOverflowgidPath, err)
-	} else if kernelOverflowgid, err = strconv.Atoi(string(bytes.TrimSpace(v))); err != nil {
-		log.Fatalf("cannot interpret %q: %v", kernelOverflowgidPath, err)
-	}
+		if v, err := os.ReadFile(kernelOverflowgidPath); err != nil {
+			msg.GetLogger().Fatalf("cannot read %q: %v", kernelOverflowgidPath, err)
+		} else if kernelOverflowgid, err = strconv.Atoi(string(bytes.TrimSpace(v))); err != nil {
+			msg.GetLogger().Fatalf("cannot interpret %q: %v", kernelOverflowgidPath, err)
+		}

-	if v, err := os.ReadFile(kernelCapLastCapPath); err != nil {
-		log.Fatalf("cannot read %q: %v", kernelCapLastCapPath, err)
-	} else if kernelCapLastCap, err = strconv.Atoi(string(bytes.TrimSpace(v))); err != nil {
-		log.Fatalf("cannot interpret %q: %v", kernelCapLastCapPath, err)
-	}
+		if v, err := os.ReadFile(kernelCapLastCapPath); err != nil {
+			msg.GetLogger().Fatalf("cannot read %q: %v", kernelCapLastCapPath, err)
+		} else if kernelCapLastCap, err = strconv.Atoi(string(bytes.TrimSpace(v))); err != nil {
+			msg.GetLogger().Fatalf("cannot interpret %q: %v", kernelCapLastCapPath, err)
+		}
+	})
 }

-func OverflowUid() int { sysctlOnce.Do(mustReadSysctl); return kernelOverflowuid }
-func OverflowGid() int { sysctlOnce.Do(mustReadSysctl); return kernelOverflowgid }
-func LastCap() uintptr { sysctlOnce.Do(mustReadSysctl); return uintptr(kernelCapLastCap) }
+func OverflowUid(msg message.Msg) int { mustReadSysctl(msg); return kernelOverflowuid }
+func OverflowGid(msg message.Msg) int { mustReadSysctl(msg); return kernelOverflowgid }
+func LastCap(msg message.Msg) uintptr { mustReadSysctl(msg); return uintptr(kernelCapLastCap) }
--- a/container/vfs/mangle_test.go
+++ b/container/vfs/mangle_test.go
@@ -7,6 +7,8 @@ import (
 )

 func TestUnmangle(t *testing.T) {
+	t.Parallel()
+
 	testCases := []struct {
 		want   string
 		sample string
@@ -17,6 +19,7 @@ func TestUnmangle(t *testing.T) {

 	for _, tc := range testCases {
 		t.Run(tc.want, func(t *testing.T) {
+			t.Parallel()
 			got := vfs.Unmangle(tc.sample)
 			if got != tc.want {
 				t.Errorf("Unmangle: %q, want %q",
--- a/container/vfs/mountinfo.go
+++ b/container/vfs/mountinfo.go
@@ -24,6 +24,32 @@ var (
 	ErrMountInfoSep    = errors.New("bad optional fields separator")
 )

+type DecoderError struct {
+	Op   string
+	Line int
+	Err  error
+}
+
+func (e *DecoderError) Unwrap() error { return e.Err }
+func (e *DecoderError) Error() string {
+	var s string
+
+	var numError *strconv.NumError
+	switch {
+	case errors.As(e.Err, &numError) && numError != nil:
+		s = "numeric field " + strconv.Quote(numError.Num) + " " + numError.Err.Error()
+
+	default:
+		s = e.Err.Error()
+	}
+
+	var atLine string
+	if e.Line >= 0 {
+		atLine = " at line " + strconv.Itoa(e.Line)
+	}
+	return e.Op + " mountinfo" + atLine + ": " + s
+}
+
 type (
 	// A MountInfoDecoder reads and decodes proc_pid_mountinfo(5) entries from an input stream.
 	MountInfoDecoder struct {
@@ -32,6 +58,7 @@ type (

 		current  *MountInfo
 		parseErr error
+		curLine  int
 		complete bool
 	}

@@ -132,9 +159,12 @@ func (d *MountInfoDecoder) Entries() iter.Seq[*MountInfoEntry] {

 func (d *MountInfoDecoder) Err() error {
 	if err := d.s.Err(); err != nil {
-		return err
+		return &DecoderError{"scan", d.curLine, err}
 	}
-	return d.parseErr
+	if d.parseErr != nil {
+		return &DecoderError{"parse", d.curLine, d.parseErr}
+	}
+	return nil
 }

 func (d *MountInfoDecoder) scan() bool {
@@ -160,6 +190,7 @@ func (d *MountInfoDecoder) scan() bool {
 		d.current.Next = m
 		d.current = d.current.Next
 	}
+	d.curLine++
 	return true
 }

