Merge release-20200522.0-145-g77c206e37 (automated)

13 files changed, 2855 insertions, 18 deletions

diff --git a/pkg/abi/linux/linux_abi_autogen_unsafe.go b/pkg/abi/linux/linux_abi_autogen_unsafe.go
index e7c11d25d..1e76a3491 100644
--- a/pkg/abi/linux/linux_abi_autogen_unsafe.go
+++ b/pkg/abi/linux/linux_abi_autogen_unsafe.go
@@ -124,7 +124,7 @@ func (s *Statx) UnmarshalBytes(src []byte) {
 // Packed implements marshal.Marshallable.Packed.
 //go:nosplit
 func (s *Statx) Packed() bool {
-    return s.Mtime.Packed() && s.Atime.Packed() && s.Btime.Packed() && s.Ctime.Packed()
+    return s.Atime.Packed() && s.Btime.Packed() && s.Ctime.Packed() && s.Mtime.Packed()
 }
 
 // MarshalUnsafe implements marshal.Marshallable.MarshalUnsafe.
@@ -138,7 +138,7 @@ func (s *Statx) MarshalUnsafe(dst []byte) {
 
 // UnmarshalUnsafe implements marshal.Marshallable.UnmarshalUnsafe.
 func (s *Statx) UnmarshalUnsafe(src []byte) {
-    if s.Atime.Packed() && s.Btime.Packed() && s.Ctime.Packed() && s.Mtime.Packed() {
+    if s.Ctime.Packed() && s.Mtime.Packed() && s.Atime.Packed() && s.Btime.Packed() {
         safecopy.CopyOut(unsafe.Pointer(s), src)
     } else {
         s.UnmarshalBytes(src)
@@ -148,7 +148,7 @@ func (s *Statx) UnmarshalUnsafe(src []byte) {
 // CopyOutN implements marshal.Marshallable.CopyOutN.
 //go:nosplit
 func (s *Statx) CopyOutN(task marshal.Task, addr usermem.Addr, limit int) (int, error) {
-    if !s.Ctime.Packed() && s.Mtime.Packed() && s.Atime.Packed() && s.Btime.Packed() {
+    if !s.Atime.Packed() && s.Btime.Packed() && s.Ctime.Packed() && s.Mtime.Packed() {
         // Type Statx doesn't have a packed layout in memory, fall back to MarshalBytes.
         buf := task.CopyScratchBuffer(s.SizeBytes()) // escapes: okay.
         s.MarshalBytes(buf) // escapes: fallback.
@@ -178,7 +178,7 @@ func (s *Statx) CopyOut(task marshal.Task, addr usermem.Addr) (int, error) {
 // CopyIn implements marshal.Marshallable.CopyIn.
 //go:nosplit
 func (s *Statx) CopyIn(task marshal.Task, addr usermem.Addr) (int, error) {
-    if !s.Ctime.Packed() && s.Mtime.Packed() && s.Atime.Packed() && s.Btime.Packed() {
+    if !s.Atime.Packed() && s.Btime.Packed() && s.Ctime.Packed() && s.Mtime.Packed() {
         // Type Statx doesn't have a packed layout in memory, fall back to UnmarshalBytes.
         buf := task.CopyScratchBuffer(s.SizeBytes()) // escapes: okay.
         length, err := task.CopyInBytes(addr, buf) // escapes: okay.
@@ -204,7 +204,7 @@ func (s *Statx) CopyIn(task marshal.Task, addr usermem.Addr) (int, error) {
 
 // WriteTo implements io.WriterTo.WriteTo.
 func (s *Statx) WriteTo(w io.Writer) (int64, error) {
-    if !s.Mtime.Packed() && s.Atime.Packed() && s.Btime.Packed() && s.Ctime.Packed() {
+    if !s.Ctime.Packed() && s.Mtime.Packed() && s.Atime.Packed() && s.Btime.Packed() {
         // Type Statx doesn't have a packed layout in memory, fall back to MarshalBytes.
         buf := make([]byte, s.SizeBytes())
         s.MarshalBytes(buf)
diff --git a/pkg/sentry/fsimpl/gofer/filesystem.go b/pkg/sentry/fsimpl/gofer/filesystem.go
index 40933b74b..3c467e313 100644
--- a/pkg/sentry/fsimpl/gofer/filesystem.go
+++ b/pkg/sentry/fsimpl/gofer/filesystem.go
@@ -118,7 +118,7 @@ func putDentrySlice(ds *[]*dentry) {
 // must be up to date.
 //
 // Postconditions: The returned dentry's cached metadata is up to date.
-func (fs *filesystem) stepLocked(ctx context.Context, rp *vfs.ResolvingPath, d *dentry, ds **[]*dentry) (*dentry, error) {
+func (fs *filesystem) stepLocked(ctx context.Context, rp *vfs.ResolvingPath, d *dentry, mayFollowSymlinks bool, ds **[]*dentry) (*dentry, error) {
 	if !d.isDir() {
 		return nil, syserror.ENOTDIR
 	}
@@ -168,7 +168,7 @@ afterSymlink:
 	if err := rp.CheckMount(&child.vfsd); err != nil {
 		return nil, err
 	}
-	if child.isSymlink() && rp.ShouldFollowSymlink() {
+	if child.isSymlink() && mayFollowSymlinks && rp.ShouldFollowSymlink() {
 		target, err := child.readlink(ctx, rp.Mount())
 		if err != nil {
 			return nil, err
@@ -275,7 +275,7 @@ func (fs *filesystem) revalidateChildLocked(ctx context.Context, vfsObj *vfs.Vir
 func (fs *filesystem) walkParentDirLocked(ctx context.Context, rp *vfs.ResolvingPath, d *dentry, ds **[]*dentry) (*dentry, error) {
 	for !rp.Final() {
 		d.dirMu.Lock()
-		next, err := fs.stepLocked(ctx, rp, d, ds)
+		next, err := fs.stepLocked(ctx, rp, d, true /* mayFollowSymlinks */, ds)
 		d.dirMu.Unlock()
 		if err != nil {
 			return nil, err
@@ -301,7 +301,7 @@ func (fs *filesystem) resolveLocked(ctx context.Context, rp *vfs.ResolvingPath,
 	}
 	for !rp.Done() {
 		d.dirMu.Lock()
-		next, err := fs.stepLocked(ctx, rp, d, ds)
+		next, err := fs.stepLocked(ctx, rp, d, true /* mayFollowSymlinks */, ds)
 		d.dirMu.Unlock()
 		if err != nil {
 			return nil, err
@@ -754,7 +754,7 @@ afterTrailingSymlink:
 	}
 	// Determine whether or not we need to create a file.
 	parent.dirMu.Lock()
-	child, err := fs.stepLocked(ctx, rp, parent, &ds)
+	child, err := fs.stepLocked(ctx, rp, parent, false /* mayFollowSymlinks */, &ds)
 	if err == syserror.ENOENT && mayCreate {
 		if parent.isSynthetic() {
 			parent.dirMu.Unlock()
@@ -939,7 +939,7 @@ func (d *dentry) createAndOpenChildLocked(ctx context.Context, rp *vfs.Resolving
 	// Filter file creation flags and O_LARGEFILE out; the create RPC already
 	// has the semantics of O_CREAT|O_EXCL, while some servers will choke on
 	// O_LARGEFILE.
-	createFlags := p9.OpenFlags(opts.Flags &^ (linux.O_CREAT | linux.O_EXCL | linux.O_NOCTTY | linux.O_TRUNC | linux.O_LARGEFILE))
+	createFlags := p9.OpenFlags(opts.Flags &^ (vfs.FileCreationFlags | linux.O_LARGEFILE))
 	fdobj, openFile, createQID, _, err := dirfile.create(ctx, name, createFlags, (p9.FileMode)(opts.Mode), (p9.UID)(creds.EffectiveKUID), (p9.GID)(creds.EffectiveKGID))
 	if err != nil {
 		dirfile.close(ctx)
diff --git a/pkg/sentry/fsimpl/kernfs/filesystem.go b/pkg/sentry/fsimpl/kernfs/filesystem.go
index 4a12ae245..8939871c1 100644
--- a/pkg/sentry/fsimpl/kernfs/filesystem.go
+++ b/pkg/sentry/fsimpl/kernfs/filesystem.go
@@ -35,7 +35,7 @@ import (
 // Preconditions: Filesystem.mu must be locked for at least reading. !rp.Done().
 //
 // Postcondition: Caller must call fs.processDeferredDecRefs*.
-func (fs *Filesystem) stepExistingLocked(ctx context.Context, rp *vfs.ResolvingPath, vfsd *vfs.Dentry) (*vfs.Dentry, error) {
+func (fs *Filesystem) stepExistingLocked(ctx context.Context, rp *vfs.ResolvingPath, vfsd *vfs.Dentry, mayFollowSymlinks bool) (*vfs.Dentry, error) {
 	d := vfsd.Impl().(*Dentry)
 	if !d.isDir() {
 		return nil, syserror.ENOTDIR
@@ -81,7 +81,7 @@ afterSymlink:
 		return nil, err
 	}
 	// Resolve any symlink at current path component.
-	if rp.ShouldFollowSymlink() && next.isSymlink() {
+	if mayFollowSymlinks && rp.ShouldFollowSymlink() && next.isSymlink() {
 		targetVD, targetPathname, err := next.inode.Getlink(ctx, rp.Mount())
 		if err != nil {
 			return nil, err
@@ -152,7 +152,7 @@ func (fs *Filesystem) walkExistingLocked(ctx context.Context, rp *vfs.ResolvingP
 	vfsd := rp.Start()
 	for !rp.Done() {
 		var err error
-		vfsd, err = fs.stepExistingLocked(ctx, rp, vfsd)
+		vfsd, err = fs.stepExistingLocked(ctx, rp, vfsd, true /* mayFollowSymlinks */)
 		if err != nil {
 			return nil, nil, err
 		}
@@ -178,7 +178,7 @@ func (fs *Filesystem) walkParentDirLocked(ctx context.Context, rp *vfs.Resolving
 	vfsd := rp.Start()
 	for !rp.Final() {
 		var err error
-		vfsd, err = fs.stepExistingLocked(ctx, rp, vfsd)
+		vfsd, err = fs.stepExistingLocked(ctx, rp, vfsd, true /* mayFollowSymlinks */)
 		if err != nil {
 			return nil, nil, err
 		}
@@ -449,7 +449,7 @@ afterTrailingSymlink:
 		return nil, syserror.ENAMETOOLONG
 	}
 	// Determine whether or not we need to create a file.
-	childVFSD, err := fs.stepExistingLocked(ctx, rp, parentVFSD)
+	childVFSD, err := fs.stepExistingLocked(ctx, rp, parentVFSD, false /* mayFollowSymlinks */)
 	if err == syserror.ENOENT {
 		// Already checked for searchability above; now check for writability.
 		if err := parentInode.CheckPermissions(ctx, rp.Credentials(), vfs.MayWrite); err != nil {
diff --git a/pkg/sentry/fsimpl/overlay/copy_up.go b/pkg/sentry/fsimpl/overlay/copy_up.go
new file mode 100644
index 000000000..8f8dcfafe
--- /dev/null
+++ b/pkg/sentry/fsimpl/overlay/copy_up.go
@@ -0,0 +1,262 @@
+// Copyright 2020 The gVisor Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package overlay
+
+import (
+	"fmt"
+	"io"
+	"sync/atomic"
+
+	"gvisor.dev/gvisor/pkg/abi/linux"
+	"gvisor.dev/gvisor/pkg/context"
+	"gvisor.dev/gvisor/pkg/fspath"
+	"gvisor.dev/gvisor/pkg/sentry/vfs"
+	"gvisor.dev/gvisor/pkg/syserror"
+	"gvisor.dev/gvisor/pkg/usermem"
+)
+
+func (d *dentry) isCopiedUp() bool {
+	return atomic.LoadUint32(&d.copiedUp) != 0
+}
+
+// copyUpLocked ensures that d exists on the upper layer, i.e. d.upperVD.Ok().
+//
+// Preconditions: filesystem.renameMu must be locked.
+func (d *dentry) copyUpLocked(ctx context.Context) error {
+	// Fast path.
+	if d.isCopiedUp() {
+		return nil
+	}
+
+	ftype := atomic.LoadUint32(&d.mode) & linux.S_IFMT
+	switch ftype {
+	case linux.S_IFREG, linux.S_IFDIR, linux.S_IFLNK, linux.S_IFBLK, linux.S_IFCHR:
+		// Can be copied-up.
+	default:
+		// Can't be copied-up.
+		return syserror.EPERM
+	}
+
+	// Ensure that our parent directory is copied-up.
+	if d.parent == nil {
+		// d is a filesystem root with no upper layer.
+		return syserror.EROFS
+	}
+	if err := d.parent.copyUpLocked(ctx); err != nil {
+		return err
+	}
+
+	d.copyMu.Lock()
+	defer d.copyMu.Unlock()
+	if d.upperVD.Ok() {
+		// Raced with another call to d.copyUpLocked().
+		return nil
+	}
+	if d.vfsd.IsDead() {
+		// Raced with deletion of d.
+		return syserror.ENOENT
+	}
+
+	// Perform copy-up.
