container/init: unwrap descriptive fatal error

These errors are printed with a descriptive message prefixed to them, so it is more readable to expose the underlying errno.

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

container/errors.go

@@ -59,6 +59,7 @@ func messagePrefixP[V any, T interface {
 	return zeroString, false
 }
 
+// MountError wraps errors returned by syscall.Mount.
 type MountError struct {
 	Source, Target, Fstype string
 
@@ -74,6 +75,7 @@ func (e *MountError) Unwrap() error {
 	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 {
@@ -90,6 +92,15 @@ func (e *MountError) Error() string {
 	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

container/init.go

@@ -178,7 +178,7 @@ func initEntrypoint(k syscallDispatcher, msg message.Msg) {
 	lastcap := k.lastcap(msg)
 
 	if err := k.mount(zeroString, fhs.Root, zeroString, MS_SILENT|MS_SLAVE|MS_REC, zeroString); err != nil {
-		k.fatalf(msg, "cannot make / rslave: %v", err)
+		k.fatalf(msg, "cannot make / rslave: %v", optionalErrorUnwrap(err))
 	}
 
 	state := &setupState{Params: &params.Params, Msg: msg}
@@ -202,7 +202,7 @@ func initEntrypoint(k syscallDispatcher, msg message.Msg) {
 	}
 
 	if err := k.mount(SourceTmpfsRootfs, intermediateHostPath, FstypeTmpfs, MS_NODEV|MS_NOSUID, zeroString); err != nil {
-		k.fatalf(msg, "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(msg, "cannot enter intermediate host path: %v", err)
@@ -212,7 +212,7 @@ func initEntrypoint(k syscallDispatcher, msg message.Msg) {
 		k.fatalf(msg, "%v", err)
 	}
 	if err := k.mount(sysrootDir, sysrootDir, zeroString, MS_SILENT|MS_BIND|MS_REC, zeroString); err != nil {
-		k.fatalf(msg, "cannot bind sysroot: %v", err)
+		k.fatalf(msg, "cannot bind sysroot: %v", optionalErrorUnwrap(err))
 	}
 
 	if err := k.mkdir(hostDir, 0755); err != nil {
@@ -246,7 +246,7 @@ func initEntrypoint(k syscallDispatcher, msg message.Msg) {
 
 	// 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(msg, "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(msg, "cannot unmount host root: %v", err)

container/params.go

@@ -31,7 +31,7 @@ func Receive(key string, e any, fdp *uintptr) (func() error, error) {
 		return nil, ErrReceiveEnv
 	} else {
 		if fd, err := strconv.Atoi(s); err != nil {
-			return nil, errors.Unwrap(err)
+			return nil, optionalErrorUnwrap(err)
 		} else {
 			setup = os.NewFile(uintptr(fd), "setup")
 			if setup == nil {