--- a/container/vfs/mountinfo_test.go
+++ b/container/vfs/mountinfo_test.go
@@ -4,6 +4,7 @@ import (
 	"encoding/json"
 	"errors"
 	"iter"
+	"os"
 	"path"
 	"reflect"
 	"slices"
@@ -15,62 +16,110 @@ import (
 	"hakurei.app/container/vfs"
 )

+func TestDecoderError(t *testing.T) {
+	t.Parallel()
+
+	testCases := []struct {
+		name    string
+		err     *vfs.DecoderError
+		want    string
+		target  error
+		targetF error
+	}{
+		{"errno", &vfs.DecoderError{Op: "parse", Line: 0xdeadbeef, Err: syscall.ENOTRECOVERABLE},
+			"parse mountinfo at line 3735928559: state not recoverable", syscall.ENOTRECOVERABLE, syscall.EROFS},
+
+		{"strconv", &vfs.DecoderError{Op: "parse", Line: 0xdeadbeef, Err: &strconv.NumError{Func: "Atoi", Num: "meow", Err: strconv.ErrSyntax}},
+			`parse mountinfo at line 3735928559: numeric field "meow" invalid syntax`, strconv.ErrSyntax, os.ErrInvalid},
+
+		{"unfold", &vfs.DecoderError{Op: "unfold", Line: -1, Err: vfs.UnfoldTargetError("/proc/nonexistent")},
+			"unfold mountinfo: mount point /proc/nonexistent never appeared in mountinfo", vfs.UnfoldTargetError("/proc/nonexistent"), os.ErrNotExist},
+	}
+	for _, tc := range testCases {
+		t.Run(tc.name, func(t *testing.T) {
+			t.Parallel()
+
+			t.Run("error", func(t *testing.T) {
+				t.Parallel()
+				if got := tc.err.Error(); got != tc.want {
+					t.Errorf("Error: %s, want %s", got, tc.want)
+				}
+			})
+
+			t.Run("is", func(t *testing.T) {
+				t.Parallel()
+				if !errors.Is(tc.err, tc.target) {
+					t.Errorf("Is: unexpected false")
+				}
+				if errors.Is(tc.err, tc.targetF) {
+					t.Errorf("Is: unexpected true")
+				}
+			})
+		})
+	}
+}
+
 func TestMountInfo(t *testing.T) {
+	t.Parallel()
+
 	testCases := []mountInfoTest{
 		{"count", sampleMountinfoBase + `
 21 20 0:53/ /mnt/test rw,relatime - tmpfs  rw
 21 16 0:17 / /sys/fs/cgroup rw,nosuid,nodev,noexec,relatime - tmpfs tmpfs rw,mode=755`,
-			vfs.ErrMountInfoFields, "", nil, nil, nil},
+			&vfs.DecoderError{Op: "parse", Line: 6, Err: vfs.ErrMountInfoFields},
+			"", nil, nil, nil},

 		{"sep", sampleMountinfoBase + `
 21 20 0:53 / /mnt/test rw,relatime shared:212 _ tmpfs  rw
 21 16 0:17 / /sys/fs/cgroup rw,nosuid,nodev,noexec,relatime - tmpfs tmpfs rw,mode=755`,
-			vfs.ErrMountInfoSep, "", nil, nil, nil},
+			&vfs.DecoderError{Op: "parse", Line: 6, Err: vfs.ErrMountInfoSep},
+			"", nil, nil, nil},