+	vfsObj := d.fs.vfsfs.VirtualFilesystem()
+	newpop := vfs.PathOperation{
+		Root:  d.parent.upperVD,
+		Start: d.parent.upperVD,
+		Path:  fspath.Parse(d.name),
+	}
+	cleanupUndoCopyUp := func() {
+		var err error
+		if ftype == linux.S_IFDIR {
+			err = vfsObj.RmdirAt(ctx, d.fs.creds, &newpop)
+		} else {
+			err = vfsObj.UnlinkAt(ctx, d.fs.creds, &newpop)
+		}
+		if err != nil {
+			ctx.Warningf("Unrecoverable overlayfs inconsistency: failed to delete upper layer file after copy-up error: %v", err)
+		}
+	}
+	switch ftype {
+	case linux.S_IFREG:
+		oldFD, err := vfsObj.OpenAt(ctx, d.fs.creds, &vfs.PathOperation{
+			Root:  d.lowerVDs[0],
+			Start: d.lowerVDs[0],
+		}, &vfs.OpenOptions{
+			Flags: linux.O_RDONLY,
+		})
+		if err != nil {
+			return err
+		}
+		defer oldFD.DecRef()
+		newFD, err := vfsObj.OpenAt(ctx, d.fs.creds, &newpop, &vfs.OpenOptions{
+			Flags: linux.O_WRONLY | linux.O_CREAT | linux.O_EXCL,
+			Mode:  linux.FileMode(d.mode &^ linux.S_IFMT),
+		})
+		if err != nil {
+			return err
+		}
+		defer newFD.DecRef()
+		bufIOSeq := usermem.BytesIOSequence(make([]byte, 32*1024)) // arbitrary buffer size
+		for {
+			readN, readErr := oldFD.Read(ctx, bufIOSeq, vfs.ReadOptions{})
+			if readErr != nil && readErr != io.EOF {
+				cleanupUndoCopyUp()
+				return readErr
+			}
+			total := int64(0)
+			for total < readN {
+				writeN, writeErr := newFD.Write(ctx, bufIOSeq.DropFirst64(total), vfs.WriteOptions{})
+				total += writeN
+				if writeErr != nil {
+					cleanupUndoCopyUp()
+					return writeErr
+				}
+			}
+			if readErr == io.EOF {
+				break
+			}
+		}
+		if err := newFD.SetStat(ctx, vfs.SetStatOptions{
+			Stat: linux.Statx{
+				Mask: linux.STATX_UID | linux.STATX_GID,
+				UID:  d.uid,
+				GID:  d.gid,
+			},
+		}); err != nil {
+			cleanupUndoCopyUp()
+			return err
+		}
+		d.upperVD = newFD.VirtualDentry()
+		d.upperVD.IncRef()
+
+	case linux.S_IFDIR:
+		if err := vfsObj.MkdirAt(ctx, d.fs.creds, &newpop, &vfs.MkdirOptions{
+			Mode: linux.FileMode(d.mode &^ linux.S_IFMT),
+		}); err != nil {
+			return err
+		}
+		if err := vfsObj.SetStatAt(ctx, d.fs.creds, &newpop, &vfs.SetStatOptions{
+			Stat: linux.Statx{
+				Mask: linux.STATX_UID | linux.STATX_GID,
+				UID:  d.uid,
+				GID:  d.gid,
+			},
+		}); err != nil {
+			cleanupUndoCopyUp()
+			return err
+		}
+		upperVD, err := vfsObj.GetDentryAt(ctx, d.fs.creds, &newpop, &vfs.GetDentryOptions{})
+		if err != nil {
+			cleanupUndoCopyUp()
+			return err
+		}
+		d.upperVD = upperVD
+
+	case linux.S_IFLNK:
+		target, err := vfsObj.ReadlinkAt(ctx, d.fs.creds, &vfs.PathOperation{
+			Root:  d.lowerVDs[0],
+			Start: d.lowerVDs[0],
+		})
+		if err != nil {
+			return err
+		}
+		if err := vfsObj.SymlinkAt(ctx, d.fs.creds, &newpop, target); err != nil {
+			return err
+		}
+		if err := vfsObj.SetStatAt(ctx, d.fs.creds, &newpop, &vfs.SetStatOptions{
+			Stat: linux.Statx{
+				Mask: linux.STATX_MODE | linux.STATX_UID | linux.STATX_GID,
+				Mode: uint16(d.mode),
+				UID:  d.uid,
+				GID:  d.gid,
+			},
+		}); err != nil {
+			cleanupUndoCopyUp()
+			return err
+		}
+		upperVD, err := vfsObj.GetDentryAt(ctx, d.fs.creds, &newpop, &vfs.GetDentryOptions{})
+		if err != nil {
+			cleanupUndoCopyUp()
+			return err
+		}
+		d.upperVD = upperVD
+
+	case linux.S_IFBLK, linux.S_IFCHR:
+		lowerStat, err := vfsObj.StatAt(ctx, d.fs.creds, &vfs.PathOperation{
+			Root:  d.lowerVDs[0],
+			Start: d.lowerVDs[0],
+		}, &vfs.StatOptions{})
+		if err != nil {
+			return err
+		}
+		if err := vfsObj.MknodAt(ctx, d.fs.creds, &newpop, &vfs.MknodOptions{
+			Mode:     linux.FileMode(d.mode),
+			DevMajor: lowerStat.RdevMajor,
+			DevMinor: lowerStat.RdevMinor,
+		}); err != nil {
+			return err
+		}
+		if err := vfsObj.SetStatAt(ctx, d.fs.creds, &newpop, &vfs.SetStatOptions{
+			Stat: linux.Statx{
+				Mask: linux.STATX_UID | linux.STATX_GID,
+				UID:  d.uid,
+				GID:  d.gid,
+			},
+		}); err != nil {
+			cleanupUndoCopyUp()
+			return err
+		}
+		upperVD, err := vfsObj.GetDentryAt(ctx, d.fs.creds, &newpop, &vfs.GetDentryOptions{})
+		if err != nil {
+			cleanupUndoCopyUp()
+			return err
+		}
+		d.upperVD = upperVD
+
+	default:
+		// Should have rejected this at the beginning of this function?
+		panic(fmt.Sprintf("unexpected file type %o", ftype))
+	}
+
+	// TODO(gvisor.dev/issue/1199): copy up xattrs
+
+	// Update the dentry's device and inode numbers (except for directories,
+	// for which these remain overlay-assigned).
+	if ftype != linux.S_IFDIR {
+		upperStat, err := vfsObj.StatAt(ctx, d.fs.creds, &vfs.PathOperation{
+			Root:  d.upperVD,
+			Start: d.upperVD,
+		}, &vfs.StatOptions{
+			Mask: linux.STATX_INO,
+		})
+		if err != nil {
+			d.upperVD.DecRef()
+			d.upperVD = vfs.VirtualDentry{}
+			cleanupUndoCopyUp()
+			return err
+		}
+		if upperStat.Mask&linux.STATX_INO == 0 {
+			d.upperVD.DecRef()
+			d.upperVD = vfs.VirtualDentry{}
+			cleanupUndoCopyUp()
+			return syserror.EREMOTE
+		}
+		atomic.StoreUint32(&d.devMajor, upperStat.DevMajor)
+		atomic.StoreUint32(&d.devMinor, upperStat.DevMinor)
+		atomic.StoreUint64(&d.ino, upperStat.Ino)
+	}
+
+	atomic.StoreUint32(&d.copiedUp, 1)
+	return nil
+}
diff --git a/pkg/sentry/fsimpl/overlay/directory.go b/pkg/sentry/fsimpl/overlay/directory.go
new file mode 100644
index 000000000..6f47167d3
--- /dev/null
+++ b/pkg/sentry/fsimpl/overlay/directory.go
@@ -0,0 +1,265 @@
+// Copyright 2020 The gVisor Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package overlay
+
+import (
+	"sync/atomic"
+
+	"gvisor.dev/gvisor/pkg/abi/linux"
+	"gvisor.dev/gvisor/pkg/context"
+	"gvisor.dev/gvisor/pkg/fspath"
+	"gvisor.dev/gvisor/pkg/sentry/vfs"
+	"gvisor.dev/gvisor/pkg/sync"
+	"gvisor.dev/gvisor/pkg/syserror"
+)
+
+func (d *dentry) isDir() bool {
+	return atomic.LoadUint32(&d.mode)&linux.S_IFMT == linux.S_IFDIR
+}
+
+// Preconditions: d.dirMu must be locked. d.isDir().
+func (d *dentry) collectWhiteoutsForRmdirLocked(ctx context.Context) (map[string]bool, error) {
+	vfsObj := d.fs.vfsfs.VirtualFilesystem()
+	var readdirErr error
+	whiteouts := make(map[string]bool)
+	var maybeWhiteouts []string
+	d.iterLayers(func(layerVD vfs.VirtualDentry, isUpper bool) bool {
+		layerFD, err := vfsObj.OpenAt(ctx, d.fs.creds, &vfs.PathOperation{
+			Root:  layerVD,
+			Start: layerVD,
+		}, &vfs.OpenOptions{
+			Flags: linux.O_RDONLY | linux.O_DIRECTORY,
+		})
+		if err != nil {
+			readdirErr = err
+			return false
+		}
+		defer layerFD.DecRef()
+
+		// Reuse slice allocated for maybeWhiteouts from a previous layer to
+		// reduce allocations.
+		maybeWhiteouts = maybeWhiteouts[:0]
+		if err := layerFD.IterDirents(ctx, vfs.IterDirentsCallbackFunc(func(dirent vfs.Dirent) error {
+			if dirent.Name == "." || dirent.Name == ".." {
+				return nil
+			}
+			if _, ok := whiteouts[dirent.Name]; ok {
+				// This file has been whited-out in a previous layer.
+				return nil
+			}
+			if dirent.Type == linux.DT_CHR {
+				// We have to determine if this is a whiteout, which doesn't
+				// count against the directory's emptiness. However, we can't
+				// do so while holding locks held by layerFD.IterDirents().
+				maybeWhiteouts = append(maybeWhiteouts, dirent.Name)
+				return nil
+			}
+			// Non-whiteout file in the directory prevents rmdir.
+			return syserror.ENOTEMPTY
+		})); err != nil {
+			readdirErr = err
+			return false
+		}
+
+		for _, maybeWhiteoutName := range maybeWhiteouts {
+			stat, err := vfsObj.StatAt(ctx, d.fs.creds, &vfs.PathOperation{
+				Root:  layerVD,
+				Start: layerVD,
+				Path:  fspath.Parse(maybeWhiteoutName),
+			}, &vfs.StatOptions{})
+			if err != nil {
+				readdirErr = err
+				return false
+			}
+			if stat.RdevMajor != 0 || stat.RdevMinor != 0 {
+				// This file is a real character device, not a whiteout.
+				readdirErr = syserror.ENOTEMPTY
+				return false
+			}
+			whiteouts[maybeWhiteoutName] = isUpper
+		}
+		// Continue iteration since we haven't found any non-whiteout files in
+		// this directory yet.
+		return true
+	})
+	return whiteouts, readdirErr
+}
+
+type directoryFD struct {
+	fileDescription
+	vfs.DirectoryFileDescriptionDefaultImpl
+	vfs.DentryMetadataFileDescriptionImpl
+
+	mu      sync.Mutex
+	off     int64
+	dirents []vfs.Dirent
+}
+
+// Release implements vfs.FileDescriptionImpl.Release.
+func (fd *directoryFD) Release() {
+}
+
+// IterDirents implements vfs.FileDescriptionImpl.IterDirents.
+func (fd *directoryFD) IterDirents(ctx context.Context, cb vfs.IterDirentsCallback) error {
+	fd.mu.Lock()
+	defer fd.mu.Unlock()
+
+	d := fd.dentry()
+	if fd.dirents == nil {
+		ds, err := d.getDirents(ctx)
+		if err != nil {
+			return err
+		}
+		fd.dirents = ds
+	}
+
+	for fd.off < int64(len(fd.dirents)) {
+		if err := cb.Handle(fd.dirents[fd.off]); err != nil {
+			return err
+		}
+		fd.off++
+	}
+	return nil
+}
+
+// Preconditions: d.isDir().
+func (d *dentry) getDirents(ctx context.Context) ([]vfs.Dirent, error) {
+	d.fs.renameMu.RLock()
+	defer d.fs.renameMu.RUnlock()
+	d.dirMu.Lock()
+	defer d.dirMu.Unlock()
+
+	if d.dirents != nil {
+		return d.dirents, nil
+	}
+
+	parent := genericParentOrSelf(d)
+	dirents := []vfs.Dirent{
+		{
+			Name:    ".",
+			Type:    linux.DT_DIR,
+			Ino:     d.ino,
+			NextOff: 1,
+		},
+		{
+			Name:    "..",
+			Type:    uint8(atomic.LoadUint32(&parent.mode) >> 12),
+			Ino:     parent.ino,
+			NextOff: 2,
+		},
+	}
+
+	// Merge dirents from all layers comprising this directory.
+	vfsObj := d.fs.vfsfs.VirtualFilesystem()
+	var readdirErr error
+	prevDirents := make(map[string]struct{})
+	var maybeWhiteouts []vfs.Dirent
+	d.iterLayers(func(layerVD vfs.VirtualDentry, isUpper bool) bool {
+		layerFD, err := vfsObj.OpenAt(ctx, d.fs.creds, &vfs.PathOperation{
+			Root:  layerVD,
+			Start: layerVD,
+		}, &vfs.OpenOptions{
+			Flags: linux.O_RDONLY | linux.O_DIRECTORY,
+		})
+		if err != nil {
+			readdirErr = err
+			return false
+		}
+		defer layerFD.DecRef()
+
+		// Reuse slice allocated for maybeWhiteouts from a previous layer to
+		// reduce allocations.
+		maybeWhiteouts = maybeWhiteouts[:0]
+		if err := layerFD.IterDirents(ctx, vfs.IterDirentsCallbackFunc(func(dirent vfs.Dirent) error {
+			if dirent.Name == "." || dirent.Name == ".." {
+				return nil
+			}
+			if _, ok := prevDirents[dirent.Name]; ok {
+				// This file is hidden by, or merged with, another file with
+				// the same name in a previous layer.
+				return nil
+			}
+			prevDirents[dirent.Name] = struct{}{}
+			if dirent.Type == linux.DT_CHR {
+				// We can't determine if this file is a whiteout while holding
+				// locks held by layerFD.IterDirents().
+				maybeWhiteouts = append(maybeWhiteouts, dirent)
+				return nil
+			}
+			dirent.NextOff = int64(len(dirents) + 1)
+			dirents = append(dirents, dirent)
+			return nil
+		})); err != nil {
+			readdirErr = err
+			return false
+		}
+
+		for _, dirent := range maybeWhiteouts {
+			stat, err := vfsObj.StatAt(ctx, d.fs.creds, &vfs.PathOperation{
+				Root:  layerVD,
+				Start: layerVD,
+				Path:  fspath.Parse(dirent.Name),
+			}, &vfs.StatOptions{})
+			if err != nil {
+				readdirErr = err
+				return false
+			}
+			if stat.RdevMajor == 0 && stat.RdevMinor == 0 {
+				// This file is a whiteout; don't emit a dirent for it.
+				continue
+			}
+			dirent.NextOff = int64(len(dirents) + 1)
+			dirents = append(dirents, dirent)
+		}
+		return true
+	})
+	if readdirErr != nil {
+		return nil, readdirErr
+	}
+
+	// Cache dirents for future directoryFDs.
+	d.dirents = dirents
+	return dirents, nil
+}
+
+// Seek implements vfs.FileDescriptionImpl.Seek.
+func (fd *directoryFD) Seek(ctx context.Context, offset int64, whence int32) (int64, error) {
+	fd.mu.Lock()
+	defer fd.mu.Unlock()
+
+	switch whence {
+	case linux.SEEK_SET:
+		if offset < 0 {
+			return 0, syserror.EINVAL
+		}
+		if offset == 0 {
+			// Ensure that the next call to fd.IterDirents() calls
+			// fd.dentry().getDirents().
+			fd.dirents = nil
+		}
+		fd.off = offset
+		return fd.off, nil
+	case linux.SEEK_CUR:
+		offset += fd.off
+		if offset < 0 {
+			return 0, syserror.EINVAL
+		}
+		// Don't clear fd.dirents in this case, even if offset == 0.
+		fd.off = offset
+		return fd.off, nil
+	default:
+		return 0, syserror.EINVAL
+	}
+}
diff --git a/pkg/sentry/fsimpl/overlay/filesystem.go b/pkg/sentry/fsimpl/overlay/filesystem.go
new file mode 100644
index 000000000..ff82e1f20
--- /dev/null
+++ b/pkg/sentry/fsimpl/overlay/filesystem.go
@@ -0,0 +1,1364 @@
+// Copyright 2020 The gVisor Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package overlay
+
+import (
+	"sync/atomic"
+
+	"gvisor.dev/gvisor/pkg/abi/linux"
+	"gvisor.dev/gvisor/pkg/context"
+	"gvisor.dev/gvisor/pkg/fspath"
+	"gvisor.dev/gvisor/pkg/sentry/kernel/auth"
+	"gvisor.dev/gvisor/pkg/sentry/socket/unix/transport"
+	"gvisor.dev/gvisor/pkg/sentry/vfs"
+	"gvisor.dev/gvisor/pkg/sync"
+	"gvisor.dev/gvisor/pkg/syserror"
+)
+
+// _OVL_XATTR_OPAQUE is an extended attribute key whose value is set to "y" for
+// opaque directories.
+// Linux: fs/overlayfs/overlayfs.h:OVL_XATTR_OPAQUE
+const _OVL_XATTR_OPAQUE = "trusted.overlay.opaque"
+
+func isWhiteout(stat *linux.Statx) bool {
+	return stat.Mode&linux.S_IFMT == linux.S_IFCHR && stat.RdevMajor == 0 && stat.RdevMinor == 0
+}
+
+// Sync implements vfs.FilesystemImpl.Sync.
+func (fs *filesystem) Sync(ctx context.Context) error {
+	if fs.opts.UpperRoot.Ok() {
+		return fs.opts.UpperRoot.Mount().Filesystem().Impl().Sync(ctx)
+	}
+	return nil
+}
+
+var dentrySlicePool = sync.Pool{
+	New: func() interface{} {
+		ds := make([]*dentry, 0, 4) // arbitrary non-zero initial capacity
+		return &ds
+	},
+}
+
+func appendDentry(ds *[]*dentry, d *dentry) *[]*dentry {
+	if ds == nil {
+		ds = dentrySlicePool.Get().(*[]*dentry)
+	}
+	*ds = append(*ds, d)
+	return ds
+}
+
+// Preconditions: ds != nil.
