Add vfs.MkdirOptions.ForSyntheticMountpoint.

PiperOrigin-RevId: 308143529

6 files changed, 455 insertions, 216 deletions

diff --git a/pkg/sentry/fsimpl/gofer/directory.go b/pkg/sentry/fsimpl/gofer/directory.go
index d02691232..c67766ab2 100644
--- a/pkg/sentry/fsimpl/gofer/directory.go
+++ b/pkg/sentry/fsimpl/gofer/directory.go
@@ -21,8 +21,10 @@ import (
 	"gvisor.dev/gvisor/pkg/abi/linux"
 	"gvisor.dev/gvisor/pkg/context"
 	"gvisor.dev/gvisor/pkg/p9"
+	"gvisor.dev/gvisor/pkg/sentry/kernel/auth"
 	"gvisor.dev/gvisor/pkg/sentry/vfs"
 	"gvisor.dev/gvisor/pkg/syserror"
+	"gvisor.dev/gvisor/pkg/usermem"
 )
 
 func (d *dentry) isDir() bool {
@@ -41,15 +43,46 @@ func (d *dentry) cacheNewChildLocked(child *dentry, name string) {
 	d.children[name] = child
 }
 
-// Preconditions: d.dirMu must be locked. d.isDir(). fs.opts.interop !=
-// InteropModeShared.
-func (d *dentry) cacheNegativeChildLocked(name string) {
+// Preconditions: d.dirMu must be locked. d.isDir().
+func (d *dentry) cacheNegativeLookupLocked(name string) {
+	// Don't cache negative lookups if InteropModeShared is in effect (since
+	// this makes remote lookup unavoidable), or if d.isSynthetic() (in which
+	// case the only files in the directory are those for which a dentry exists
+	// in d.children). Instead, just delete any previously-cached dentry.
+	if d.fs.opts.interop == InteropModeShared || d.isSynthetic() {
+		delete(d.children, name)
+		return
+	}
 	if d.children == nil {
 		d.children = make(map[string]*dentry)
 	}
 	d.children[name] = nil
 }
 
+// createSyntheticDirectory creates a synthetic directory with the given name
+// in d.
+//
+// Preconditions: d.dirMu must be locked. d.isDir(). d does not already contain
+// a child with the given name.
+func (d *dentry) createSyntheticDirectoryLocked(name string, mode linux.FileMode, kuid auth.KUID, kgid auth.KGID) {
+	d2 := &dentry{
+		refs:      1, // held by d
+		fs:        d.fs,
+		mode:      uint32(mode) | linux.S_IFDIR,
+		uid:       uint32(kuid),
+		gid:       uint32(kgid),
+		blockSize: usermem.PageSize, // arbitrary
+		handle: handle{
+			fd: -1,
+		},
+	}
+	d2.pf.dentry = d2
+	d2.vfsd.Init(d2)
+
+	d.cacheNewChildLocked(d2, name)
+	d.syntheticChildren++
+}
+
 type directoryFD struct {
 	fileDescription
 	vfs.DirectoryFileDescriptionDefaultImpl
@@ -77,7 +110,7 @@ func (fd *directoryFD) IterDirents(ctx context.Context, cb vfs.IterDirentsCallba
 		fd.dirents = ds
 	}
 
-	if d.fs.opts.interop != InteropModeShared {
+	if d.cachedMetadataAuthoritative() {
 		d.touchAtime(fd.vfsfd.Mount())
 	}
 
@@ -108,10 +141,10 @@ func (d *dentry) getDirents(ctx context.Context) ([]vfs.Dirent, error) {
 	// filesystem.renameMu is needed for d.parent, and must be locked before
 	// dentry.dirMu.
 	d.fs.renameMu.RLock()
+	defer d.fs.renameMu.RUnlock()
 	d.dirMu.Lock()
 	defer d.dirMu.Unlock()
 	if d.dirents != nil {
-		d.fs.renameMu.RUnlock()
 		return d.dirents, nil
 	}
 