 		{"id", sampleMountinfoBase + `
 id 20 0:53 / /mnt/test rw,relatime shared:212 - tmpfs  rw
 21 16 0:17 / /sys/fs/cgroup rw,nosuid,nodev,noexec,relatime - tmpfs tmpfs rw,mode=755`,
-			strconv.ErrSyntax, "", nil, nil, nil},
+			&vfs.DecoderError{Op: "parse", Line: 6, Err: &strconv.NumError{Func: "Atoi", Num: "id", Err: strconv.ErrSyntax}},
+			"", nil, nil, nil},

 		{"parent", sampleMountinfoBase + `
 21 parent 0:53 / /mnt/test rw,relatime shared:212 - tmpfs  rw
 21 16 0:17 / /sys/fs/cgroup rw,nosuid,nodev,noexec,relatime - tmpfs tmpfs rw,mode=755`,
-			strconv.ErrSyntax, "", nil, nil, nil},
+			&vfs.DecoderError{Op: "parse", Line: 6, Err: &strconv.NumError{Func: "Atoi", Num: "parent", Err: strconv.ErrSyntax}}, "", nil, nil, nil},

 		{"devno", sampleMountinfoBase + `
 21 20 053 / /mnt/test rw,relatime shared:212 - tmpfs  rw
 21 16 0:17 / /sys/fs/cgroup rw,nosuid,nodev,noexec,relatime - tmpfs tmpfs rw,mode=755`,
-			nil, "unexpected EOF", nil, nil, nil},
+			nil, "parse mountinfo at line 6: unexpected EOF", nil, nil, nil},

 		{"maj", sampleMountinfoBase + `
 21 20 maj:53 / /mnt/test rw,relatime shared:212 - tmpfs  rw
 21 16 0:17 / /sys/fs/cgroup rw,nosuid,nodev,noexec,relatime - tmpfs tmpfs rw,mode=755`,
-			nil, "expected integer", nil, nil, nil},
+			nil, "parse mountinfo at line 6: expected integer", nil, nil, nil},

 		{"min", sampleMountinfoBase + `
 21 20 0:min / /mnt/test rw,relatime shared:212 - tmpfs  rw
 21 16 0:17 / /sys/fs/cgroup rw,nosuid,nodev,noexec,relatime - tmpfs tmpfs rw,mode=755`,
-			nil, "expected integer", nil, nil, nil},
+			nil, "parse mountinfo at line 6: expected integer", nil, nil, nil},

 		{"mountroot", sampleMountinfoBase + `
 21 20 0:53  /mnt/test rw,relatime - tmpfs  rw
 21 16 0:17 / /sys/fs/cgroup rw,nosuid,nodev,noexec,relatime - tmpfs tmpfs rw,mode=755`,
-			vfs.ErrMountInfoEmpty, "", nil, nil, nil},
+			&vfs.DecoderError{Op: "parse", Line: 6, Err: vfs.ErrMountInfoEmpty}, "", nil, nil, nil},

 		{"target", sampleMountinfoBase + `
 21 20 0:53 /  rw,relatime - tmpfs  rw
 21 16 0:17 / /sys/fs/cgroup rw,nosuid,nodev,noexec,relatime - tmpfs tmpfs rw,mode=755`,
-			vfs.ErrMountInfoEmpty, "", nil, nil, nil},
+			&vfs.DecoderError{Op: "parse", Line: 6, Err: vfs.ErrMountInfoEmpty}, "", nil, nil, nil},

 		{"vfs options", sampleMountinfoBase + `
 21 20 0:53 / /mnt/test  - tmpfs  rw
 21 16 0:17 / /sys/fs/cgroup rw,nosuid,nodev,noexec,relatime - tmpfs tmpfs rw,mode=755`,
-			vfs.ErrMountInfoEmpty, "", nil, nil, nil},
+			&vfs.DecoderError{Op: "parse", Line: 6, Err: vfs.ErrMountInfoEmpty}, "", nil, nil, nil},

 		{"FS type", sampleMountinfoBase + `
-21 20 0:53 / /mnt/test rw,relatime -   rw
-21 16 0:17 / /sys/fs/cgroup rw,nosuid,nodev,noexec,relatime - tmpfs tmpfs rw,mode=755`,
-			vfs.ErrMountInfoEmpty, "", nil, nil, nil},
+21 16 0:17 / /sys/fs/cgroup rw,nosuid,nodev,noexec,relatime - tmpfs tmpfs rw,mode=755
+21 20 0:53 / /mnt/test rw,relatime -   rw`,
+			&vfs.DecoderError{Op: "parse", Line: 7, Err: vfs.ErrMountInfoEmpty}, "", nil, nil, nil},