+func putDentrySlice(ds *[]*dentry) {
+	// Allow dentries to be GC'd.
+	for i := range *ds {
+		(*ds)[i] = nil
+	}
+	*ds = (*ds)[:0]
+	dentrySlicePool.Put(ds)
+}
+
+// renameMuRUnlockAndCheckDrop calls fs.renameMu.RUnlock(), then calls
+// dentry.checkDropLocked on all dentries in *ds with fs.renameMu locked for
+// writing.
+//
+// ds is a pointer-to-pointer since defer evaluates its arguments immediately,
+// but dentry slices are allocated lazily, and it's much easier to say "defer
+// fs.renameMuRUnlockAndCheckDrop(&ds)" than "defer func() {
+// fs.renameMuRUnlockAndCheckDrop(ds) }()" to work around this.
+func (fs *filesystem) renameMuRUnlockAndCheckDrop(ds **[]*dentry) {
+	fs.renameMu.RUnlock()
+	if *ds == nil {
+		return
+	}
+	if len(**ds) != 0 {
+		fs.renameMu.Lock()
+		for _, d := range **ds {
+			d.checkDropLocked()
+		}
+		fs.renameMu.Unlock()
+	}
+	putDentrySlice(*ds)
+}
+
+func (fs *filesystem) renameMuUnlockAndCheckDrop(ds **[]*dentry) {
+	if *ds == nil {
+		fs.renameMu.Unlock()
+		return
+	}
+	for _, d := range **ds {
+		d.checkDropLocked()
+	}
+	fs.renameMu.Unlock()
+	putDentrySlice(*ds)
+}
+
+// stepLocked resolves rp.Component() to an existing file, starting from the
+// given directory.
+//
+// Dentries which may have a reference count of zero, and which therefore
+// should be dropped once traversal is complete, are appended to ds.
+//
+// Preconditions: fs.renameMu must be locked. d.dirMu must be locked.
+// !rp.Done().
+func (fs *filesystem) stepLocked(ctx context.Context, rp *vfs.ResolvingPath, d *dentry, mayFollowSymlinks bool, ds **[]*dentry) (*dentry, error) {
+	if !d.isDir() {
+		return nil, syserror.ENOTDIR
+	}
+	if err := d.checkPermissions(rp.Credentials(), vfs.MayExec); err != nil {
+		return nil, err
+	}
+afterSymlink:
+	name := rp.Component()
+	if name == "." {
+		rp.Advance()
+		return d, nil
+	}
+	if name == ".." {
+		if isRoot, err := rp.CheckRoot(&d.vfsd); err != nil {
+			return nil, err
+		} else if isRoot || d.parent == nil {
+			rp.Advance()
+			return d, nil
+		}
+		if err := rp.CheckMount(&d.parent.vfsd); err != nil {
+			return nil, err
+		}
+		rp.Advance()
+		return d.parent, nil
+	}
+	child, err := fs.getChildLocked(ctx, d, name, ds)
+	if err != nil {
+		return nil, err
+	}
+	if err := rp.CheckMount(&child.vfsd); err != nil {
+		return nil, err
+	}
+	if child.isSymlink() && mayFollowSymlinks && rp.ShouldFollowSymlink() {
+		target, err := child.readlink(ctx)
+		if err != nil {
+			return nil, err
+		}
+		if err := rp.HandleSymlink(target); err != nil {
+			return nil, err
+		}
+		goto afterSymlink // don't check the current directory again
+	}
+	rp.Advance()
+	return child, nil
+}
+
+// Preconditions: fs.renameMu must be locked. d.dirMu must be locked.
+func (fs *filesystem) getChildLocked(ctx context.Context, parent *dentry, name string, ds **[]*dentry) (*dentry, error) {
+	if child, ok := parent.children[name]; ok {
+		return child, nil
+	}
+	child, err := fs.lookupLocked(ctx, parent, name)
+	if err != nil {
+		return nil, err
+	}
+	if parent.children == nil {
+		parent.children = make(map[string]*dentry)
+	}
+	parent.children[name] = child
+	// child's refcount is initially 0, so it may be dropped after traversal.
+	*ds = appendDentry(*ds, child)
+	return child, nil
+}
+
+// Preconditions: fs.renameMu must be locked. parent.dirMu must be locked.
+func (fs *filesystem) lookupLocked(ctx context.Context, parent *dentry, name string) (*dentry, error) {
+	childPath := fspath.Parse(name)
+	child := fs.newDentry()
+	existsOnAnyLayer := false
+	var lookupErr error
+
+	vfsObj := fs.vfsfs.VirtualFilesystem()
+	parent.iterLayers(func(parentVD vfs.VirtualDentry, isUpper bool) bool {
+		childVD, err := vfsObj.GetDentryAt(ctx, fs.creds, &vfs.PathOperation{
+			Root:  parentVD,
+			Start: parentVD,
+			Path:  childPath,
+		}, &vfs.GetDentryOptions{})
+		if err == syserror.ENOENT || err == syserror.ENAMETOOLONG {
+			// The file doesn't exist on this layer. Proceed to the next one.
+			return true
+		}
+		if err != nil {
+			lookupErr = err
+			return false
+		}
+
+		mask := uint32(linux.STATX_TYPE)
+		if !existsOnAnyLayer {
+			// Mode, UID, GID, and (for non-directories) inode number come from
+			// the topmost layer on which the file exists.
+			mask |= linux.STATX_MODE | linux.STATX_UID | linux.STATX_GID | linux.STATX_INO
+		}
+		stat, err := vfsObj.StatAt(ctx, fs.creds, &vfs.PathOperation{
+			Root:  childVD,
+			Start: childVD,
+		}, &vfs.StatOptions{
+			Mask: mask,
+		})
+		if err != nil {
+			lookupErr = err
+			return false
+		}
+		if stat.Mask&mask != mask {
+			lookupErr = syserror.EREMOTE
+			return false
+		}
+
+		if isWhiteout(&stat) {
+			// This is a whiteout, so it "doesn't exist" on this layer, and
+			// layers below this one are ignored.
+			return false
+		}
+		isDir := stat.Mode&linux.S_IFMT == linux.S_IFDIR
+		if existsOnAnyLayer && !isDir {
+			// Directories are not merged with non-directory files from lower
+			// layers; instead, layers including and below the first
+			// non-directory file are ignored. (This file must be a directory
+			// on previous layers, since lower layers aren't searched for
+			// non-directory files.)
+			return false
+		}
+
+		// Update child to include this layer.
+		if isUpper {
+			child.upperVD = childVD
+			child.copiedUp = 1
+		} else {
+			child.lowerVDs = append(child.lowerVDs, childVD)
+		}
+		if !existsOnAnyLayer {
+			existsOnAnyLayer = true
+			child.mode = uint32(stat.Mode)
+			child.uid = stat.UID
+			child.gid = stat.GID
+			child.devMajor = stat.DevMajor
+			child.devMinor = stat.DevMinor
+			child.ino = stat.Ino
+		}
+
+		// For non-directory files, only the topmost layer that contains a file
+		// matters.
+		if !isDir {
+			return false
+		}
+
+		// Directories are merged with directories from lower layers if they
+		// are not explicitly opaque.
+		opaqueVal, err := vfsObj.GetxattrAt(ctx, fs.creds, &vfs.PathOperation{
+			Root:  childVD,
+			Start: childVD,
+		}, &vfs.GetxattrOptions{
+			Name: _OVL_XATTR_OPAQUE,
+			Size: 1,
+		})
+		return !(err == nil && opaqueVal == "y")
+	})
+
+	if lookupErr != nil {
+		child.destroyLocked()
+		return nil, lookupErr
+	}
+	if !existsOnAnyLayer {
+		child.destroyLocked()
+		return nil, syserror.ENOENT
+	}
+
+	// Device and inode numbers were copied from the topmost layer above;
+	// override them if necessary.
+	if child.isDir() {
+		child.devMajor = linux.UNNAMED_MAJOR
+		child.devMinor = fs.dirDevMinor
+		child.ino = fs.newDirIno()
+	} else if !child.upperVD.Ok() {
+		child.devMajor = linux.UNNAMED_MAJOR
+		child.devMinor = fs.lowerDevMinors[child.lowerVDs[0].Mount().Filesystem()]
+	}
+
+	parent.IncRef()
+	child.parent = parent
+	child.name = name
+	return child, nil
+}
+
+// lookupLayerLocked is similar to lookupLocked, but only returns information
+// about the file rather than a dentry.
+//
+// Preconditions: fs.renameMu must be locked. parent.dirMu must be locked.
+func (fs *filesystem) lookupLayerLocked(ctx context.Context, parent *dentry, name string) (lookupLayer, error) {
+	childPath := fspath.Parse(name)
+	lookupLayer := lookupLayerNone
+	var lookupErr error
+
+	parent.iterLayers(func(parentVD vfs.VirtualDentry, isUpper bool) bool {
+		stat, err := fs.vfsfs.VirtualFilesystem().StatAt(ctx, fs.creds, &vfs.PathOperation{
+			Root:  parentVD,
+			Start: parentVD,
+			Path:  childPath,
+		}, &vfs.StatOptions{
+			Mask: linux.STATX_TYPE,
+		})
+		if err == syserror.ENOENT || err == syserror.ENAMETOOLONG {
+			// The file doesn't exist on this layer. Proceed to the next
+			// one.
+			return true
+		}
+		if err != nil {
+			lookupErr = err
+			return false
+		}
+		if stat.Mask&linux.STATX_TYPE == 0 {
+			// Linux's overlayfs tends to return EREMOTE in cases where a file
+			// is unusable for reasons that are not better captured by another
+			// errno.
+			lookupErr = syserror.EREMOTE
+			return false
+		}
+		if isWhiteout(&stat) {
+			// This is a whiteout, so it "doesn't exist" on this layer, and
+			// layers below this one are ignored.
+			if isUpper {
+				lookupLayer = lookupLayerUpperWhiteout
+			}
+			return false
+		}
+		// The file exists; we can stop searching.
+		if isUpper {
+			lookupLayer = lookupLayerUpper
+		} else {
+			lookupLayer = lookupLayerLower
+		}
+		return false
+	})
+
+	return lookupLayer, lookupErr
+}
+
+type lookupLayer int
+
+const (
+	// lookupLayerNone indicates that no file exists at the given path on the
+	// upper layer, and is either whited out or does not exist on lower layers.
+	// Therefore, the file does not exist in the overlay filesystem, and file
+	// creation may proceed normally (if an upper layer exists).
+	lookupLayerNone lookupLayer = iota
+
+	// lookupLayerLower indicates that no file exists at the given path on the
+	// upper layer, but exists on a lower layer. Therefore, the file exists in
+	// the overlay filesystem, but must be copied-up before mutation.
+	lookupLayerLower
+
+	// lookupLayerUpper indicates that a non-whiteout file exists at the given
+	// path on the upper layer. Therefore, the file exists in the overlay
+	// filesystem, and is already copied-up.
+	lookupLayerUpper
+
+	// lookupLayerUpperWhiteout indicates that a whiteout exists at the given
+	// path on the upper layer. Therefore, the file does not exist in the
+	// overlay filesystem, and file creation must remove the whiteout before
+	// proceeding.
+	lookupLayerUpperWhiteout
+)
+
+func (ll lookupLayer) existsInOverlay() bool {
+	return ll == lookupLayerLower || ll == lookupLayerUpper
+}
+
+// walkParentDirLocked resolves all but the last path component of rp to an
+// existing directory, starting from the given directory (which is usually
+// rp.Start().Impl().(*dentry)). It does not check that the returned directory
+// is searchable by the provider of rp.
+//
+// Preconditions: fs.renameMu must be locked. !rp.Done().
+func (fs *filesystem) walkParentDirLocked(ctx context.Context, rp *vfs.ResolvingPath, d *dentry, ds **[]*dentry) (*dentry, error) {
+	for !rp.Final() {
+		d.dirMu.Lock()
+		next, err := fs.stepLocked(ctx, rp, d, true /* mayFollowSymlinks */, ds)
+		d.dirMu.Unlock()
+		if err != nil {
+			return nil, err
+		}
+		d = next
+	}
+	if !d.isDir() {
+		return nil, syserror.ENOTDIR
+	}
+	return d, nil
+}
+
+// resolveLocked resolves rp to an existing file.
+//
+// Preconditions: fs.renameMu must be locked.
+func (fs *filesystem) resolveLocked(ctx context.Context, rp *vfs.ResolvingPath, ds **[]*dentry) (*dentry, error) {
+	d := rp.Start().Impl().(*dentry)
+	for !rp.Done() {
+		d.dirMu.Lock()
+		next, err := fs.stepLocked(ctx, rp, d, true /* mayFollowSymlinks */, ds)
+		d.dirMu.Unlock()
+		if err != nil {
+			return nil, err
+		}
+		d = next
+	}
+	if rp.MustBeDir() && !d.isDir() {
+		return nil, syserror.ENOTDIR
+	}
+	return d, nil
+}
+
+// doCreateAt checks that creating a file at rp is permitted, then invokes
+// create to do so.
+//
+// Preconditions: !rp.Done(). For the final path component in rp,
+// !rp.ShouldFollowSymlink().
+func (fs *filesystem) doCreateAt(ctx context.Context, rp *vfs.ResolvingPath, dir bool, create func(parent *dentry, name string, haveUpperWhiteout bool) error) error {
+	var ds *[]*dentry
+	fs.renameMu.RLock()
+	defer fs.renameMuRUnlockAndCheckDrop(&ds)
+	start := rp.Start().Impl().(*dentry)
+	parent, err := fs.walkParentDirLocked(ctx, rp, start, &ds)
+	if err != nil {
+		return err
+	}
+	if err := parent.checkPermissions(rp.Credentials(), vfs.MayWrite|vfs.MayExec); err != nil {
+		return err
+	}
+	name := rp.Component()
+	if name == "." || name == ".." {
+		return syserror.EEXIST
+	}
+	if !dir && rp.MustBeDir() {
+		return syserror.ENOENT
+	}
+	if parent.vfsd.IsDead() {
+		return syserror.ENOENT
+	}
+	mnt := rp.Mount()
+	if err := mnt.CheckBeginWrite(); err != nil {
+		return err
+	}
+	defer mnt.EndWrite()
+	parent.dirMu.Lock()
+	defer parent.dirMu.Unlock()
+
+	// Determine if a file already exists at name.
+	if _, ok := parent.children[name]; ok {
+		return syserror.EEXIST
+	}
+	childLayer, err := fs.lookupLayerLocked(ctx, parent, name)
+	if err != nil {
+		return err
+	}
+	if childLayer.existsInOverlay() {
+		return syserror.EEXIST
+	}
+
+	// Ensure that the parent directory is copied-up so that we can create the
+	// new file in the upper layer.
+	if err := parent.copyUpLocked(ctx); err != nil {
+		return err
+	}
+
+	// Finally create the new file.
+	if err := create(parent, name, childLayer == lookupLayerUpperWhiteout); err != nil {
+		return err
+	}
+	parent.dirents = nil
+	return nil
+}
+
+// Preconditions: pop's parent directory has been copied up.
+func (fs *filesystem) createWhiteout(ctx context.Context, vfsObj *vfs.VirtualFilesystem, pop *vfs.PathOperation) error {
+	return vfsObj.MknodAt(ctx, fs.creds, pop, &vfs.MknodOptions{
+		Mode: linux.S_IFCHR, // permissions == include/linux/fs.h:WHITEOUT_MODE == 0
+		// DevMajor == DevMinor == 0, from include/linux/fs.h:WHITEOUT_DEV
+	})
+}
+
+func (fs *filesystem) cleanupRecreateWhiteout(ctx context.Context, vfsObj *vfs.VirtualFilesystem, pop *vfs.PathOperation) {
+	if err := fs.createWhiteout(ctx, vfsObj, pop); err != nil {
+		ctx.Warningf("Unrecoverable overlayfs inconsistency: failed to recreate whiteout after failed file creation: %v", err)
+	}
+}
+
+// AccessAt implements vfs.Filesystem.Impl.AccessAt.