@@ -132,51 +165,81 @@ func (d *dentry) getDirents(ctx context.Context) ([]vfs.Dirent, error) {
 			NextOff: 2,
 		},
 	}
-	d.fs.renameMu.RUnlock()
-	off := uint64(0)
-	const count = 64 * 1024 // for consistency with the vfs1 client
-	d.handleMu.RLock()
-	defer d.handleMu.RUnlock()
-	if !d.handleReadable {
-		// This should not be possible because a readable handle should have
-		// been opened when the calling directoryFD was opened.
-		panic("gofer.dentry.getDirents called without a readable handle")
-	}
-	for {
-		p9ds, err := d.handle.file.readdir(ctx, off, count)
-		if err != nil {
-			return nil, err
+	var realChildren map[string]struct{}
+	if !d.isSynthetic() {
+		if d.syntheticChildren != 0 && d.fs.opts.interop == InteropModeShared {
+			// Record the set of children d actually has so that we don't emit
+			// duplicate entries for synthetic children.
+			realChildren = make(map[string]struct{})
 		}
-		if len(p9ds) == 0 {
-			// Cache dirents for future directoryFDs if permitted.
-			if d.fs.opts.interop != InteropModeShared {
-				d.dirents = dirents
+		off := uint64(0)
+		const count = 64 * 1024 // for consistency with the vfs1 client
+		d.handleMu.RLock()
+		if !d.handleReadable {
+			// This should not be possible because a readable handle should
+			// have been opened when the calling directoryFD was opened.
+			d.handleMu.RUnlock()
+			panic("gofer.dentry.getDirents called without a readable handle")
+		}
+		for {
+			p9ds, err := d.handle.file.readdir(ctx, off, count)
+			if err != nil {
+				d.handleMu.RUnlock()
+				return nil, err
+			}
+			if len(p9ds) == 0 {
+				d.handleMu.RUnlock()
+				break
+			}
+			for _, p9d := range p9ds {
+				if p9d.Name == "." || p9d.Name == ".." {
+					continue
+				}
+				dirent := vfs.Dirent{
+					Name:    p9d.Name,
+					Ino:     p9d.QID.Path,
+					NextOff: int64(len(dirents) + 1),
+				}
+				// p9 does not expose 9P2000.U's DMDEVICE, DMNAMEDPIPE, or
+				// DMSOCKET.
+				switch p9d.Type {
+				case p9.TypeSymlink:
+					dirent.Type = linux.DT_LNK
+				case p9.TypeDir:
+					dirent.Type = linux.DT_DIR
+				default:
+					dirent.Type = linux.DT_REG
+				}
+				dirents = append(dirents, dirent)
+				if realChildren != nil {
+					realChildren[p9d.Name] = struct{}{}
+				}
 			}
-			return dirents, nil
+			off = p9ds[len(p9ds)-1].Offset
 		}
-		for _, p9d := range p9ds {
-			if p9d.Name == "." || p9d.Name == ".." {
+	}
+	// Emit entries for synthetic children.
+	if d.syntheticChildren != 0 {
+		for _, child := range d.children {
+			if child == nil || !child.isSynthetic() {
 				continue
 			}
-			dirent := vfs.Dirent{
-				Name:    p9d.Name,
-				Ino:     p9d.QID.Path,
-				NextOff: int64(len(dirents) + 1),
-			}
-			// p9 does not expose 9P2000.U's DMDEVICE, DMNAMEDPIPE, or
-			// DMSOCKET.
-			switch p9d.Type {
-			case p9.TypeSymlink:
-				dirent.Type = linux.DT_LNK
-			case p9.TypeDir:
-				dirent.Type = linux.DT_DIR
-			default:
-				dirent.Type = linux.DT_REG
+			if _, ok := realChildren[child.name]; ok {
+				continue
 			}
-			dirents = append(dirents, dirent)
+			dirents = append(dirents, vfs.Dirent{
+				Name:    child.name,
+				Type:    uint8(atomic.LoadUint32(&child.mode) >> 12),
+				Ino:     child.ino,
+				NextOff: int64(len(dirents) + 1),
+			})
 		}
-		off = p9ds[len(p9ds)-1].Offset
 	}
+	// Cache dirents for future directoryFDs if permitted.
+	if d.cachedMetadataAuthoritative() {
+		d.dirents = dirents
+	}
+	return dirents, nil
 }
 