 		{"base", sampleMountinfoBase, nil, "", []*wantMountInfo{
 			m(15, 20, 0, 3, "/", "/proc", "rw,relatime", o(), "proc", "/proc", "rw", syscall.MS_RELATIME, nil),
@@ -146,7 +195,10 @@ id 20 0:53 / /mnt/test rw,relatime shared:212 - tmpfs  rw

 	for _, tc := range testCases {
 		t.Run(tc.name, func(t *testing.T) {
+			t.Parallel()
+
 			t.Run("decode", func(t *testing.T) {
+				t.Parallel()
 				var got *vfs.MountInfo
 				d := vfs.NewMountInfoDecoder(strings.NewReader(tc.sample))
 				err := d.Decode(&got)
@@ -165,6 +217,7 @@ id 20 0:53 / /mnt/test rw,relatime shared:212 - tmpfs  rw
 			})

 			t.Run("iter", func(t *testing.T) {
+				t.Parallel()
 				d := vfs.NewMountInfoDecoder(strings.NewReader(tc.sample))
 				tc.check(t, d, "Entries",
 					d.Entries(), d.Err)
@@ -176,6 +229,7 @@ id 20 0:53 / /mnt/test rw,relatime shared:212 - tmpfs  rw
 			})

 			t.Run("yield", func(t *testing.T) {
+				t.Parallel()
 				d := vfs.NewMountInfoDecoder(strings.NewReader(tc.sample))
 				v := false
 				d.Entries()(func(entry *vfs.MountInfoEntry) bool { v = !v; return v })
@@ -266,9 +320,9 @@ func (tc *mountInfoTest) check(t *testing.T, d *vfs.MountInfoDecoder, funcName s
 		})
 	} else if tc.wantNode != nil || tc.wantCollectF != nil {
 		panic("invalid test case")
-	} else if _, err := d.Unfold("/"); !errors.Is(err, tc.wantErr) {
+	} else if _, err := d.Unfold("/"); !reflect.DeepEqual(err, tc.wantErr) {
 		if tc.wantError == "" {
-			t.Errorf("Unfold: error = %v, wantErr %v",
+			t.Errorf("Unfold: error = %#v, wantErr %#v",
 				err, tc.wantErr)
 		} else if err != nil && err.Error() != tc.wantError {
 			t.Errorf("Unfold: error = %q, wantError %q",
@@ -276,9 +330,9 @@ func (tc *mountInfoTest) check(t *testing.T, d *vfs.MountInfoDecoder, funcName s
 		}
 	}

-	if err := gotErr(); !errors.Is(err, tc.wantErr) {
+	if err := gotErr(); !reflect.DeepEqual(err, tc.wantErr) {
 		if tc.wantError == "" {
-			t.Errorf("%s: error = %v, wantErr %v",
+			t.Errorf("%s: error = %#v, wantErr %#v",
 				funcName, err, tc.wantErr)
 		} else if err != nil && err.Error() != tc.wantError {
 			t.Errorf("%s: error = %q, wantError %q",
--- a/container/vfs/unfold.go
+++ b/container/vfs/unfold.go
@@ -4,9 +4,14 @@ import (
 	"iter"
 	"path"
 	"strings"
-	"syscall"
 )

+type UnfoldTargetError string
+
+func (e UnfoldTargetError) Error() string {
+	return "mount point " + string(e) + " never appeared in mountinfo"
+}
+
 // MountInfoNode positions a [MountInfoEntry] in its mount hierarchy.
 type MountInfoNode struct {
 	*MountInfoEntry
@@ -65,7 +70,8 @@ func (d *MountInfoDecoder) Unfold(target string) (*MountInfoNode, error) {
 	}

 	if targetIndex == -1 {
-		return nil, syscall.ESTALE
+		// target does not exist in parsed mountinfo
+		return nil, &DecoderError{Op: "unfold", Line: -1, Err: UnfoldTargetError(targetClean)}
 	}

 	for _, cur := range mountinfo {
--- a/container/vfs/unfold_test.go
+++ b/container/vfs/unfold_test.go
@@ -1,17 +1,17 @@
 package vfs_test

 import (
-	"errors"
 	"reflect"
 	"slices"
 	"strings"
-	"syscall"
 	"testing"