+func (fs *filesystem) AccessAt(ctx context.Context, rp *vfs.ResolvingPath, creds *auth.Credentials, ats vfs.AccessTypes) error {
+	var ds *[]*dentry
+	fs.renameMu.RLock()
+	defer fs.renameMuRUnlockAndCheckDrop(&ds)
+	d, err := fs.resolveLocked(ctx, rp, &ds)
+	if err != nil {
+		return err
+	}
+	return d.checkPermissions(creds, ats)
+}
+
+// BoundEndpointAt implements vfs.FilesystemImpl.BoundEndpointAt.
+func (fs *filesystem) BoundEndpointAt(ctx context.Context, rp *vfs.ResolvingPath, opts vfs.BoundEndpointOptions) (transport.BoundEndpoint, error) {
+	var ds *[]*dentry
+	fs.renameMu.RLock()
+	defer fs.renameMuRUnlockAndCheckDrop(&ds)
+	d, err := fs.resolveLocked(ctx, rp, &ds)
+	if err != nil {
+		return nil, err
+	}
+	if err := d.checkPermissions(rp.Credentials(), vfs.MayWrite); err != nil {
+		return nil, err
+	}
+	layerVD := d.topLayer()
+	return fs.vfsfs.VirtualFilesystem().BoundEndpointAt(ctx, fs.creds, &vfs.PathOperation{
+		Root:  layerVD,
+		Start: layerVD,
+	}, &opts)
+}
+
+// GetDentryAt implements vfs.FilesystemImpl.GetDentryAt.
+func (fs *filesystem) GetDentryAt(ctx context.Context, rp *vfs.ResolvingPath, opts vfs.GetDentryOptions) (*vfs.Dentry, error) {
+	var ds *[]*dentry
+	fs.renameMu.RLock()
+	defer fs.renameMuRUnlockAndCheckDrop(&ds)
+	d, err := fs.resolveLocked(ctx, rp, &ds)
+	if err != nil {
+		return nil, err
+	}
+	if opts.CheckSearchable {
+		if !d.isDir() {
+			return nil, syserror.ENOTDIR
+		}
+		if err := d.checkPermissions(rp.Credentials(), vfs.MayExec); err != nil {
+			return nil, err
+		}
+	}
+	d.IncRef()
+	return &d.vfsd, nil
+}
+
+// GetParentDentryAt implements vfs.FilesystemImpl.GetParentDentryAt.
+func (fs *filesystem) GetParentDentryAt(ctx context.Context, rp *vfs.ResolvingPath) (*vfs.Dentry, error) {
+	var ds *[]*dentry
+	fs.renameMu.RLock()
+	defer fs.renameMuRUnlockAndCheckDrop(&ds)
+	start := rp.Start().Impl().(*dentry)
+	d, err := fs.walkParentDirLocked(ctx, rp, start, &ds)
+	if err != nil {
+		return nil, err
+	}
+	d.IncRef()
+	return &d.vfsd, nil
+}
+
+// LinkAt implements vfs.FilesystemImpl.LinkAt.
+func (fs *filesystem) LinkAt(ctx context.Context, rp *vfs.ResolvingPath, vd vfs.VirtualDentry) error {
+	return fs.doCreateAt(ctx, rp, false /* dir */, func(parent *dentry, childName string, haveUpperWhiteout bool) error {
+		if rp.Mount() != vd.Mount() {
+			return syserror.EXDEV
+		}
+		old := vd.Dentry().Impl().(*dentry)
+		if old.isDir() {
+			return syserror.EPERM
+		}
+		if err := old.copyUpLocked(ctx); err != nil {
+			return err
+		}
+		vfsObj := fs.vfsfs.VirtualFilesystem()
+		newpop := vfs.PathOperation{
+			Root:  parent.upperVD,
+			Start: parent.upperVD,
+			Path:  fspath.Parse(childName),
+		}
+		if haveUpperWhiteout {
+			if err := vfsObj.UnlinkAt(ctx, fs.creds, &newpop); err != nil {
+				return err
+			}
+		}
+		if err := vfsObj.LinkAt(ctx, fs.creds, &vfs.PathOperation{
+			Root:  old.upperVD,
+			Start: old.upperVD,
+		}, &newpop); err != nil {
+			if haveUpperWhiteout {
+				fs.cleanupRecreateWhiteout(ctx, vfsObj, &newpop)
+			}
+			return err
+		}
+		creds := rp.Credentials()
+		if err := vfsObj.SetStatAt(ctx, fs.creds, &newpop, &vfs.SetStatOptions{
+			Stat: linux.Statx{
+				Mask: linux.STATX_UID | linux.STATX_GID,
+				UID:  uint32(creds.EffectiveKUID),
+				GID:  uint32(creds.EffectiveKGID),
+			},
+		}); err != nil {
+			if cleanupErr := vfsObj.UnlinkAt(ctx, fs.creds, &newpop); cleanupErr != nil {
+				ctx.Warningf("Unrecoverable overlayfs inconsistency: failed to delete upper layer file after LinkAt metadata update failure: %v", cleanupErr)
+			} else if haveUpperWhiteout {
+				fs.cleanupRecreateWhiteout(ctx, vfsObj, &newpop)
+			}
+			return err
+		}
+		return nil
+	})
+}
+
+// MkdirAt implements vfs.FilesystemImpl.MkdirAt.
+func (fs *filesystem) MkdirAt(ctx context.Context, rp *vfs.ResolvingPath, opts vfs.MkdirOptions) error {
+	return fs.doCreateAt(ctx, rp, true /* dir */, func(parent *dentry, childName string, haveUpperWhiteout bool) error {
+		vfsObj := fs.vfsfs.VirtualFilesystem()
+		pop := vfs.PathOperation{
+			Root:  parent.upperVD,
+			Start: parent.upperVD,
+			Path:  fspath.Parse(childName),
+		}
+		if haveUpperWhiteout {
+			if err := vfsObj.UnlinkAt(ctx, fs.creds, &pop); err != nil {
+				return err
+			}
+		}
+		if err := vfsObj.MkdirAt(ctx, fs.creds, &pop, &opts); err != nil {
+			if haveUpperWhiteout {
+				fs.cleanupRecreateWhiteout(ctx, vfsObj, &pop)
+			}
+			return err
+		}
+		creds := rp.Credentials()
+		if err := vfsObj.SetStatAt(ctx, fs.creds, &pop, &vfs.SetStatOptions{
+			Stat: linux.Statx{
+				Mask: linux.STATX_UID | linux.STATX_GID,
+				UID:  uint32(creds.EffectiveKUID),
+				GID:  uint32(creds.EffectiveKGID),
+			},
+		}); err != nil {
+			if cleanupErr := vfsObj.RmdirAt(ctx, fs.creds, &pop); cleanupErr != nil {
+				ctx.Warningf("Unrecoverable overlayfs inconsistency: failed to delete upper layer directory after MkdirAt metadata update failure: %v", cleanupErr)
+			} else if haveUpperWhiteout {
+				fs.cleanupRecreateWhiteout(ctx, vfsObj, &pop)
+			}
+			return err
+		}
+		if haveUpperWhiteout {
+			// There may be directories on lower layers (previously hidden by
+			// the whiteout) that the new directory should not be merged with.
+			// Mark it opaque to prevent merging.
+			if err := vfsObj.SetxattrAt(ctx, fs.creds, &pop, &vfs.SetxattrOptions{
+				Name:  _OVL_XATTR_OPAQUE,
+				Value: "y",
+			}); err != nil {
+				if cleanupErr := vfsObj.RmdirAt(ctx, fs.creds, &pop); cleanupErr != nil {
+					ctx.Warningf("Unrecoverable overlayfs inconsistency: failed to delete upper layer directory after MkdirAt set-opaque failure: %v", cleanupErr)
+				} else {
+					fs.cleanupRecreateWhiteout(ctx, vfsObj, &pop)
+				}
+				return err
+			}
+		}
+		return nil
+	})
+}
+
+// MknodAt implements vfs.FilesystemImpl.MknodAt.
+func (fs *filesystem) MknodAt(ctx context.Context, rp *vfs.ResolvingPath, opts vfs.MknodOptions) error {
+	return fs.doCreateAt(ctx, rp, false /* dir */, func(parent *dentry, childName string, haveUpperWhiteout bool) error {
+		// Disallow attempts to create whiteouts.
+		if opts.Mode&linux.S_IFMT == linux.S_IFCHR && opts.DevMajor == 0 && opts.DevMinor == 0 {
+			return syserror.EPERM
+		}
+		vfsObj := fs.vfsfs.VirtualFilesystem()
+		pop := vfs.PathOperation{
+			Root:  parent.upperVD,
+			Start: parent.upperVD,
+			Path:  fspath.Parse(childName),
+		}
+		if haveUpperWhiteout {
+			if err := vfsObj.UnlinkAt(ctx, fs.creds, &pop); err != nil {
+				return err
+			}
+		}
+		if err := vfsObj.MknodAt(ctx, fs.creds, &pop, &opts); err != nil {
+			if haveUpperWhiteout {
+				fs.cleanupRecreateWhiteout(ctx, vfsObj, &pop)
+			}
+			return err
+		}
+		creds := rp.Credentials()
+		if err := vfsObj.SetStatAt(ctx, fs.creds, &pop, &vfs.SetStatOptions{
+			Stat: linux.Statx{
+				Mask: linux.STATX_UID | linux.STATX_GID,
+				UID:  uint32(creds.EffectiveKUID),
+				GID:  uint32(creds.EffectiveKGID),
+			},
+		}); err != nil {
+			if cleanupErr := vfsObj.UnlinkAt(ctx, fs.creds, &pop); cleanupErr != nil {
+				ctx.Warningf("Unrecoverable overlayfs inconsistency: failed to delete upper layer file after MknodAt metadata update failure: %v", cleanupErr)
+			} else if haveUpperWhiteout {
+				fs.cleanupRecreateWhiteout(ctx, vfsObj, &pop)
+			}
+			return err
+		}
+		return nil
+	})
+}
+
+// OpenAt implements vfs.FilesystemImpl.OpenAt.
+func (fs *filesystem) OpenAt(ctx context.Context, rp *vfs.ResolvingPath, opts vfs.OpenOptions) (*vfs.FileDescription, error) {
+	mayCreate := opts.Flags&linux.O_CREAT != 0
+	mustCreate := opts.Flags&(linux.O_CREAT|linux.O_EXCL) == (linux.O_CREAT | linux.O_EXCL)
+
+	var ds *[]*dentry
+	fs.renameMu.RLock()
+	defer fs.renameMuRUnlockAndCheckDrop(&ds)
+
+	start := rp.Start().Impl().(*dentry)
+	if rp.Done() {
+		if mustCreate {
+			return nil, syserror.EEXIST
+		}
+		return start.openLocked(ctx, rp, &opts)
+	}
+
+afterTrailingSymlink:
+	parent, err := fs.walkParentDirLocked(ctx, rp, start, &ds)
+	if err != nil {
+		return nil, err
+	}
+	// Check for search permission in the parent directory.
+	if err := parent.checkPermissions(rp.Credentials(), vfs.MayExec); err != nil {
+		return nil, err
+	}
+	// Determine whether or not we need to create a file.
+	parent.dirMu.Lock()
+	child, err := fs.stepLocked(ctx, rp, parent, false /* mayFollowSymlinks */, &ds)
+	if err == syserror.ENOENT && mayCreate {
+		fd, err := fs.createAndOpenLocked(ctx, rp, parent, &opts, &ds)
+		parent.dirMu.Unlock()
+		return fd, err
+	}
+	if err != nil {
+		parent.dirMu.Unlock()
+		return nil, err
+	}
+	// Open existing child or follow symlink.
+	parent.dirMu.Unlock()
+	if mustCreate {
+		return nil, syserror.EEXIST
+	}
+	if child.isSymlink() && rp.ShouldFollowSymlink() {
+		target, err := child.readlink(ctx)
+		if err != nil {
+			return nil, err
+		}
+		if err := rp.HandleSymlink(target); err != nil {
+			return nil, err
+		}
+		start = parent
+		goto afterTrailingSymlink
+	}
+	return child.openLocked(ctx, rp, &opts)
+}
+
+// Preconditions: fs.renameMu must be locked.
+func (d *dentry) openLocked(ctx context.Context, rp *vfs.ResolvingPath, opts *vfs.OpenOptions) (*vfs.FileDescription, error) {
+	ats := vfs.AccessTypesForOpenFlags(opts)
+	if err := d.checkPermissions(rp.Credentials(), ats); err != nil {
+		return nil, err
+	}
+	if ats.MayWrite() {
+		if err := d.copyUpLocked(ctx); err != nil {
+			return nil, err
+		}
+	}
+	mnt := rp.Mount()
+
+	// Directory FDs open FDs from each layer when directory entries are read,
+	// so they don't require opening an FD from d.topLayer() up front.
+	ftype := atomic.LoadUint32(&d.mode) & linux.S_IFMT
+	if ftype == linux.S_IFDIR {
+		// Can't open directories with O_CREAT.
+		if opts.Flags&linux.O_CREAT != 0 {
+			return nil, syserror.EISDIR
+		}
+		// Can't open directories writably.
+		if ats&vfs.MayWrite != 0 {
+			return nil, syserror.EISDIR
+		}
+		if opts.Flags&linux.O_DIRECT != 0 {
+			return nil, syserror.EINVAL
+		}
+		fd := &directoryFD{}
+		fd.LockFD.Init(&d.locks)
+		if err := fd.vfsfd.Init(fd, opts.Flags, mnt, &d.vfsd, &vfs.FileDescriptionOptions{
+			UseDentryMetadata: true,
+		}); err != nil {
+			return nil, err
+		}
+		return &fd.vfsfd, nil
+	}
+
+	layerVD, isUpper := d.topLayerInfo()
+	layerFD, err := rp.VirtualFilesystem().OpenAt(ctx, d.fs.creds, &vfs.PathOperation{
+		Root:  layerVD,
+		Start: layerVD,
+	}, opts)
+	if err != nil {
+		return nil, err
+	}
+	layerFlags := layerFD.StatusFlags()
+	fd := &nonDirectoryFD{
+		copiedUp:    isUpper,
+		cachedFD:    layerFD,
+		cachedFlags: layerFlags,
+	}
+	fd.LockFD.Init(&d.locks)
+	layerFDOpts := layerFD.Options()
+	if err := fd.vfsfd.Init(fd, layerFlags, mnt, &d.vfsd, &layerFDOpts); err != nil {
+		layerFD.DecRef()
+		return nil, err
+	}
+	return &fd.vfsfd, nil
+}
+
+// Preconditions: parent.dirMu must be locked. parent does not already contain
+// a child named rp.Component().
+func (fs *filesystem) createAndOpenLocked(ctx context.Context, rp *vfs.ResolvingPath, parent *dentry, opts *vfs.OpenOptions, ds **[]*dentry) (*vfs.FileDescription, error) {
+	creds := rp.Credentials()
+	if err := parent.checkPermissions(creds, vfs.MayWrite); err != nil {
+		return nil, err
+	}
+	if parent.vfsd.IsDead() {
+		return nil, syserror.ENOENT
+	}
+	mnt := rp.Mount()
+	if err := mnt.CheckBeginWrite(); err != nil {
+		return nil, err
+	}
+	defer mnt.EndWrite()
+
+	if err := parent.copyUpLocked(ctx); err != nil {
+		return nil, err
+	}
+
+	vfsObj := fs.vfsfs.VirtualFilesystem()
+	childName := rp.Component()
+	pop := vfs.PathOperation{
+		Root:  parent.upperVD,
+		Start: parent.upperVD,
+		Path:  fspath.Parse(childName),
+	}
+	// We don't know if a whiteout exists on the upper layer; speculatively
+	// unlink it.
+	//
+	// TODO(gvisor.dev/issue/1199): Modify OpenAt => stepLocked so that we do
+	// know whether a whiteout exists.
