internal/app: remove seal interface

This further cleans up the package for the restructure.

Signed-off-by: Ophestra <cat@gensokyo.uk>

internal/app/shim.go

@@ -0,0 +1,183 @@
+package app
+
+import (
+	"context"
+	"errors"
+	"io"
+	"log"
+	"os"
+	"os/exec"
+	"os/signal"
+	"runtime"
+	"sync/atomic"
+	"syscall"
+	"time"
+
+	"hakurei.app/container"
+	"hakurei.app/container/seccomp"
+	"hakurei.app/internal"
+	"hakurei.app/internal/hlog"
+)
+
+//#include "shim-signal.h"
+import "C"
+
+const shimEnv = "HAKUREI_SHIM"
+
+type shimParams struct {
+	// monitor pid, checked against ppid in signal handler
+	Monitor int
+
+	// duration to wait for after interrupting a container's initial process before the container is killed;
+	// zero value defaults to [DefaultShimWaitDelay], values exceeding [MaxShimWaitDelay] becomes [MaxShimWaitDelay]
+	WaitDelay time.Duration
+
+	// finalised container params
+	Container *container.Params
+
+	// verbosity pass through
+	Verbose bool
+}
+
+const (
+	// ShimExitRequest is returned when the monitor process requests shim exit.
+	ShimExitRequest = 254
+	// ShimExitOrphan is returned when the shim is orphaned before monitor delivers a signal.
+	ShimExitOrphan = 3
+
+	DefaultShimWaitDelay = 5 * time.Second
+	MaxShimWaitDelay     = 30 * time.Second
+)
+
+// ShimMain is the main function of the shim process and runs as the unconstrained target user.
+func ShimMain() {
+	hlog.Prepare("shim")
+
+	if err := container.SetDumpable(container.SUID_DUMP_DISABLE); err != nil {
+		log.Fatalf("cannot set SUID_DUMP_DISABLE: %s", err)
+	}
+
+	var (
+		params     shimParams
+		closeSetup func() error
+	)
+	if f, err := container.Receive(shimEnv, &params, nil); err != nil {
+		if errors.Is(err, syscall.EBADF) {
+			log.Fatal("invalid config descriptor")
+		}
+		if errors.Is(err, container.ErrNotSet) {
+			log.Fatal("HAKUREI_SHIM not set")
+		}
+
+		log.Fatalf("cannot receive shim setup params: %v", err)
+	} else {
+		internal.InstallOutput(params.Verbose)
+		closeSetup = f
+	}
+
+	var signalPipe io.ReadCloser
+	// the Go runtime does not expose siginfo_t so SIGCONT is handled in C to check si_pid
+	if r, w, err := os.Pipe(); err != nil {
+		log.Fatalf("cannot pipe: %v", err)
+	} else if _, err = C.hakurei_shim_setup_cont_signal(C.pid_t(params.Monitor), C.int(w.Fd())); err != nil {
+		log.Fatalf("cannot install SIGCONT handler: %v", err)
+	} else {
+		defer runtime.KeepAlive(w)
+		signalPipe = r
+	}
+
+	// pdeath_signal delivery is checked as if the dying process called kill(2), see kernel/exit.c
+	if _, _, errno := syscall.Syscall(syscall.SYS_PRCTL, syscall.PR_SET_PDEATHSIG, uintptr(syscall.SIGCONT), 0); errno != 0 {
+		log.Fatalf("cannot set parent-death signal: %v", errno)
+	}
+
+	// signal handler outcome
+	var cancelContainer atomic.Pointer[context.CancelFunc]
+	go func() {
+		buf := make([]byte, 1)
+		for {
+			if _, err := signalPipe.Read(buf); err != nil {
+				log.Fatalf("cannot read from signal pipe: %v", err)
+			}
+
+			switch buf[0] {
+			case 0: // got SIGCONT from monitor: shim exit requested
+				if fp := cancelContainer.Load(); params.Container.ForwardCancel && fp != nil && *fp != nil {
+					(*fp)()
+					// shim now bound by ShimWaitDelay, implemented below
+					continue
+				}
+
+				// setup has not completed, terminate immediately
+				hlog.Resume()
+				os.Exit(ShimExitRequest)
+				return
+
+			case 1: // got SIGCONT after adoption: monitor died before delivering signal
+				hlog.BeforeExit()
+				os.Exit(ShimExitOrphan)
+				return
+
+			case 2: // unreachable
+				log.Println("sa_sigaction got invalid siginfo")
+
+			case 3: // got SIGCONT from unexpected process: hopefully the terminal driver
+				log.Println("got SIGCONT from unexpected process")
+
+			default: // unreachable
+				log.Fatalf("got invalid message %d from signal handler", buf[0])
+			}
+		}
+	}()
+
+	if params.Container == nil || params.Container.Ops == nil {
+		log.Fatal("invalid container params")
+	}
+
+	// close setup socket
+	if err := closeSetup(); err != nil {
+		log.Printf("cannot close setup pipe: %v", err)
+		// not fatal
+	}
+
+	ctx, stop := signal.NotifyContext(context.Background(), os.Interrupt, syscall.SIGTERM)
+	cancelContainer.Store(&stop)
+	z := container.New(ctx)
+	z.Params = *params.Container
+	z.Stdin, z.Stdout, z.Stderr = os.Stdin, os.Stdout, os.Stderr
+
+	z.WaitDelay = params.WaitDelay
+	if z.WaitDelay == 0 {
+		z.WaitDelay = DefaultShimWaitDelay
+	}
+	if z.WaitDelay > MaxShimWaitDelay {
+		z.WaitDelay = MaxShimWaitDelay
+	}
+
+	if err := z.Start(); err != nil {
+		hlog.PrintBaseError(err, "cannot start container:")
+		os.Exit(1)
+	}
+	if err := z.Serve(); err != nil {
+		hlog.PrintBaseError(err, "cannot configure container:")
+	}
+
+	if err := seccomp.Load(
+		seccomp.Preset(seccomp.PresetStrict, seccomp.AllowMultiarch),
+		seccomp.AllowMultiarch,
+	); err != nil {
+		log.Fatalf("cannot load syscall filter: %v", err)
+	}
+
+	if err := z.Wait(); err != nil {
+		var exitError *exec.ExitError
+		if !errors.As(err, &exitError) {
+			if errors.Is(err, context.Canceled) {
+				os.Exit(2)
+			}
+			log.Printf("wait: %v", err)
+			os.Exit(127)
+		}
+		os.Exit(exitError.ExitCode())
+	}
+}