 	"hakurei.app/container/vfs"
 )

 func TestUnfold(t *testing.T) {
+	t.Parallel()
+
 	testCases := []struct {
 		name    string
 		sample  string
@@ -26,7 +26,7 @@ func TestUnfold(t *testing.T) {
 			"no match",
 			sampleMountinfoBase,
 			"/mnt",
-			syscall.ESTALE, nil, nil, nil,
+			&vfs.DecoderError{Op: "unfold", Line: -1, Err: vfs.UnfoldTargetError("/mnt")}, nil, nil, nil,
 		},
 		{
 			"cover",
@@ -52,10 +52,12 @@ func TestUnfold(t *testing.T) {

 	for _, tc := range testCases {
 		t.Run(tc.name, func(t *testing.T) {
+			t.Parallel()
+
 			d := vfs.NewMountInfoDecoder(strings.NewReader(tc.sample))
 			got, err := d.Unfold(tc.target)

-			if !errors.Is(err, tc.wantErr) {
+			if !reflect.DeepEqual(err, tc.wantErr) {
 				t.Errorf("Unfold: error = %v, wantErr %v",
 					err, tc.wantErr)
 			}
--- a/dist/comp/_hakurei
+++ b/dist/comp/_hakurei
@@ -54,8 +54,8 @@ __hakurei_instances() {
 {
  local -a _hakurei_cmds
  _hakurei_cmds=(
-    "app:Load app from configuration file"
-    "run:Configure and start a permissive default sandbox"
+    "app:Load and start container from configuration file"
+    "run:Configure and start a permissive container"
    "show:Show live or local app configuration"
    "ps:List active instances"
    "version:Display version information"
--- a/dist/install.sh
+++ b/dist/install.sh
@@ -4,7 +4,7 @@ cd "$(dirname -- "$0")" || exit 1
 install -vDm0755 "bin/hakurei" "${HAKUREI_INSTALL_PREFIX}/usr/bin/hakurei"
 install -vDm0755 "bin/hpkg" "${HAKUREI_INSTALL_PREFIX}/usr/bin/hpkg"

-install -vDm6511 "bin/hsu" "${HAKUREI_INSTALL_PREFIX}/usr/bin/hsu"
+install -vDm4511 "bin/hsu" "${HAKUREI_INSTALL_PREFIX}/usr/bin/hsu"
 if [ ! -f "${HAKUREI_INSTALL_PREFIX}/etc/hsurc" ]; then
    install -vDm0400 "hsurc.default" "${HAKUREI_INSTALL_PREFIX}/etc/hsurc"
 fi
--- a/flake.lock
+++ b/flake.lock
@@ -7,11 +7,11 @@
        ]
      },
      "locked": {
-        "lastModified": 1753479839,
-        "narHash": "sha256-E/rPVh7vyPMJUFl2NAew+zibNGfVbANr8BP8nLRbLkQ=",
+        "lastModified": 1756679287,
+        "narHash": "sha256-Xd1vOeY9ccDf5VtVK12yM0FS6qqvfUop8UQlxEB+gTQ=",
        "owner": "nix-community",
        "repo": "home-manager",
-        "rev": "0b9bf983db4d064764084cd6748efb1ab8297d1e",
+        "rev": "07fc025fe10487dd80f2ec694f1cd790e752d0e8",
        "type": "github"
      },
      "original": {
@@ -23,11 +23,11 @@
    },
    "nixpkgs": {
      "locked": {
-        "lastModified": 1753345091,
-        "narHash": "sha256-CdX2Rtvp5I8HGu9swBmYuq+ILwRxpXdJwlpg8jvN4tU=",
+        "lastModified": 1757020766,
+        "narHash": "sha256-PLoSjHRa2bUbi1x9HoXgTx2AiuzNXs54c8omhadyvp0=",
        "owner": "NixOS",
        "repo": "nixpkgs",
-        "rev": "3ff0e34b1383648053bba8ed03f201d3466f90c9",
+        "rev": "fe83bbdde2ccdc2cb9573aa846abe8363f79a97a",
        "type": "github"
      },
      "original": {
--- a/helper/args_test.go
+++ b/helper/args_test.go
@@ -11,27 +11,29 @@ import (
 )