+	var haveUpperWhiteout bool
+	switch err := vfsObj.UnlinkAt(ctx, fs.creds, &pop); err {
+	case nil:
+		haveUpperWhiteout = true
+	case syserror.ENOENT:
+		haveUpperWhiteout = false
+	default:
+		return nil, err
+	}
+	// Create the file on the upper layer, and get an FD representing it.
+	upperFD, err := vfsObj.OpenAt(ctx, fs.creds, &pop, &vfs.OpenOptions{
+		Flags: opts.Flags&^vfs.FileCreationFlags | linux.O_CREAT | linux.O_EXCL,
+		Mode:  opts.Mode,
+	})
+	if err != nil {
+		if haveUpperWhiteout {
+			fs.cleanupRecreateWhiteout(ctx, vfsObj, &pop)
+		}
+		return nil, err
+	}
+	// Change the file's owner to the caller. We can't use upperFD.SetStat()
+	// because it will pick up creds from ctx.
+	if err := vfsObj.SetStatAt(ctx, fs.creds, &pop, &vfs.SetStatOptions{
+		Stat: linux.Statx{
+			Mask: linux.STATX_UID | linux.STATX_GID,
+			UID:  uint32(creds.EffectiveKUID),
+			GID:  uint32(creds.EffectiveKGID),
+		},
+	}); err != nil {
+		if cleanupErr := vfsObj.UnlinkAt(ctx, fs.creds, &pop); cleanupErr != nil {
+			ctx.Warningf("Unrecoverable overlayfs inconsistency: failed to delete upper layer file after OpenAt(O_CREAT) metadata update failure: %v", cleanupErr)
+		} else if haveUpperWhiteout {
+			fs.cleanupRecreateWhiteout(ctx, vfsObj, &pop)
+		}
+		return nil, err
+	}
+	// Re-lookup to get a dentry representing the new file, which is needed for
+	// the returned FD.
+	child, err := fs.getChildLocked(ctx, parent, childName, ds)
+	if err != nil {
+		if cleanupErr := vfsObj.UnlinkAt(ctx, fs.creds, &pop); cleanupErr != nil {
+			ctx.Warningf("Unrecoverable overlayfs inconsistency: failed to delete upper layer file after OpenAt(O_CREAT) dentry lookup failure: %v", cleanupErr)
+		} else if haveUpperWhiteout {
+			fs.cleanupRecreateWhiteout(ctx, vfsObj, &pop)
+		}
+		return nil, err
+	}
+	// Finally construct the overlay FD.
+	upperFlags := upperFD.StatusFlags()
+	fd := &nonDirectoryFD{
+		copiedUp:    true,
+		cachedFD:    upperFD,
+		cachedFlags: upperFlags,
+	}
+	fd.LockFD.Init(&child.locks)
+	upperFDOpts := upperFD.Options()
+	if err := fd.vfsfd.Init(fd, upperFlags, mnt, &child.vfsd, &upperFDOpts); err != nil {
+		upperFD.DecRef()
+		// Don't bother with cleanup; the file was created successfully, we
+		// just can't open it anymore for some reason.
+		return nil, err
+	}
+	return &fd.vfsfd, nil
+}
+
+// ReadlinkAt implements vfs.FilesystemImpl.ReadlinkAt.
+func (fs *filesystem) ReadlinkAt(ctx context.Context, rp *vfs.ResolvingPath) (string, error) {
+	var ds *[]*dentry
+	fs.renameMu.RLock()
+	defer fs.renameMuRUnlockAndCheckDrop(&ds)
+	d, err := fs.resolveLocked(ctx, rp, &ds)
+	if err != nil {
+		return "", err
+	}
+	layerVD := d.topLayer()
+	return fs.vfsfs.VirtualFilesystem().ReadlinkAt(ctx, d.fs.creds, &vfs.PathOperation{
+		Root:  layerVD,
+		Start: layerVD,
+	})
+}
+
+// RenameAt implements vfs.FilesystemImpl.RenameAt.
+func (fs *filesystem) RenameAt(ctx context.Context, rp *vfs.ResolvingPath, oldParentVD vfs.VirtualDentry, oldName string, opts vfs.RenameOptions) error {
+	if opts.Flags != 0 {
+		return syserror.EINVAL
+	}
+
+	var ds *[]*dentry
+	fs.renameMu.Lock()
+	defer fs.renameMuUnlockAndCheckDrop(&ds)
+	newParent, err := fs.walkParentDirLocked(ctx, rp, rp.Start().Impl().(*dentry), &ds)
+	if err != nil {
+		return err
+	}
+	newName := rp.Component()
+	if newName == "." || newName == ".." {
+		return syserror.EBUSY
+	}
+	mnt := rp.Mount()
+	if mnt != oldParentVD.Mount() {
+		return syserror.EXDEV
+	}
+	if err := mnt.CheckBeginWrite(); err != nil {
+		return err
+	}
+	defer mnt.EndWrite()
+
+	// FIXME(gvisor.dev/issue/1199): Actually implement rename.
+	_ = newParent
+	return syserror.EXDEV
+}
+
+// RmdirAt implements vfs.FilesystemImpl.RmdirAt.
+func (fs *filesystem) RmdirAt(ctx context.Context, rp *vfs.ResolvingPath) error {
+	var ds *[]*dentry
+	fs.renameMu.RLock()
+	defer fs.renameMuRUnlockAndCheckDrop(&ds)
+	start := rp.Start().Impl().(*dentry)
+	parent, err := fs.walkParentDirLocked(ctx, rp, start, &ds)
+	if err != nil {
+		return err
+	}
+	if err := parent.checkPermissions(rp.Credentials(), vfs.MayWrite|vfs.MayExec); err != nil {
+		return err
+	}
+	if err := rp.Mount().CheckBeginWrite(); err != nil {
+		return err
+	}
+	defer rp.Mount().EndWrite()
+	name := rp.Component()
+	if name == "." {
+		return syserror.EINVAL
+	}
+	if name == ".." {
+		return syserror.ENOTEMPTY
+	}
+	vfsObj := rp.VirtualFilesystem()
+	mntns := vfs.MountNamespaceFromContext(ctx)
+	defer mntns.DecRef()
+	parent.dirMu.Lock()
+	defer parent.dirMu.Unlock()
+
+	// Ensure that parent is copied-up before potentially holding child.copyMu
+	// below.
+	if err := parent.copyUpLocked(ctx); err != nil {
+		return err
+	}
+
+	// Unlike UnlinkAt, we need a dentry representing the child directory being
+	// removed in order to verify that it's empty.
+	child, err := fs.getChildLocked(ctx, parent, name, &ds)
+	if err != nil {
+		return err
+	}
+	if !child.isDir() {
+		return syserror.ENOTDIR
+	}
+	child.dirMu.Lock()
+	defer child.dirMu.Unlock()
+	whiteouts, err := child.collectWhiteoutsForRmdirLocked(ctx)
+	if err != nil {
+		return err
+	}
+	child.copyMu.RLock()
+	defer child.copyMu.RUnlock()
+	if err := vfsObj.PrepareDeleteDentry(mntns, &child.vfsd); err != nil {
+		return err
+	}
+
+	pop := vfs.PathOperation{
+		Root:  parent.upperVD,
+		Start: parent.upperVD,
+		Path:  fspath.Parse(name),
+	}
+	if child.upperVD.Ok() {
+		cleanupRecreateWhiteouts := func() {
+			if !child.upperVD.Ok() {
+				return
+			}
+			for whiteoutName, whiteoutUpper := range whiteouts {
+				if !whiteoutUpper {
+					continue
+				}
+				if err := fs.createWhiteout(ctx, vfsObj, &vfs.PathOperation{
+					Root:  child.upperVD,
+					Start: child.upperVD,
+					Path:  fspath.Parse(whiteoutName),
+				}); err != nil && err != syserror.EEXIST {
+					ctx.Warningf("Unrecoverable overlayfs inconsistency: failed to recreate deleted whiteout after RmdirAt failure: %v", err)
+				}
+			}
+		}
+		// Remove existing whiteouts on the upper layer.
+		for whiteoutName, whiteoutUpper := range whiteouts {
+			if !whiteoutUpper {
+				continue
+			}
+			if err := vfsObj.UnlinkAt(ctx, fs.creds, &vfs.PathOperation{
+				Root:  child.upperVD,
+				Start: child.upperVD,
+				Path:  fspath.Parse(whiteoutName),
+			}); err != nil {
+				cleanupRecreateWhiteouts()
+				vfsObj.AbortDeleteDentry(&child.vfsd)
+				return err
+			}
+		}
+		// Remove the existing directory on the upper layer.
+		if err := vfsObj.RmdirAt(ctx, fs.creds, &pop); err != nil {
+			cleanupRecreateWhiteouts()
+			vfsObj.AbortDeleteDentry(&child.vfsd)
+			return err
+		}
+	}
+	if err := fs.createWhiteout(ctx, vfsObj, &pop); err != nil {
+		// Don't attempt to recover from this: the original directory is
+		// already gone, so any dentries representing it are invalid, and
+		// creating a new directory won't undo that.
+		ctx.Warningf("Unrecoverable overlayfs inconsistency: failed to create whiteout during RmdirAt: %v", err)
+		vfsObj.AbortDeleteDentry(&child.vfsd)
+		return err
+	}
+
+	vfsObj.CommitDeleteDentry(&child.vfsd)
+	delete(parent.children, name)
+	ds = appendDentry(ds, child)
+	parent.dirents = nil
+	return nil
+}
+
+// SetStatAt implements vfs.FilesystemImpl.SetStatAt.
+func (fs *filesystem) SetStatAt(ctx context.Context, rp *vfs.ResolvingPath, opts vfs.SetStatOptions) error {
+	var ds *[]*dentry
+	fs.renameMu.RLock()
+	defer fs.renameMuRUnlockAndCheckDrop(&ds)
+	d, err := fs.resolveLocked(ctx, rp, &ds)
+	if err != nil {
+		return err
+	}
+
+	mode := linux.FileMode(atomic.LoadUint32(&d.mode))
+	if err := vfs.CheckSetStat(ctx, rp.Credentials(), &opts.Stat, mode, auth.KUID(atomic.LoadUint32(&d.uid)), auth.KGID(atomic.LoadUint32(&d.gid))); err != nil {
+		return err
+	}
+	mnt := rp.Mount()
+	if err := mnt.CheckBeginWrite(); err != nil {
+		return err
+	}
+	defer mnt.EndWrite()
+	if err := d.copyUpLocked(ctx); err != nil {
+		return err
+	}
+	// Changes to d's attributes are serialized by d.copyMu.
+	d.copyMu.Lock()
+	defer d.copyMu.Unlock()
+	if err := d.fs.vfsfs.VirtualFilesystem().SetStatAt(ctx, d.fs.creds, &vfs.PathOperation{
+		Root:  d.upperVD,
+		Start: d.upperVD,
+	}, &opts); err != nil {
+		return err
+	}
+	d.updateAfterSetStatLocked(&opts)
+	return nil
+}
+
+// StatAt implements vfs.FilesystemImpl.StatAt.
+func (fs *filesystem) StatAt(ctx context.Context, rp *vfs.ResolvingPath, opts vfs.StatOptions) (linux.Statx, error) {
+	var ds *[]*dentry
+	fs.renameMu.RLock()
+	defer fs.renameMuRUnlockAndCheckDrop(&ds)
+	d, err := fs.resolveLocked(ctx, rp, &ds)
+	if err != nil {
+		return linux.Statx{}, err
+	}
+
+	var stat linux.Statx
+	if layerMask := opts.Mask &^ statInternalMask; layerMask != 0 {
+		layerVD := d.topLayer()
+		stat, err = fs.vfsfs.VirtualFilesystem().StatAt(ctx, fs.creds, &vfs.PathOperation{
+			Root:  layerVD,
+			Start: layerVD,
+		}, &vfs.StatOptions{
+			Mask: layerMask,
+			Sync: opts.Sync,
+		})
+		if err != nil {
+			return linux.Statx{}, err
+		}
+	}
+	d.statInternalTo(ctx, &opts, &stat)
+	return stat, nil
+}
+
+// StatFSAt implements vfs.FilesystemImpl.StatFSAt.
+func (fs *filesystem) StatFSAt(ctx context.Context, rp *vfs.ResolvingPath) (linux.Statfs, error) {
+	var ds *[]*dentry
+	fs.renameMu.RLock()
+	defer fs.renameMuRUnlockAndCheckDrop(&ds)
+	_, err := fs.resolveLocked(ctx, rp, &ds)
+	if err != nil {
+		return linux.Statfs{}, err
+	}
+	return fs.statFS(ctx)
+}
+
+// SymlinkAt implements vfs.FilesystemImpl.SymlinkAt.
+func (fs *filesystem) SymlinkAt(ctx context.Context, rp *vfs.ResolvingPath, target string) error {
+	return fs.doCreateAt(ctx, rp, false /* dir */, func(parent *dentry, childName string, haveUpperWhiteout bool) error {
+		vfsObj := fs.vfsfs.VirtualFilesystem()
+		pop := vfs.PathOperation{
+			Root:  parent.upperVD,
+			Start: parent.upperVD,
+			Path:  fspath.Parse(childName),
+		}
+		if haveUpperWhiteout {
+			if err := vfsObj.UnlinkAt(ctx, fs.creds, &pop); err != nil {
+				return err
+			}
+		}
+		if err := vfsObj.SymlinkAt(ctx, fs.creds, &pop, target); err != nil {
+			if haveUpperWhiteout {
+				fs.cleanupRecreateWhiteout(ctx, vfsObj, &pop)
+			}
+			return err
+		}
+		creds := rp.Credentials()
+		if err := vfsObj.SetStatAt(ctx, fs.creds, &pop, &vfs.SetStatOptions{
+			Stat: linux.Statx{
+				Mask: linux.STATX_UID | linux.STATX_GID,
+				UID:  uint32(creds.EffectiveKUID),
+				GID:  uint32(creds.EffectiveKGID),
+			},
+		}); err != nil {
+			if cleanupErr := vfsObj.UnlinkAt(ctx, fs.creds, &pop); cleanupErr != nil {
+				ctx.Warningf("Unrecoverable overlayfs inconsistency: failed to delete upper layer file after SymlinkAt metadata update failure: %v", cleanupErr)
+			} else if haveUpperWhiteout {
+				fs.cleanupRecreateWhiteout(ctx, vfsObj, &pop)
+			}
+			return err
+		}
+		return nil
+	})
+}
+
+// UnlinkAt implements vfs.FilesystemImpl.UnlinkAt.
+func (fs *filesystem) UnlinkAt(ctx context.Context, rp *vfs.ResolvingPath) error {
+	var ds *[]*dentry
+	fs.renameMu.RLock()
+	defer fs.renameMuRUnlockAndCheckDrop(&ds)
+	start := rp.Start().Impl().(*dentry)
+	parent, err := fs.walkParentDirLocked(ctx, rp, start, &ds)
+	if err != nil {
+		return err
+	}
+	if err := parent.checkPermissions(rp.Credentials(), vfs.MayWrite|vfs.MayExec); err != nil {
+		return err
+	}
+	if err := rp.Mount().CheckBeginWrite(); err != nil {
+		return err
+	}
+	defer rp.Mount().EndWrite()
+	name := rp.Component()
+	if name == "." || name == ".." {
+		return syserror.EISDIR
+	}
+	if rp.MustBeDir() {
+		return syserror.ENOTDIR
+	}
+	vfsObj := rp.VirtualFilesystem()
+	mntns := vfs.MountNamespaceFromContext(ctx)
+	defer mntns.DecRef()
+	parent.dirMu.Lock()
+	defer parent.dirMu.Unlock()
+
+	// Ensure that parent is copied-up before potentially holding child.copyMu
+	// below.
+	if err := parent.copyUpLocked(ctx); err != nil {
+		return err
+	}
+
+	child := parent.children[name]
+	var childLayer lookupLayer
+	if child != nil {
+		if child.isDir() {
+			return syserror.EISDIR
+		}
+		if err := vfsObj.PrepareDeleteDentry(mntns, &child.vfsd); err != nil {
+			return err
+		}
+		// Hold child.copyMu to prevent it from being copied-up during
+		// deletion.