 // Seek implements vfs.FileDescriptionImpl.Seek.
diff --git a/pkg/sentry/fsimpl/gofer/filesystem.go b/pkg/sentry/fsimpl/gofer/filesystem.go
index eba4aabe8..98ccb42fd 100644
--- a/pkg/sentry/fsimpl/gofer/filesystem.go
+++ b/pkg/sentry/fsimpl/gofer/filesystem.go
@@ -29,14 +29,16 @@ import (
 
 // Sync implements vfs.FilesystemImpl.Sync.
 func (fs *filesystem) Sync(ctx context.Context) error {
-	// Snapshot current dentries and special files.
+	// Snapshot current syncable dentries and special files.
 	fs.syncMu.Lock()
-	ds := make([]*dentry, 0, len(fs.dentries))
-	for d := range fs.dentries {
+	ds := make([]*dentry, 0, len(fs.syncableDentries))
+	for d := range fs.syncableDentries {
+		d.IncRef()
 		ds = append(ds, d)
 	}
 	sffds := make([]*specialFileFD, 0, len(fs.specialFileFDs))
 	for sffd := range fs.specialFileFDs {
+		sffd.vfsfd.IncRef()
 		sffds = append(sffds, sffd)
 	}
 	fs.syncMu.Unlock()
@@ -47,9 +49,6 @@ func (fs *filesystem) Sync(ctx context.Context) error {
 
 	// Sync regular files.
 	for _, d := range ds {
-		if !d.TryIncRef() {
-			continue
-		}
 		err := d.syncSharedHandle(ctx)
 		d.DecRef()
 		if err != nil && retErr == nil {
@@ -60,9 +59,6 @@ func (fs *filesystem) Sync(ctx context.Context) error {
 	// Sync special files, which may be writable but do not use dentry shared
 	// handles (so they won't be synced by the above).
 	for _, sffd := range sffds {
-		if !sffd.vfsfd.TryIncRef() {
-			continue
-		}
 		err := sffd.Sync(ctx)
 		sffd.vfsfd.DecRef()
 		if err != nil && retErr == nil {
@@ -114,8 +110,8 @@ func putDentrySlice(ds *[]*dentry) {
 // to *ds.
 //
 // Preconditions: fs.renameMu must be locked. d.dirMu must be locked.
-// !rp.Done(). If fs.opts.interop == InteropModeShared, then d's cached
-// metadata must be up to date.
+// !rp.Done(). If !d.cachedMetadataAuthoritative(), then d's cached metadata
+// 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) {
@@ -148,7 +144,7 @@ afterSymlink:
 		if err := rp.CheckMount(&d.parent.vfsd); err != nil {
 			return nil, err
 		}
-		if fs.opts.interop == InteropModeShared && d != d.parent {
+		if d != d.parent && !d.cachedMetadataAuthoritative() {
 			_, attrMask, attr, err := d.parent.file.getAttr(ctx, dentryAttrMask())
 			if err != nil {
 				return nil, err
@@ -195,7 +191,7 @@ func (fs *filesystem) getChildLocked(ctx context.Context, vfsObj *vfs.VirtualFil
 		return nil, syserror.ENAMETOOLONG
 	}
 	child, ok := parent.children[name]
-	if ok && fs.opts.interop != InteropModeShared {
+	if (ok && fs.opts.interop != InteropModeShared) || parent.isSynthetic() {
 		// Whether child is nil or not, it is cached information that is
 		// assumed to be correct.
 		return child, nil
@@ -206,7 +202,7 @@ func (fs *filesystem) getChildLocked(ctx context.Context, vfsObj *vfs.VirtualFil
 	return fs.revalidateChildLocked(ctx, vfsObj, parent, name, child, ds)
 }
 