 func TestArgsString(t *testing.T) {
+	t.Parallel()
 	wantString := strings.Join(wantArgs, " ")
 	if got := argsWt.(fmt.Stringer).String(); got != wantString {
-		t.Errorf("String: %q, want %q",
-			got, wantString)
+		t.Errorf("String: %q, want %q", got, wantString)
 	}
 }

 func TestNewCheckedArgs(t *testing.T) {
+	t.Parallel()
+
 	args := []string{"\x00"}
 	if _, err := helper.NewCheckedArgs(args); !errors.Is(err, syscall.EINVAL) {
-		t.Errorf("NewCheckedArgs: error = %v, wantErr %v",
-			err, syscall.EINVAL)
+		t.Errorf("NewCheckedArgs: error = %v, wantErr %v", err, syscall.EINVAL)
 	}

 	t.Run("must panic", func(t *testing.T) {
+		t.Parallel()
+
 		badPayload := []string{"\x00"}
 		defer func() {
 			wantPanic := "invalid argument"
 			if r := recover(); r != wantPanic {
-				t.Errorf("MustNewCheckedArgs: panic = %v, wantPanic %v",
-					r, wantPanic)
+				t.Errorf("MustNewCheckedArgs: panic = %v, wantPanic %v", r, wantPanic)
 			}
 		}()
 		helper.MustNewCheckedArgs(badPayload)
--- a/helper/container.go
+++ b/helper/container.go
@@ -10,13 +10,16 @@ import (
 	"sync"

 	"hakurei.app/container"
+	"hakurei.app/container/check"
 	"hakurei.app/helper/proc"
+	"hakurei.app/message"
 )

 // New initialises a Helper instance with wt as the null-terminated argument writer.
 func New(
 	ctx context.Context,
-	pathname *container.Absolute, name string,
+	msg message.Msg,
+	pathname *check.Absolute, name string,
 	wt io.WriterTo,
 	stat bool,
 	argF func(argsFd, statFd int) []string,
@@ -26,7 +29,7 @@ func New(
 	var args []string
 	h := new(helperContainer)
 	h.helperFiles, args = newHelperFiles(ctx, wt, stat, argF, extraFiles)
-	h.Container = container.NewCommand(ctx, pathname, name, args...)
+	h.Container = container.NewCommand(ctx, msg, pathname, name, args...)
 	h.WaitDelay = WaitDelay
 	if cmdF != nil {
 		cmdF(h.Container)
--- a/helper/container_test.go
+++ b/helper/container_test.go
@@ -7,14 +7,16 @@ import (
 	"testing"

 	"hakurei.app/container"
+	"hakurei.app/container/check"
+	"hakurei.app/container/fhs"
 	"hakurei.app/helper"
 )

 func TestContainer(t *testing.T) {
-	t.Run("start empty container", func(t *testing.T) {
-		h := helper.New(t.Context(), container.MustAbs(container.Nonexistent), "hakurei", argsWt, false, argF, nil, nil)
+	t.Run("start invalid container", func(t *testing.T) {
+		h := helper.New(t.Context(), nil, check.MustAbs(container.Nonexistent), "hakurei", argsWt, false, argF, nil, nil)

-		wantErr := "container: starting an empty container"
+		wantErr := "container: starting an invalid container"
 		if err := h.Start(); err == nil || err.Error() != wantErr {
 			t.Errorf("Start: error = %v, wantErr %q",
 				err, wantErr)
@@ -22,7 +24,7 @@ func TestContainer(t *testing.T) {
 	})

 	t.Run("valid new helper nil check", func(t *testing.T) {
-		if got := helper.New(t.Context(), container.MustAbs(container.Nonexistent), "hakurei", argsWt, false, argF, nil, nil); got == nil {
+		if got := helper.New(t.Context(), nil, check.MustAbs(container.Nonexistent), "hakurei", argsWt, false, argF, nil, nil); got == nil {
 			t.Errorf("New(%q, %q) got nil",
 				argsWt, "hakurei")
 			return
@@ -31,12 +33,12 @@ func TestContainer(t *testing.T) {