+		child.copyMu.RLock()
+		defer child.copyMu.RUnlock()
+		if child.upperVD.Ok() {
+			childLayer = lookupLayerUpper
+		} else {
+			childLayer = lookupLayerLower
+		}
+	} else {
+		// Determine if the file being unlinked actually exists. Holding
+		// parent.dirMu prevents a dentry from being instantiated for the file,
+		// which in turn prevents it from being copied-up, so this result is
+		// stable.
+		childLayer, err = fs.lookupLayerLocked(ctx, parent, name)
+		if err != nil {
+			return err
+		}
+		if !childLayer.existsInOverlay() {
+			return syserror.ENOENT
+		}
+	}
+
+	pop := vfs.PathOperation{
+		Root:  parent.upperVD,
+		Start: parent.upperVD,
+		Path:  fspath.Parse(name),
+	}
+	if childLayer == lookupLayerUpper {
+		// Remove the existing file on the upper layer.
+		if err := vfsObj.UnlinkAt(ctx, fs.creds, &pop); err != nil {
+			if child != nil {
+				vfsObj.AbortDeleteDentry(&child.vfsd)
+			}
+			return err
+		}
+	}
+	if err := fs.createWhiteout(ctx, vfsObj, &pop); err != nil {
+		ctx.Warningf("Unrecoverable overlayfs inconsistency: failed to create whiteout during UnlinkAt: %v", err)
+		if child != nil {
+			vfsObj.AbortDeleteDentry(&child.vfsd)
+		}
+		return err
+	}
+
+	if child != nil {
+		vfsObj.CommitDeleteDentry(&child.vfsd)
+		delete(parent.children, name)
+		ds = appendDentry(ds, child)
+	}
+	parent.dirents = nil
+	return nil
+}
+
+// ListxattrAt implements vfs.FilesystemImpl.ListxattrAt.
+func (fs *filesystem) ListxattrAt(ctx context.Context, rp *vfs.ResolvingPath, size uint64) ([]string, error) {
+	var ds *[]*dentry
+	fs.renameMu.RLock()
+	defer fs.renameMuRUnlockAndCheckDrop(&ds)
+	_, err := fs.resolveLocked(ctx, rp, &ds)
+	if err != nil {
+		return nil, err
+	}
+	// TODO(gvisor.dev/issue/1199): Linux overlayfs actually allows listxattr,
+	// but not any other xattr syscalls. For now we just reject all of them.
+	return nil, syserror.ENOTSUP
+}
+
+// GetxattrAt implements vfs.FilesystemImpl.GetxattrAt.
+func (fs *filesystem) GetxattrAt(ctx context.Context, rp *vfs.ResolvingPath, opts vfs.GetxattrOptions) (string, error) {
+	var ds *[]*dentry
+	fs.renameMu.RLock()
+	defer fs.renameMuRUnlockAndCheckDrop(&ds)
+	_, err := fs.resolveLocked(ctx, rp, &ds)
+	if err != nil {
+		return "", err
+	}
+	return "", syserror.ENOTSUP
+}
+
+// SetxattrAt implements vfs.FilesystemImpl.SetxattrAt.
+func (fs *filesystem) SetxattrAt(ctx context.Context, rp *vfs.ResolvingPath, opts vfs.SetxattrOptions) error {
+	var ds *[]*dentry
+	fs.renameMu.RLock()
+	defer fs.renameMuRUnlockAndCheckDrop(&ds)
+	_, err := fs.resolveLocked(ctx, rp, &ds)
+	if err != nil {
+		return err
+	}
+	return syserror.ENOTSUP
+}
+
+// RemovexattrAt implements vfs.FilesystemImpl.RemovexattrAt.
+func (fs *filesystem) RemovexattrAt(ctx context.Context, rp *vfs.ResolvingPath, name string) error {
+	var ds *[]*dentry
+	fs.renameMu.RLock()
+	defer fs.renameMuRUnlockAndCheckDrop(&ds)
+	_, err := fs.resolveLocked(ctx, rp, &ds)
+	if err != nil {
+		return err
+	}
+	return syserror.ENOTSUP
+}
+
+// PrependPath implements vfs.FilesystemImpl.PrependPath.
+func (fs *filesystem) PrependPath(ctx context.Context, vfsroot, vd vfs.VirtualDentry, b *fspath.Builder) error {
+	fs.renameMu.RLock()
+	defer fs.renameMu.RUnlock()
+	return genericPrependPath(vfsroot, vd.Mount(), vd.Dentry().Impl().(*dentry), b)
+}
diff --git a/pkg/sentry/fsimpl/overlay/fstree.go b/pkg/sentry/fsimpl/overlay/fstree.go
new file mode 100644
index 000000000..1fa1b4ad3
--- /dev/null
+++ b/pkg/sentry/fsimpl/overlay/fstree.go
@@ -0,0 +1,46 @@
+package overlay
+
+import (
+	"gvisor.dev/gvisor/pkg/fspath"
+	"gvisor.dev/gvisor/pkg/sentry/vfs"
+)
+
+// IsAncestorDentry returns true if d is an ancestor of d2; that is, d is
+// either d2's parent or an ancestor of d2's parent.
+func genericIsAncestorDentry(d, d2 *dentry) bool {
+	for d2 != nil {
+		if d2.parent == d {
+			return true
+		}
+		if d2.parent == d2 {
+			return false
+		}
+		d2 = d2.parent
+	}
+	return false
+}
+
+// ParentOrSelf returns d.parent. If d.parent is nil, ParentOrSelf returns d.
+func genericParentOrSelf(d *dentry) *dentry {
+	if d.parent != nil {
+		return d.parent
+	}
+	return d
+}
+
+// PrependPath is a generic implementation of FilesystemImpl.PrependPath().
+func genericPrependPath(vfsroot vfs.VirtualDentry, mnt *vfs.Mount, d *dentry, b *fspath.Builder) error {
+	for {
+		if mnt == vfsroot.Mount() && &d.vfsd == vfsroot.Dentry() {
+			return vfs.PrependPathAtVFSRootError{}
+		}
+		if &d.vfsd == mnt.Root() {
+			return nil
+		}
+		if d.parent == nil {
+			return vfs.PrependPathAtNonMountRootError{}
+		}
+		b.PrependComponent(d.name)
+		d = d.parent
+	}
+}
diff --git a/pkg/sentry/fsimpl/overlay/non_directory.go b/pkg/sentry/fsimpl/overlay/non_directory.go
new file mode 100644
index 000000000..a3c1f7a8d
--- /dev/null
+++ b/pkg/sentry/fsimpl/overlay/non_directory.go
@@ -0,0 +1,266 @@
+// Copyright 2020 The gVisor Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package overlay
+
+import (
+	"sync/atomic"
+
+	"gvisor.dev/gvisor/pkg/abi/linux"
+	"gvisor.dev/gvisor/pkg/context"
+	"gvisor.dev/gvisor/pkg/sentry/kernel/auth"
+	"gvisor.dev/gvisor/pkg/sentry/memmap"
+	"gvisor.dev/gvisor/pkg/sentry/vfs"
+	"gvisor.dev/gvisor/pkg/sync"
+	"gvisor.dev/gvisor/pkg/usermem"
+)
+
+func (d *dentry) isSymlink() bool {
+	return atomic.LoadUint32(&d.mode)&linux.S_IFMT == linux.S_IFLNK
+}
+
+func (d *dentry) readlink(ctx context.Context) (string, error) {
+	layerVD := d.topLayer()
+	return d.fs.vfsfs.VirtualFilesystem().ReadlinkAt(ctx, d.fs.creds, &vfs.PathOperation{
+		Root:  layerVD,
+		Start: layerVD,
+	})
+}
+
+type nonDirectoryFD struct {
+	fileDescription
+
+	// If copiedUp is false, cachedFD represents
+	// fileDescription.dentry().lowerVDs[0]; otherwise, cachedFD represents
+	// fileDescription.dentry().upperVD. cachedFlags is the last known value of
+	// cachedFD.StatusFlags(). copiedUp, cachedFD, and cachedFlags are
+	// protected by mu.
+	mu          sync.Mutex
+	copiedUp    bool
+	cachedFD    *vfs.FileDescription
+	cachedFlags uint32
+}
+
+func (fd *nonDirectoryFD) getCurrentFD(ctx context.Context) (*vfs.FileDescription, error) {
+	fd.mu.Lock()
+	defer fd.mu.Unlock()
+	wrappedFD, err := fd.currentFDLocked(ctx)
+	if err != nil {
+		return nil, err
+	}
+	wrappedFD.IncRef()
+	return wrappedFD, nil
+}
+
+func (fd *nonDirectoryFD) currentFDLocked(ctx context.Context) (*vfs.FileDescription, error) {
+	d := fd.dentry()
+	statusFlags := fd.vfsfd.StatusFlags()
+	if !fd.copiedUp && d.isCopiedUp() {
+		// Switch to the copied-up file.
+		upperVD := d.topLayer()
+		upperFD, err := fd.filesystem().vfsfs.VirtualFilesystem().OpenAt(ctx, d.fs.creds, &vfs.PathOperation{
+			Root:  upperVD,
+			Start: upperVD,
+		}, &vfs.OpenOptions{
+			Flags: statusFlags,
+		})
+		if err != nil {
+			return nil, err
+		}
+		oldOff, oldOffErr := fd.cachedFD.Seek(ctx, 0, linux.SEEK_CUR)
+		if oldOffErr == nil {
+			if _, err := upperFD.Seek(ctx, oldOff, linux.SEEK_SET); err != nil {
+				upperFD.DecRef()
+				return nil, err
+			}
+		}
+		fd.cachedFD.DecRef()
+		fd.copiedUp = true
+		fd.cachedFD = upperFD
+		fd.cachedFlags = statusFlags
+	} else if fd.cachedFlags != statusFlags {
+		if err := fd.cachedFD.SetStatusFlags(ctx, d.fs.creds, statusFlags); err != nil {
+			return nil, err
+		}
+		fd.cachedFlags = statusFlags
+	}
+	return fd.cachedFD, nil
+}
+
+// Release implements vfs.FileDescriptionImpl.Release.
+func (fd *nonDirectoryFD) Release() {
+	fd.cachedFD.DecRef()
+	fd.cachedFD = nil
+}
+
+// OnClose implements vfs.FileDescriptionImpl.OnClose.
+func (fd *nonDirectoryFD) OnClose(ctx context.Context) error {
+	// Linux doesn't define ovl_file_operations.flush at all (i.e. its
+	// equivalent to OnClose is a no-op). We pass through to
+	// fd.cachedFD.OnClose() without upgrading if fd.dentry() has been
+	// copied-up, since OnClose is mostly used to define post-close writeback,
+	// and if fd.cachedFD hasn't been updated then it can't have been used to
+	// mutate fd.dentry() anyway.
+	fd.mu.Lock()
+	if statusFlags := fd.vfsfd.StatusFlags(); fd.cachedFlags != statusFlags {
+		if err := fd.cachedFD.SetStatusFlags(ctx, fd.filesystem().creds, statusFlags); err != nil {
+			fd.mu.Unlock()
+			return err
+		}
+		fd.cachedFlags = statusFlags
+	}
+	wrappedFD := fd.cachedFD
+	defer wrappedFD.IncRef()
+	fd.mu.Unlock()
+	return wrappedFD.OnClose(ctx)
+}
+
+// Stat implements vfs.FileDescriptionImpl.Stat.
+func (fd *nonDirectoryFD) Stat(ctx context.Context, opts vfs.StatOptions) (linux.Statx, error) {
+	var stat linux.Statx
+	if layerMask := opts.Mask &^ statInternalMask; layerMask != 0 {
+		wrappedFD, err := fd.getCurrentFD(ctx)
+		if err != nil {
+			return linux.Statx{}, err
+		}
+		stat, err = wrappedFD.Stat(ctx, vfs.StatOptions{
+			Mask: layerMask,
+			Sync: opts.Sync,
+		})
+		wrappedFD.DecRef()
+		if err != nil {
+			return linux.Statx{}, err
+		}
+	}
+	fd.dentry().statInternalTo(ctx, &opts, &stat)
+	return stat, nil
+}
+
+// SetStat implements vfs.FileDescriptionImpl.SetStat.
+func (fd *nonDirectoryFD) SetStat(ctx context.Context, opts vfs.SetStatOptions) error {
+	d := fd.dentry()
+	mode := linux.FileMode(atomic.LoadUint32(&d.mode))
+	if err := vfs.CheckSetStat(ctx, auth.CredentialsFromContext(ctx), &opts.Stat, mode, auth.KUID(atomic.LoadUint32(&d.uid)), auth.KGID(atomic.LoadUint32(&d.gid))); err != nil {
+		return err
+	}
+	mnt := fd.vfsfd.Mount()
+	if err := mnt.CheckBeginWrite(); err != nil {
+		return err
+	}
+	defer mnt.EndWrite()
+	if err := d.copyUpLocked(ctx); err != nil {
+		return err
+	}
+	// Changes to d's attributes are serialized by d.copyMu.
+	d.copyMu.Lock()
+	defer d.copyMu.Unlock()
+	wrappedFD, err := fd.currentFDLocked(ctx)
+	if err != nil {
+		return err
+	}
+	if err := wrappedFD.SetStat(ctx, opts); err != nil {
+		return err
+	}
+	d.updateAfterSetStatLocked(&opts)
+	return nil
+}
+
+// StatFS implements vfs.FileDesciptionImpl.StatFS.
+func (fd *nonDirectoryFD) StatFS(ctx context.Context) (linux.Statfs, error) {
+	return fd.filesystem().statFS(ctx)
+}
+
+// PRead implements vfs.FileDescriptionImpl.PRead.
+func (fd *nonDirectoryFD) PRead(ctx context.Context, dst usermem.IOSequence, offset int64, opts vfs.ReadOptions) (int64, error) {
+	wrappedFD, err := fd.getCurrentFD(ctx)
+	if err != nil {
+		return 0, err
+	}
+	defer wrappedFD.DecRef()
+	return wrappedFD.PRead(ctx, dst, offset, opts)
+}
+
+// Read implements vfs.FileDescriptionImpl.Read.
+func (fd *nonDirectoryFD) Read(ctx context.Context, dst usermem.IOSequence, opts vfs.ReadOptions) (int64, error) {
+	// Hold fd.mu during the read to serialize the file offset.
+	fd.mu.Lock()
+	defer fd.mu.Unlock()
+	wrappedFD, err := fd.currentFDLocked(ctx)
+	if err != nil {
+		return 0, err
+	}
+	return wrappedFD.Read(ctx, dst, opts)
+}
+
+// PWrite implements vfs.FileDescriptionImpl.PWrite.
+func (fd *nonDirectoryFD) PWrite(ctx context.Context, src usermem.IOSequence, offset int64, opts vfs.WriteOptions) (int64, error) {
+	wrappedFD, err := fd.getCurrentFD(ctx)
+	if err != nil {
+		return 0, err
+	}
+	defer wrappedFD.DecRef()
+	return wrappedFD.PWrite(ctx, src, offset, opts)
+}
+
+// Write implements vfs.FileDescriptionImpl.Write.
+func (fd *nonDirectoryFD) Write(ctx context.Context, src usermem.IOSequence, opts vfs.WriteOptions) (int64, error) {
+	// Hold fd.mu during the write to serialize the file offset.
+	fd.mu.Lock()
+	defer fd.mu.Unlock()
+	wrappedFD, err := fd.currentFDLocked(ctx)
+	if err != nil {
+		return 0, err
+	}
+	return wrappedFD.Write(ctx, src, opts)
+}
+
+// Seek implements vfs.FileDescriptionImpl.Seek.
+func (fd *nonDirectoryFD) Seek(ctx context.Context, offset int64, whence int32) (int64, error) {
+	// Hold fd.mu during the seek to serialize the file offset.