-// Preconditions: As for getChildLocked.
+// Preconditions: As for getChildLocked. !parent.isSynthetic().
 func (fs *filesystem) revalidateChildLocked(ctx context.Context, vfsObj *vfs.VirtualFilesystem, parent *dentry, name string, child *dentry, ds **[]*dentry) (*dentry, error) {
 	qid, file, attrMask, attr, err := parent.file.walkGetAttrOne(ctx, name)
 	if err != nil && err != syserror.ENOENT {
@@ -220,24 +216,41 @@ func (fs *filesystem) revalidateChildLocked(ctx context.Context, vfsObj *vfs.Vir
 			child.updateFromP9Attrs(attrMask, &attr)
 			return child, nil
 		}
-		// The file at this path has changed or no longer exists. Remove
-		// the stale dentry from the tree, and re-evaluate its caching
-		// status (i.e. if it has 0 references, drop it).
+		if file.isNil() && child.isSynthetic() {
+			// We have a synthetic file, and no remote file has arisen to
+			// replace it.
+			return child, nil
+		}
+		// The file at this path has changed or no longer exists. Mark the
+		// dentry invalidated, and re-evaluate its caching status (i.e. if it
+		// has 0 references, drop it). Wait to update parent.children until we
+		// know what to replace the existing dentry with (i.e. one of the
+		// returns below), to avoid a redundant map access.
 		vfsObj.InvalidateDentry(&child.vfsd)
+		if child.isSynthetic() {
+			// Normally we don't mark invalidated dentries as deleted since
+			// they may still exist (but at a different path), and also for
+			// consistency with Linux. However, synthetic files are guaranteed
+			// to become unreachable if their dentries are invalidated, so
+			// treat their invalidation as deletion.
+			child.setDeleted()
+			parent.syntheticChildren--
+			child.decRefLocked()
+			parent.dirents = nil
+		}
 		*ds = appendDentry(*ds, child)
 	}
 	if file.isNil() {
 		// No file exists at this path now. Cache the negative lookup if
 		// allowed.
-		if fs.opts.interop != InteropModeShared {
-			parent.cacheNegativeChildLocked(name)
-		}
+		parent.cacheNegativeLookupLocked(name)
 		return nil, nil
 	}
 	// Create a new dentry representing the file.
 	child, err = fs.newDentry(ctx, file, qid, attrMask, &attr)
 	if err != nil {
 		file.close(ctx)
+		delete(parent.children, name)
 		return nil, err
 	}
 	parent.cacheNewChildLocked(child, name)
@@ -252,8 +265,9 @@ func (fs *filesystem) revalidateChildLocked(ctx context.Context, vfsObj *vfs.Vir
 // 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(). If fs.opts.interop ==
-// InteropModeShared, then d's cached metadata must be up to date.
+// Preconditions: fs.renameMu must be locked. !rp.Done(). If
+// !d.cachedMetadataAuthoritative(), then d's cached metadata must be up to
+// date.
 func (fs *filesystem) walkParentDirLocked(ctx context.Context, rp *vfs.ResolvingPath, d *dentry, ds **[]*dentry) (*dentry, error) {
 	for !rp.Final() {
 		d.dirMu.Lock()
@@ -275,7 +289,7 @@ func (fs *filesystem) walkParentDirLocked(ctx context.Context, rp *vfs.Resolving
 // 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)
-	if fs.opts.interop == InteropModeShared {
+	if !d.cachedMetadataAuthoritative() {
 		// Get updated metadata for rp.Start() as required by fs.stepLocked().
 		if err := d.updateFromGetattr(ctx); err != nil {
 			return nil, err
@@ -297,16 +311,17 @@ func (fs *filesystem) resolveLocked(ctx context.Context, rp *vfs.ResolvingPath,
 }
 