 	t.Run("implementation compliance", func(t *testing.T) {
 		testHelper(t, func(ctx context.Context, setOutput func(stdoutP, stderrP *io.Writer), stat bool) helper.Helper {
-			return helper.New(ctx, container.MustAbs(os.Args[0]), "helper", argsWt, stat, argF, func(z *container.Container) {
+			return helper.New(ctx, nil, check.MustAbs(os.Args[0]), "helper", argsWt, stat, argF, func(z *container.Container) {
 				setOutput(&z.Stdout, &z.Stderr)
 				z.
-					Bind(container.AbsFHSRoot, container.AbsFHSRoot, 0).
-					Proc(container.AbsFHSProc).
-					Dev(container.AbsFHSDev, true)
+					Bind(fhs.AbsRoot, fhs.AbsRoot, 0).
+					Proc(fhs.AbsProc).
+					Dev(fhs.AbsDev, true)
 			}, nil)
 		})
 	})
--- a/helper/helper_test.go
+++ b/helper/helper_test.go
@@ -6,8 +6,10 @@ import (
 	"fmt"
 	"io"
 	"os"
+	"reflect"
 	"strconv"
 	"strings"
+	"syscall"
 	"testing"
 	"time"

@@ -47,6 +49,10 @@ func argFChecked(argsFd, statFd int) (args []string) {
 	return
 }

+const (
+	containerTimeout = 30 * time.Second
+)
+
 // this function tests an implementation of the helper.Helper interface
 func testHelper(t *testing.T, createHelper func(ctx context.Context, setOutput func(stdoutP, stderrP *io.Writer), stat bool) helper.Helper) {
 	oldWaitDelay := helper.WaitDelay
@@ -54,18 +60,15 @@ func testHelper(t *testing.T, createHelper func(ctx context.Context, setOutput f
 	t.Cleanup(func() { helper.WaitDelay = oldWaitDelay })

 	t.Run("start helper with status channel and wait", func(t *testing.T) {
-		ctx, cancel := context.WithTimeout(t.Context(), 5*time.Second)
+		ctx, cancel := context.WithTimeout(t.Context(), containerTimeout)
 		stdout := new(strings.Builder)
 		h := createHelper(ctx, func(stdoutP, stderrP *io.Writer) { *stdoutP, *stderrP = stdout, os.Stderr }, true)

 		t.Run("wait not yet started helper", func(t *testing.T) {
-			defer func() {
-				r := recover()
-				if r == nil {
-					t.Fatalf("Wait did not panic")
-				}
-			}()
-			panic(fmt.Sprintf("unreachable: %v", h.Wait()))
+			if err := h.Wait(); !reflect.DeepEqual(err, syscall.EINVAL) &&
+				!reflect.DeepEqual(err, errors.New("exec: not started")) {
+				t.Errorf("Wait: error = %v", err)
+			}
 		})

 		t.Log("starting helper stub")
@@ -108,7 +111,7 @@ func testHelper(t *testing.T, createHelper func(ctx context.Context, setOutput f
 	})

 	t.Run("start helper and wait", func(t *testing.T) {
-		ctx, cancel := context.WithTimeout(t.Context(), 5*time.Second)
+		ctx, cancel := context.WithTimeout(t.Context(), containerTimeout)
 		defer cancel()
 		stdout := new(strings.Builder)
 		h := createHelper(ctx, func(stdoutP, stderrP *io.Writer) { *stdoutP, *stderrP = stdout, os.Stderr }, false)
--- a/helper/proc/files.go
+++ b/helper/proc/files.go
@@ -6,11 +6,18 @@ import (
 	"os/exec"
 	"sync/atomic"
 	"syscall"
+	"testing"
 	"time"
 )

 var FulfillmentTimeout = 2 * time.Second

+func init() {
+	if testing.Testing() {
+		FulfillmentTimeout *= 10
+	}
+}
+
 // A File is an extra file with deferred initialisation.
 type File interface {
 	// Init initialises File state. Init must not be called more than once.
--- a/Show More
+++ b/Show More