+	fd.mu.Lock()
+	defer fd.mu.Unlock()
+	wrappedFD, err := fd.currentFDLocked(ctx)
+	if err != nil {
+		return 0, err
+	}
+	return wrappedFD.Seek(ctx, offset, whence)
+}
+
+// Sync implements vfs.FileDescriptionImpl.Sync.
+func (fd *nonDirectoryFD) Sync(ctx context.Context) error {
+	fd.mu.Lock()
+	if !fd.dentry().isCopiedUp() {
+		fd.mu.Unlock()
+		return nil
+	}
+	wrappedFD, err := fd.currentFDLocked(ctx)
+	if err != nil {
+		fd.mu.Unlock()
+		return err
+	}
+	wrappedFD.IncRef()
+	defer wrappedFD.DecRef()
+	fd.mu.Unlock()
+	return wrappedFD.Sync(ctx)
+}
+
+// ConfigureMMap implements vfs.FileDescriptionImpl.ConfigureMMap.
+func (fd *nonDirectoryFD) ConfigureMMap(ctx context.Context, opts *memmap.MMapOpts) error {
+	wrappedFD, err := fd.getCurrentFD(ctx)
+	if err != nil {
+		return err
+	}
+	defer wrappedFD.DecRef()
+	return wrappedFD.ConfigureMMap(ctx, opts)
+}
diff --git a/pkg/sentry/fsimpl/overlay/overlay.go b/pkg/sentry/fsimpl/overlay/overlay.go
new file mode 100644
index 000000000..e660d0e2c
--- /dev/null
+++ b/pkg/sentry/fsimpl/overlay/overlay.go
@@ -0,0 +1,612 @@
+// Copyright 2020 The gVisor Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+// Package overlay provides an overlay filesystem implementation, which
+// synthesizes a filesystem by composing one or more immutable filesystems
+// ("lower layers") with an optional mutable filesystem ("upper layer").
+//
+// Lock order:
+//
+// directoryFD.mu / nonDirectoryFD.mu
+//   filesystem.renameMu
+//     dentry.dirMu
+//       dentry.copyMu
+//
+// Locking dentry.dirMu in multiple dentries requires that parent dentries are
+// locked before child dentries, and that filesystem.renameMu is locked to
+// stabilize this relationship.
+package overlay
+
+import (
+	"strings"
+	"sync/atomic"
+
+	"gvisor.dev/gvisor/pkg/abi/linux"
+	"gvisor.dev/gvisor/pkg/context"
+	"gvisor.dev/gvisor/pkg/fspath"
+	"gvisor.dev/gvisor/pkg/sentry/kernel/auth"
+	"gvisor.dev/gvisor/pkg/sentry/vfs"
+	"gvisor.dev/gvisor/pkg/sentry/vfs/lock"
+	"gvisor.dev/gvisor/pkg/sync"
+	"gvisor.dev/gvisor/pkg/syserror"
+)
+
+// Name is the default filesystem name.
+const Name = "overlay"
+
+// FilesystemType implements vfs.FilesystemType.
+type FilesystemType struct{}
+
+// Name implements vfs.FilesystemType.Name.
+func (FilesystemType) Name() string {
+	return Name
+}
+
+// FilesystemOptions may be passed as vfs.GetFilesystemOptions.InternalData to
+// FilesystemType.GetFilesystem.
+type FilesystemOptions struct {
+	// Callers passing FilesystemOptions to
+	// overlay.FilesystemType.GetFilesystem() are responsible for ensuring that
+	// the vfs.Mounts comprising the layers of the overlay filesystem do not
+	// contain submounts.
+
+	// If UpperRoot.Ok(), it is the root of the writable upper layer of the
+	// overlay.
+	UpperRoot vfs.VirtualDentry
+
+	// LowerRoots contains the roots of the immutable lower layers of the
+	// overlay. LowerRoots is immutable.
+	LowerRoots []vfs.VirtualDentry
+}
+
+// filesystem implements vfs.FilesystemImpl.
+type filesystem struct {
+	vfsfs vfs.Filesystem
+
+	// Immutable options.
+	opts FilesystemOptions
+
+	// creds is a copy of the filesystem's creator's credentials, which are
+	// used for accesses to the filesystem's layers. creds is immutable.
+	creds *auth.Credentials
+
+	// dirDevMinor is the device minor number used for directories. dirDevMinor
+	// is immutable.
+	dirDevMinor uint32
+
+	// lowerDevMinors maps lower layer filesystems to device minor numbers
+	// assigned to non-directory files originating from that filesystem.
+	// lowerDevMinors is immutable.
+	lowerDevMinors map[*vfs.Filesystem]uint32
+
+	// renameMu synchronizes renaming with non-renaming operations in order to
+	// ensure consistent lock ordering between dentry.dirMu in different
+	// dentries.
+	renameMu sync.RWMutex
+
+	// lastDirIno is the last inode number assigned to a directory. lastDirIno
+	// is accessed using atomic memory operations.
+	lastDirIno uint64
+}
+
+// GetFilesystem implements vfs.FilesystemType.GetFilesystem.
+func (fstype FilesystemType) GetFilesystem(ctx context.Context, vfsObj *vfs.VirtualFilesystem, creds *auth.Credentials, source string, opts vfs.GetFilesystemOptions) (*vfs.Filesystem, *vfs.Dentry, error) {
+	mopts := vfs.GenericParseMountOptions(opts.Data)
+	fsoptsRaw := opts.InternalData
+	fsopts, haveFSOpts := fsoptsRaw.(FilesystemOptions)
+	if fsoptsRaw != nil && !haveFSOpts {
+		ctx.Warningf("overlay.FilesystemType.GetFilesystem: GetFilesystemOptions.InternalData has type %T, wanted overlay.FilesystemOptions or nil", fsoptsRaw)
+		return nil, nil, syserror.EINVAL
+	}
+	if haveFSOpts {
+		if len(fsopts.LowerRoots) == 0 {
+			ctx.Warningf("overlay.FilesystemType.GetFilesystem: LowerRoots must be non-empty")
+			return nil, nil, syserror.EINVAL
+		}
+		if len(fsopts.LowerRoots) < 2 && !fsopts.UpperRoot.Ok() {
+			ctx.Warningf("overlay.FilesystemType.GetFilesystem: at least two LowerRoots are required when UpperRoot is unspecified")
+			return nil, nil, syserror.EINVAL
+		}
+		// We don't enforce a maximum number of lower layers when not
+		// configured by applications; the sandbox owner can have an overlay
+		// filesystem with any number of lower layers.
+	} else {
+		vfsroot := vfs.RootFromContext(ctx)
+		defer vfsroot.DecRef()
+		upperPathname, ok := mopts["upperdir"]
+		if ok {
+			delete(mopts, "upperdir")
+			// Linux overlayfs also requires a workdir when upperdir is
+			// specified; we don't, so silently ignore this option.
+			delete(mopts, "workdir")
+			upperPath := fspath.Parse(upperPathname)
+			if !upperPath.Absolute {
+				ctx.Warningf("overlay.FilesystemType.GetFilesystem: upperdir %q must be absolute", upperPathname)
+				return nil, nil, syserror.EINVAL
+			}
+			upperRoot, err := vfsObj.GetDentryAt(ctx, creds, &vfs.PathOperation{
+				Root:               vfsroot,
+				Start:              vfsroot,
+				Path:               upperPath,
+				FollowFinalSymlink: true,
+			}, &vfs.GetDentryOptions{
+				CheckSearchable: true,
+			})
+			if err != nil {
+				ctx.Warningf("overlay.FilesystemType.GetFilesystem: failed to resolve upperdir %q: %v", upperPathname, err)
+				return nil, nil, err
+			}
+			defer upperRoot.DecRef()
+			privateUpperRoot, err := clonePrivateMount(vfsObj, upperRoot, false /* forceReadOnly */)
+			if err != nil {
+				ctx.Warningf("overlay.FilesystemType.GetFilesystem: failed to make private bind mount of upperdir %q: %v", upperPathname, err)
+				return nil, nil, err
+			}
+			defer privateUpperRoot.DecRef()
+			fsopts.UpperRoot = privateUpperRoot
+		}
+		lowerPathnamesStr, ok := mopts["lowerdir"]
+		if !ok {
+			ctx.Warningf("overlay.FilesystemType.GetFilesystem: missing required option lowerdir")
+			return nil, nil, syserror.EINVAL
+		}
+		delete(mopts, "lowerdir")
+		lowerPathnames := strings.Split(lowerPathnamesStr, ":")
+		const maxLowerLayers = 500 // Linux: fs/overlay/super.c:OVL_MAX_STACK
+		if len(lowerPathnames) < 2 && !fsopts.UpperRoot.Ok() {
+			ctx.Warningf("overlay.FilesystemType.GetFilesystem: at least two lowerdirs are required when upperdir is unspecified")
+			return nil, nil, syserror.EINVAL
+		}
+		if len(lowerPathnames) > maxLowerLayers {
+			ctx.Warningf("overlay.FilesystemType.GetFilesystem: %d lowerdirs specified, maximum %d", len(lowerPathnames), maxLowerLayers)
+			return nil, nil, syserror.EINVAL
+		}
+		for _, lowerPathname := range lowerPathnames {
+			lowerPath := fspath.Parse(lowerPathname)
+			if !lowerPath.Absolute {
+				ctx.Warningf("overlay.FilesystemType.GetFilesystem: lowerdir %q must be absolute", lowerPathname)
+				return nil, nil, syserror.EINVAL
+			}
+			lowerRoot, err := vfsObj.GetDentryAt(ctx, creds, &vfs.PathOperation{
+				Root:               vfsroot,
+				Start:              vfsroot,
+				Path:               lowerPath,
+				FollowFinalSymlink: true,
+			}, &vfs.GetDentryOptions{
+				CheckSearchable: true,
+			})
+			if err != nil {
+				ctx.Warningf("overlay.FilesystemType.GetFilesystem: failed to resolve lowerdir %q: %v", lowerPathname, err)
+				return nil, nil, err
+			}
+			defer lowerRoot.DecRef()
+			privateLowerRoot, err := clonePrivateMount(vfsObj, lowerRoot, true /* forceReadOnly */)
+			if err != nil {
+				ctx.Warningf("overlay.FilesystemType.GetFilesystem: failed to make private bind mount of lowerdir %q: %v", lowerPathname, err)
+				return nil, nil, err
+			}
+			defer privateLowerRoot.DecRef()
+			fsopts.LowerRoots = append(fsopts.LowerRoots, privateLowerRoot)
+		}
+	}
+	if len(mopts) != 0 {
+		ctx.Warningf("overlay.FilesystemType.GetFilesystem: unused options: %v", mopts)
+		return nil, nil, syserror.EINVAL
+	}
+
+	// Allocate device numbers.
+	dirDevMinor, err := vfsObj.GetAnonBlockDevMinor()
+	if err != nil {
+		return nil, nil, err
+	}
+	lowerDevMinors := make(map[*vfs.Filesystem]uint32)
+	for _, lowerRoot := range fsopts.LowerRoots {
+		lowerFS := lowerRoot.Mount().Filesystem()
+		if _, ok := lowerDevMinors[lowerFS]; !ok {
+			devMinor, err := vfsObj.GetAnonBlockDevMinor()
+			if err != nil {
+				vfsObj.PutAnonBlockDevMinor(dirDevMinor)
+				for _, lowerDevMinor := range lowerDevMinors {
+					vfsObj.PutAnonBlockDevMinor(lowerDevMinor)
+				}
+				return nil, nil, err
+			}
+			lowerDevMinors[lowerFS] = devMinor
+		}
+	}
+
+	// Take extra references held by the filesystem.
+	if fsopts.UpperRoot.Ok() {
+		fsopts.UpperRoot.IncRef()
+	}
+	for _, lowerRoot := range fsopts.LowerRoots {
+		lowerRoot.IncRef()
+	}
+
+	fs := &filesystem{
+		opts:           fsopts,
+		creds:          creds.Fork(),
+		dirDevMinor:    dirDevMinor,
+		lowerDevMinors: lowerDevMinors,
+	}
+	fs.vfsfs.Init(vfsObj, &fstype, fs)
+
+	// Construct the root dentry.
+	root := fs.newDentry()
+	root.refs = 1
+	if fs.opts.UpperRoot.Ok() {
+		fs.opts.UpperRoot.IncRef()
+		root.copiedUp = 1
+		root.upperVD = fs.opts.UpperRoot
+	}
+	for _, lowerRoot := range fs.opts.LowerRoots {
+		lowerRoot.IncRef()
+		root.lowerVDs = append(root.lowerVDs, lowerRoot)
+	}
+	rootTopVD := root.topLayer()
+	// Get metadata from the topmost layer. See fs.lookupLocked().
+	const rootStatMask = linux.STATX_TYPE | linux.STATX_MODE | linux.STATX_UID | linux.STATX_GID | linux.STATX_INO
+	rootStat, err := vfsObj.StatAt(ctx, creds, &vfs.PathOperation{
+		Root:  rootTopVD,
+		Start: rootTopVD,
+	}, &vfs.StatOptions{
+		Mask: rootStatMask,
+	})
+	if err != nil {
+		root.destroyLocked()
+		fs.vfsfs.DecRef()
+		return nil, nil, err
+	}
+	if rootStat.Mask&rootStatMask != rootStatMask {
+		root.destroyLocked()
+		fs.vfsfs.DecRef()
+		return nil, nil, syserror.EREMOTE
+	}
+	if isWhiteout(&rootStat) {
+		ctx.Warningf("overlay.FilesystemType.GetFilesystem: filesystem root is a whiteout")
+		root.destroyLocked()
+		fs.vfsfs.DecRef()
+		return nil, nil, syserror.EINVAL
+	}
+	root.mode = uint32(rootStat.Mode)
+	root.uid = rootStat.UID
+	root.gid = rootStat.GID
+	if rootStat.Mode&linux.S_IFMT == linux.S_IFDIR {
+		root.devMajor = linux.UNNAMED_MAJOR
+		root.devMinor = fs.dirDevMinor
+		root.ino = fs.newDirIno()
+	} else if !root.upperVD.Ok() {
+		root.devMajor = linux.UNNAMED_MAJOR
+		root.devMinor = fs.lowerDevMinors[root.lowerVDs[0].Mount().Filesystem()]
+		root.ino = rootStat.Ino
+	} else {
+		root.devMajor = rootStat.DevMajor
+		root.devMinor = rootStat.DevMinor
+		root.ino = rootStat.Ino
+	}
+
+	return &fs.vfsfs, &root.vfsd, nil
+}
+
+// clonePrivateMount creates a non-recursive bind mount rooted at vd, not
+// associated with any MountNamespace, and returns the root of the new mount.
+// (This is required to ensure that each layer of an overlay comprises only a
+// single mount, and therefore can't cross into e.g. the overlay filesystem
+// itself, risking lock recursion.) A reference is held on the returned
+// VirtualDentry.
+func clonePrivateMount(vfsObj *vfs.VirtualFilesystem, vd vfs.VirtualDentry, forceReadOnly bool) (vfs.VirtualDentry, error) {
+	oldmnt := vd.Mount()
+	opts := oldmnt.Options()
+	if forceReadOnly {
+		opts.ReadOnly = true
+	}
+	newmnt, err := vfsObj.NewDisconnectedMount(oldmnt.Filesystem(), vd.Dentry(), &opts)
+	if err != nil {
+		return vfs.VirtualDentry{}, err
+	}
+	return vfs.MakeVirtualDentry(newmnt, vd.Dentry()), nil
+}
+
+// Release implements vfs.FilesystemImpl.Release.
+func (fs *filesystem) Release() {
+	vfsObj := fs.vfsfs.VirtualFilesystem()
+	vfsObj.PutAnonBlockDevMinor(fs.dirDevMinor)
+	for _, lowerDevMinor := range fs.lowerDevMinors {
+		vfsObj.PutAnonBlockDevMinor(lowerDevMinor)
+	}
+	if fs.opts.UpperRoot.Ok() {
+		fs.opts.UpperRoot.DecRef()
+	}
+	for _, lowerRoot := range fs.opts.LowerRoots {
+		lowerRoot.DecRef()
+	}
+}
+
+func (fs *filesystem) statFS(ctx context.Context) (linux.Statfs, error) {
+	// Always statfs the root of the topmost layer. Compare Linux's
+	// fs/overlayfs/super.c:ovl_statfs().
