Implement Sync() to directories

Updates #1035, #1199

PiperOrigin-RevId: 317028108

pkg/sentry/fsimpl/gofer/directory.go

@@ -299,3 +299,8 @@ func (fd *directoryFD) Seek(ctx context.Context, offset int64, whence int32) (in
 		return 0, syserror.EINVAL
 	}
 }
+
+// Sync implements vfs.FileDescriptionImpl.Sync.
+func (fd *directoryFD) Sync(ctx context.Context) error {
+	return fd.dentry().handle.sync(ctx)
+}

pkg/sentry/fsimpl/host/host.go

@@ -690,7 +690,8 @@ func (f *fileDescription) Seek(_ context.Context, offset int64, whence int32) (i
 
 // Sync implements FileDescriptionImpl.
 func (f *fileDescription) Sync(context.Context) error {
-	// TODO(gvisor.dev/issue/1672): Currently we do not support the SyncData optimization, so we always sync everything.
+	// TODO(gvisor.dev/issue/1672): Currently we do not support the SyncData
+	//  optimization, so we always sync everything.
 	return unix.Fsync(f.inode.hostFD)
 }
 

pkg/sentry/fsimpl/overlay/directory.go

@@ -263,3 +263,25 @@ func (fd *directoryFD) Seek(ctx context.Context, offset int64, whence int32) (in
 		return 0, syserror.EINVAL
 	}
 }
+
+// Sync implements vfs.FileDescriptionImpl.Sync. Forwards sync to the upper
+// layer, if there is one. The lower layer doesn't need to sync because it
+// never changes.
+func (fd *directoryFD) Sync(ctx context.Context) error {
+	d := fd.dentry()
+	if !d.isCopiedUp() {
+		return nil
+	}
+	vfsObj := d.fs.vfsfs.VirtualFilesystem()
+	pop := vfs.PathOperation{
+		Root:  d.upperVD,
+		Start: d.upperVD,
+	}
+	upperFD, err := vfsObj.OpenAt(ctx, d.fs.creds, &pop, &vfs.OpenOptions{Flags: linux.O_RDONLY | linux.O_DIRECTORY})
+	if err != nil {
+		return err
+	}
+	err = upperFD.Sync(ctx)
+	upperFD.DecRef()
+	return err
+}

pkg/sentry/fsimpl/tmpfs/regular_file.go

@@ -360,11 +360,6 @@ func (fd *regularFileFD) Seek(ctx context.Context, offset int64, whence int32) (
 	return offset, nil
 }
 
-// Sync implements vfs.FileDescriptionImpl.Sync.
-func (fd *regularFileFD) Sync(ctx context.Context) error {
-	return nil
-}
-
 // ConfigureMMap implements vfs.FileDescriptionImpl.ConfigureMMap.
 func (fd *regularFileFD) ConfigureMMap(ctx context.Context, opts *memmap.MMapOpts) error {
 	file := fd.inode().impl.(*regularFile)

pkg/sentry/fsimpl/tmpfs/tmpfs.go

@@ -778,3 +778,9 @@ func (fd *fileDescription) LockPOSIX(ctx context.Context, uid fslock.UniqueID, t
 func (fd *fileDescription) UnlockPOSIX(ctx context.Context, uid fslock.UniqueID, start, length uint64, whence int16) error {
 	return fd.Locks().UnlockPOSIX(ctx, &fd.vfsfd, uid, start, length, whence)
 }
+
+// Sync implements vfs.FileDescriptionImpl.Sync. It does nothing because all
+// filesystem state is in-memory.
+func (*fileDescription) Sync(context.Context) error {
+	return nil
+}