 // doCreateAt checks that creating a file at rp is permitted, then invokes
-// create to do so.
+// createInRemoteDir (if the parent directory is a real remote directory) or
+// createInSyntheticDir (if the parent directory is synthetic) 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) error) error {
+func (fs *filesystem) doCreateAt(ctx context.Context, rp *vfs.ResolvingPath, dir bool, createInRemoteDir func(parent *dentry, name string) error, createInSyntheticDir func(parent *dentry, name string) error) error {
 	var ds *[]*dentry
 	fs.renameMu.RLock()
 	defer fs.renameMuRUnlockAndCheckCaching(&ds)
 	start := rp.Start().Impl().(*dentry)
-	if fs.opts.interop == InteropModeShared {
+	if !start.cachedMetadataAuthoritative() {
 		// Get updated metadata for start as required by
 		// fs.walkParentDirLocked().
 		if err := start.updateFromGetattr(ctx); err != nil {
@@ -340,6 +355,20 @@ func (fs *filesystem) doCreateAt(ctx context.Context, rp *vfs.ResolvingPath, dir
 	defer mnt.EndWrite()
 	parent.dirMu.Lock()
 	defer parent.dirMu.Unlock()
+	if parent.isSynthetic() {
+		if child := parent.children[name]; child != nil {
+			return syserror.EEXIST
+		}
+		if createInSyntheticDir == nil {
+			return syserror.EPERM
+		}
+		if err := createInSyntheticDir(parent, name); err != nil {
+			return err
+		}
+		parent.touchCMtime()
+		parent.dirents = nil
+		return nil
+	}
 	if fs.opts.interop == InteropModeShared {
 		// The existence of a dentry at name would be inconclusive because the
 		// file it represents may have been deleted from the remote filesystem,
@@ -348,21 +377,21 @@ func (fs *filesystem) doCreateAt(ctx context.Context, rp *vfs.ResolvingPath, dir
 		// will fail with EEXIST like we would have. If the RPC succeeds, and a
 		// stale dentry exists, the dentry will fail revalidation next time
 		// it's used.
-		return create(parent, name)
+		return createInRemoteDir(parent, name)
 	}
 	if child := parent.children[name]; child != nil {
 		return syserror.EEXIST
 	}
 	// No cached dentry exists; however, there might still be an existing file
 	// at name. As above, we attempt the file creation RPC anyway.
-	if err := create(parent, name); err != nil {
+	if err := createInRemoteDir(parent, name); err != nil {
 		return err
 	}
+	if child, ok := parent.children[name]; ok && child == nil {
+		// Delete the now-stale negative dentry.
+		delete(parent.children, name)
+	}
 	parent.touchCMtime()
-	// Either parent.children[name] doesn't exist (in which case this is a
-	// no-op) or is nil (in which case this erases the now-stale information
-	// that the file doesn't exist).
-	delete(parent.children, name)
 	parent.dirents = nil
 	return nil
 }
@@ -373,7 +402,7 @@ func (fs *filesystem) unlinkAt(ctx context.Context, rp *vfs.ResolvingPath, dir b
 	fs.renameMu.RLock()
 	defer fs.renameMuRUnlockAndCheckCaching(&ds)
 	start := rp.Start().Impl().(*dentry)
-	if fs.opts.interop == InteropModeShared {
+	if !start.cachedMetadataAuthoritative() {
 		// Get updated metadata for start as required by
 		// fs.walkParentDirLocked().
 		if err := start.updateFromGetattr(ctx); err != nil {
@@ -421,8 +450,10 @@ func (fs *filesystem) unlinkAt(ctx context.Context, rp *vfs.ResolvingPath, dir b
 	// only revalidating the dentry if that fails (indicating that the existing
 	// dentry is a mount point).
 	if child != nil {
+		child.dirMu.Lock()
+		defer child.dirMu.Unlock()
 		if err := vfsObj.PrepareDeleteDentry(mntns, &child.vfsd); err != nil {
-			if fs.opts.interop != InteropModeShared {
+			if parent.cachedMetadataAuthoritative() {
 				return err
 			}
 			child, err = fs.revalidateChildLocked(ctx, vfsObj, parent, name, child, &ds)
@@ -437,13 +468,37 @@ func (fs *filesystem) unlinkAt(ctx context.Context, rp *vfs.ResolvingPath, dir b
 		}
 	}
 	flags := uint32(0)
+	// If a dentry exists, use it for best-effort checks on its deletability.
 	if dir {
-		if child != nil && !child.isDir() {
-			vfsObj.AbortDeleteDentry(&child.vfsd)
-			return syserror.ENOTDIR
+		if child != nil {
+			// child must be an empty directory.
+			if child.syntheticChildren != 0 {
+				// This is definitely not an empty directory, irrespective of
+				// fs.opts.interop.
+				vfsObj.AbortDeleteDentry(&child.vfsd)
+				return syserror.ENOTEMPTY
+			}
+			// If InteropModeShared is in effect and the first call to
+			// PrepareDeleteDentry above succeeded, then child wasn't
+			// revalidated (so we can't expect its file type to be correct) and
+			// individually revalidating its children (to confirm that they
+			// still exist) would be a waste of time.
+			if child.cachedMetadataAuthoritative() {
+				if !child.isDir() {
+					vfsObj.AbortDeleteDentry(&child.vfsd)
+					return syserror.ENOTDIR
+				}
+				for _, grandchild := range child.children {
+					if grandchild != nil {
+						vfsObj.AbortDeleteDentry(&child.vfsd)
+						return syserror.ENOTEMPTY
+					}
+				}
+			}
 		}
 		flags = linux.AT_REMOVEDIR
 	} else {
+		// child must be a non-directory file.
 		if child != nil && child.isDir() {
 			vfsObj.AbortDeleteDentry(&child.vfsd)
 			return syserror.EISDIR
@@ -455,28 +510,36 @@ func (fs *filesystem) unlinkAt(ctx context.Context, rp *vfs.ResolvingPath, dir b
 			return syserror.ENOTDIR
 		}
 	}
-	err = parent.file.unlinkAt(ctx, name, flags)
-	if err != nil {
-		if child != nil {
-			vfsObj.AbortDeleteDentry(&child.vfsd)
-		}
-		return err
-	}
-	if fs.opts.interop != InteropModeShared {
-		parent.touchCMtime()
-		if dir {
-			parent.decLinks()
+	if parent.isSynthetic() {
+		if child == nil {
+			return syserror.ENOENT
 		}
-		parent.cacheNegativeChildLocked(name)
-		parent.dirents = nil
 	} else {
-		delete(parent.children, name)
+		err = parent.file.unlinkAt(ctx, name, flags)
+		if err != nil {
+			if child != nil {
+				vfsObj.AbortDeleteDentry(&child.vfsd)
+			}
+			return err
+		}
 	}
 	if child != nil {
-		child.setDeleted()
 		vfsObj.CommitDeleteDentry(&child.vfsd)
+		child.setDeleted()
+		if child.isSynthetic() {
+			parent.syntheticChildren--
+			child.decRefLocked()
+		}
 		ds = appendDentry(ds, child)
 	}
+	parent.cacheNegativeLookupLocked(name)
+	if parent.cachedMetadataAuthoritative() {
+		parent.dirents = nil
+		parent.touchCMtime()
+		if dir {
+			parent.decLinks()
+		}
+	}
 	return nil
 }
 