+	var rootVD vfs.VirtualDentry
+	if fs.opts.UpperRoot.Ok() {
+		rootVD = fs.opts.UpperRoot
+	} else {
+		rootVD = fs.opts.LowerRoots[0]
+	}
+	fsstat, err := fs.vfsfs.VirtualFilesystem().StatFSAt(ctx, fs.creds, &vfs.PathOperation{
+		Root:  rootVD,
+		Start: rootVD,
+	})
+	if err != nil {
+		return linux.Statfs{}, err
+	}
+	fsstat.Type = linux.OVERLAYFS_SUPER_MAGIC
+	return fsstat, nil
+}
+
+func (fs *filesystem) newDirIno() uint64 {
+	return atomic.AddUint64(&fs.lastDirIno, 1)
+}
+
+// dentry implements vfs.DentryImpl.
+type dentry struct {
+	vfsd vfs.Dentry
+
+	refs int64
+
+	// fs is the owning filesystem. fs is immutable.
+	fs *filesystem
+
+	// mode, uid, and gid are the file mode, owner, and group of the file in
+	// the topmost layer (and therefore the overlay file as well), and are used
+	// for permission checks on this dentry. These fields are protected by
+	// copyMu and accessed using atomic memory operations.
+	mode uint32
+	uid  uint32
+	gid  uint32
+
+	// copiedUp is 1 if this dentry has been copied-up (i.e. upperVD.Ok()) and
+	// 0 otherwise. copiedUp is accessed using atomic memory operations.
+	copiedUp uint32
+
+	// parent is the dentry corresponding to this dentry's parent directory.
+	// name is this dentry's name in parent. If this dentry is a filesystem
+	// root, parent is nil and name is the empty string. parent and name are
+	// protected by fs.renameMu.
+	parent *dentry
+	name   string
+
+	// If this dentry represents a directory, children maps the names of
+	// children for which dentries have been instantiated to those dentries,
+	// and dirents (if not nil) is a cache of dirents as returned by
+	// directoryFDs representing this directory. children is protected by
+	// dirMu.
+	dirMu    sync.Mutex
+	children map[string]*dentry
+	dirents  []vfs.Dirent
+
+	// upperVD and lowerVDs are the files from the overlay filesystem's layers
+	// that comprise the file on the overlay filesystem.
+	//
+	// If !upperVD.Ok(), it can transition to a valid vfs.VirtualDentry (i.e.
+	// be copied up) with copyMu locked for writing; otherwise, it is
+	// immutable. lowerVDs is always immutable.
+	copyMu   sync.RWMutex
+	upperVD  vfs.VirtualDentry
+	lowerVDs []vfs.VirtualDentry
+
+	// inlineLowerVDs backs lowerVDs in the common case where len(lowerVDs) <=
+	// len(inlineLowerVDs).
+	inlineLowerVDs [1]vfs.VirtualDentry
+
+	// devMajor, devMinor, and ino are the device major/minor and inode numbers
+	// used by this dentry. These fields are protected by copyMu and accessed
+	// using atomic memory operations.
+	devMajor uint32
+	devMinor uint32
+	ino      uint64
+
+	locks lock.FileLocks
+}
+
+// newDentry creates a new dentry. The dentry initially has no references; it
+// is the caller's responsibility to set the dentry's reference count and/or
+// call dentry.destroy() as appropriate. The dentry is initially invalid in
+// that it contains no layers; the caller is responsible for setting them.
+func (fs *filesystem) newDentry() *dentry {
+	d := &dentry{
+		fs: fs,
+	}
+	d.lowerVDs = d.inlineLowerVDs[:0]
+	d.vfsd.Init(d)
+	return d
+}
+
+// IncRef implements vfs.DentryImpl.IncRef.
+func (d *dentry) IncRef() {
+	// d.refs may be 0 if d.fs.renameMu is locked, which serializes against
+	// d.checkDropLocked().
+	atomic.AddInt64(&d.refs, 1)
+}
+
+// TryIncRef implements vfs.DentryImpl.TryIncRef.
+func (d *dentry) TryIncRef() bool {
+	for {
+		refs := atomic.LoadInt64(&d.refs)
+		if refs <= 0 {
+			return false
+		}
+		if atomic.CompareAndSwapInt64(&d.refs, refs, refs+1) {
+			return true
+		}
+	}
+}
+
+// DecRef implements vfs.DentryImpl.DecRef.
+func (d *dentry) DecRef() {
+	if refs := atomic.AddInt64(&d.refs, -1); refs == 0 {
+		d.fs.renameMu.Lock()
+		d.checkDropLocked()
+		d.fs.renameMu.Unlock()
+	} else if refs < 0 {
+		panic("overlay.dentry.DecRef() called without holding a reference")
+	}
+}
+
+// checkDropLocked should be called after d's reference count becomes 0 or it
+// becomes deleted.
+//
+// Preconditions: d.fs.renameMu must be locked for writing.
+func (d *dentry) checkDropLocked() {
+	// Dentries with a positive reference count must be retained. (The only way
+	// to obtain a reference on a dentry with zero references is via path
+	// resolution, which requires renameMu, so if d.refs is zero then it will
+	// remain zero while we hold renameMu for writing.) Dentries with a
+	// negative reference count have already been destroyed.
+	if atomic.LoadInt64(&d.refs) != 0 {
+		return
+	}
+	// Refs is still zero; destroy it.
+	d.destroyLocked()
+	return
+}
+
+// destroyLocked destroys the dentry.
+//
+// Preconditions: d.fs.renameMu must be locked for writing. d.refs == 0.
+func (d *dentry) destroyLocked() {
+	switch atomic.LoadInt64(&d.refs) {
+	case 0:
+		// Mark the dentry destroyed.
+		atomic.StoreInt64(&d.refs, -1)
+	case -1:
+		panic("overlay.dentry.destroyLocked() called on already destroyed dentry")
+	default:
+		panic("overlay.dentry.destroyLocked() called with references on the dentry")
+	}
+
+	if d.upperVD.Ok() {
+		d.upperVD.DecRef()
+	}
+	for _, lowerVD := range d.lowerVDs {
+		lowerVD.DecRef()
+	}
+
+	if d.parent != nil {
+		d.parent.dirMu.Lock()
+		if !d.vfsd.IsDead() {
+			delete(d.parent.children, d.name)
+		}
+		d.parent.dirMu.Unlock()
+		// Drop the reference held by d on its parent without recursively
+		// locking d.fs.renameMu.
+		if refs := atomic.AddInt64(&d.parent.refs, -1); refs == 0 {
+			d.parent.checkDropLocked()
+		} else if refs < 0 {
+			panic("overlay.dentry.DecRef() called without holding a reference")
+		}
+	}
+}
+
+// InotifyWithParent implements vfs.DentryImpl.InotifyWithParent.
+func (d *dentry) InotifyWithParent(events uint32, cookie uint32, et vfs.EventType) {
+	// TODO(gvisor.dev/issue/1479): Implement inotify.
+}
+
+// Watches implements vfs.DentryImpl.Watches.
+func (d *dentry) Watches() *vfs.Watches {
+	// TODO(gvisor.dev/issue/1479): Implement inotify.
+	return nil
+}
+
+// iterLayers invokes yield on each layer comprising d, from top to bottom. If
+// any call to yield returns false, iterLayer stops iteration.
+func (d *dentry) iterLayers(yield func(vd vfs.VirtualDentry, isUpper bool) bool) {
+	if d.isCopiedUp() {
+		if !yield(d.upperVD, true) {
+			return
+		}
+	}
+	for _, lowerVD := range d.lowerVDs {
+		if !yield(lowerVD, false) {
+			return
+		}
+	}
+}
+
+func (d *dentry) topLayerInfo() (vd vfs.VirtualDentry, isUpper bool) {
+	if d.isCopiedUp() {
+		return d.upperVD, true
+	}
+	return d.lowerVDs[0], false
+}
+
+func (d *dentry) topLayer() vfs.VirtualDentry {
+	vd, _ := d.topLayerInfo()
+	return vd
+}
+
+func (d *dentry) checkPermissions(creds *auth.Credentials, ats vfs.AccessTypes) error {
+	return vfs.GenericCheckPermissions(creds, ats, linux.FileMode(atomic.LoadUint32(&d.mode)), auth.KUID(atomic.LoadUint32(&d.uid)), auth.KGID(atomic.LoadUint32(&d.gid)))
+}
+
+// statInternalMask is the set of stat fields that is set by
+// dentry.statInternalTo().
+const statInternalMask = linux.STATX_TYPE | linux.STATX_MODE | linux.STATX_UID | linux.STATX_GID | linux.STATX_INO
+
+// statInternalTo writes fields to stat that are stored in d, and therefore do
+// not requiring invoking StatAt on the overlay's layers.
+func (d *dentry) statInternalTo(ctx context.Context, opts *vfs.StatOptions, stat *linux.Statx) {
+	stat.Mask |= statInternalMask
+	if d.isDir() {
+		// Linux sets nlink to 1 for merged directories
+		// (fs/overlayfs/inode.c:ovl_getattr()); we set it to 2 because this is
+		// correct more often ("." and the directory's entry in its parent),
+		// and some of our tests expect this.
+		stat.Nlink = 2
+	}
+	stat.UID = atomic.LoadUint32(&d.uid)
+	stat.GID = atomic.LoadUint32(&d.gid)
+	stat.Mode = uint16(atomic.LoadUint32(&d.mode))
+	stat.Ino = atomic.LoadUint64(&d.ino)
+	stat.DevMajor = atomic.LoadUint32(&d.devMajor)
+	stat.DevMinor = atomic.LoadUint32(&d.devMinor)
+}
+
+// Preconditions: d.copyMu must be locked for writing.
+func (d *dentry) updateAfterSetStatLocked(opts *vfs.SetStatOptions) {
+	if opts.Stat.Mask&linux.STATX_MODE != 0 {
+		atomic.StoreUint32(&d.mode, (d.mode&linux.S_IFMT)|uint32(opts.Stat.Mode&^linux.S_IFMT))
+	}
+	if opts.Stat.Mask&linux.STATX_UID != 0 {
+		atomic.StoreUint32(&d.uid, opts.Stat.UID)
+	}
+	if opts.Stat.Mask&linux.STATX_GID != 0 {
+		atomic.StoreUint32(&d.gid, opts.Stat.GID)
+	}
+}
+
+// fileDescription is embedded by overlay implementations of
+// vfs.FileDescriptionImpl.
+type fileDescription struct {
+	vfsfd vfs.FileDescription
+	vfs.FileDescriptionDefaultImpl
+	vfs.LockFD
+}
+
+func (fd *fileDescription) filesystem() *filesystem {
+	return fd.vfsfd.Mount().Filesystem().Impl().(*filesystem)
+}
+
+func (fd *fileDescription) dentry() *dentry {
+	return fd.vfsfd.Dentry().Impl().(*dentry)
+}
diff --git a/pkg/sentry/fsimpl/overlay/overlay_state_autogen.go b/pkg/sentry/fsimpl/overlay/overlay_state_autogen.go
new file mode 100644
index 000000000..0c184964f
--- /dev/null
+++ b/pkg/sentry/fsimpl/overlay/overlay_state_autogen.go
@@ -0,0 +1,3 @@
+// automatically generated by stateify.
+
+package overlay
diff --git a/pkg/sentry/vfs/file_description.go b/pkg/sentry/vfs/file_description.go
index 97b9b18d7..13c48824e 100644
--- a/pkg/sentry/vfs/file_description.go
+++ b/pkg/sentry/vfs/file_description.go
@@ -108,6 +108,10 @@ type FileDescriptionOptions struct {
 	UseDentryMetadata bool
 }
 
+// FileCreationFlags are the set of flags passed to FileDescription.Init() but
+// omitted from FileDescription.StatusFlags().
+const FileCreationFlags = linux.O_CREAT | linux.O_EXCL | linux.O_NOCTTY | linux.O_TRUNC
+
 // Init must be called before first use of fd. If it succeeds, it takes
 // references on mnt and d. flags is the initial file description flags, which
 // is usually the full set of flags passed to open(2).
@@ -122,8 +126,8 @@ func (fd *FileDescription) Init(impl FileDescriptionImpl, flags uint32, mnt *Mou
 	fd.refs = 1
 
 	// Remove "file creation flags" to mirror the behavior from file.f_flags in
-	// fs/open.c:do_dentry_open
-	fd.statusFlags = flags &^ (linux.O_CREAT | linux.O_EXCL | linux.O_NOCTTY | linux.O_TRUNC)
+	// fs/open.c:do_dentry_open.
+	fd.statusFlags = flags &^ FileCreationFlags
 	fd.vd = VirtualDentry{
 		mount:  mnt,
 		dentry: d,
@@ -471,6 +475,15 @@ type IterDirentsCallback interface {
 	Handle(dirent Dirent) error
 }
 
+// IterDirentsCallbackFunc implements IterDirentsCallback for a function with
+// the semantics of IterDirentsCallback.Handle.
+type IterDirentsCallbackFunc func(dirent Dirent) error
+
+// Handle implements IterDirentsCallback.Handle.
+func (f IterDirentsCallbackFunc) Handle(dirent Dirent) error {
+	return f(dirent)
+}
+
 // OnClose is called when a file descriptor representing the FileDescription is
 // closed. Returning a non-nil error should not prevent the file descriptor
 // from being closed.
diff --git a/pkg/syserror/syserror.go b/pkg/syserror/syserror.go
index f86db0999..c73072c42 100644
--- a/pkg/syserror/syserror.go
+++ b/pkg/syserror/syserror.go
@@ -72,6 +72,7 @@ var (
 	EPERM        = error(syscall.EPERM)
 	EPIPE        = error(syscall.EPIPE)
 	ERANGE       = error(syscall.ERANGE)
+	EREMOTE      = error(syscall.EREMOTE)
 	EROFS        = error(syscall.EROFS)
 	ESPIPE       = error(syscall.ESPIPE)
 	ESRCH        = error(syscall.ESRCH)
diff --git a/runsc/boot/vfs.go b/runsc/boot/vfs.go
index 8eeb43e79..d1653b279 100644
--- a/runsc/boot/vfs.go
+++ b/runsc/boot/vfs.go
@@ -30,6 +30,7 @@ import (
 	"gvisor.dev/gvisor/pkg/sentry/fsimpl/devpts"
 	"gvisor.dev/gvisor/pkg/sentry/fsimpl/devtmpfs"
 	"gvisor.dev/gvisor/pkg/sentry/fsimpl/gofer"
+	"gvisor.dev/gvisor/pkg/sentry/fsimpl/overlay"
 	"gvisor.dev/gvisor/pkg/sentry/fsimpl/proc"
 	"gvisor.dev/gvisor/pkg/sentry/fsimpl/sys"
 	"gvisor.dev/gvisor/pkg/sentry/fsimpl/tmpfs"
@@ -53,6 +54,10 @@ func registerFilesystems(ctx context.Context, vfsObj *vfs.VirtualFilesystem, cre
 	vfsObj.MustRegisterFilesystemType(gofer.Name, &gofer.FilesystemType{}, &vfs.RegisterFilesystemTypeOptions{
 		AllowUserList: true,
 	})
+	vfsObj.MustRegisterFilesystemType(overlay.Name, &overlay.FilesystemType{}, &vfs.RegisterFilesystemTypeOptions{
+		AllowUserMount: true,
+		AllowUserList:  true,
+	})
 	vfsObj.MustRegisterFilesystemType(proc.Name, &proc.FilesystemType{}, &vfs.RegisterFilesystemTypeOptions{
 		AllowUserMount: true,
 		AllowUserList:  true,