@@ -554,7 +617,7 @@ func (fs *filesystem) GetParentDentryAt(ctx context.Context, rp *vfs.ResolvingPa
 	fs.renameMu.RLock()
 	defer fs.renameMuRUnlockAndCheckCaching(&ds)
 	start := rp.Start().Impl().(*dentry)
-	if fs.opts.interop == InteropModeShared {
+	if !start.cachedMetadataAuthoritative() {
 		// Get updated metadata for start as required by
 		// fs.walkParentDirLocked().
 		if err := start.updateFromGetattr(ctx); err != nil {
@@ -577,20 +640,32 @@ func (fs *filesystem) LinkAt(ctx context.Context, rp *vfs.ResolvingPath, vd vfs.
 		}
 		// 9P2000.L supports hard links, but we don't.
 		return syserror.EPERM
-	})
+	}, nil)
 }
 
 // MkdirAt implements vfs.FilesystemImpl.MkdirAt.
 func (fs *filesystem) MkdirAt(ctx context.Context, rp *vfs.ResolvingPath, opts vfs.MkdirOptions) error {
+	creds := rp.Credentials()
 	return fs.doCreateAt(ctx, rp, true /* dir */, func(parent *dentry, name string) error {
-		creds := rp.Credentials()
 		if _, err := parent.file.mkdir(ctx, name, (p9.FileMode)(opts.Mode), (p9.UID)(creds.EffectiveKUID), (p9.GID)(creds.EffectiveKGID)); err != nil {
-			return err
+			if !opts.ForSyntheticMountpoint || err == syserror.EEXIST {
+				return err
+			}
+			ctx.Infof("Failed to create remote directory %q: %v; falling back to synthetic directory", name, err)
+			parent.createSyntheticDirectoryLocked(name, opts.Mode, creds.EffectiveKUID, creds.EffectiveKGID)
 		}
 		if fs.opts.interop != InteropModeShared {
 			parent.incLinks()
 		}
 		return nil
+	}, func(parent *dentry, name string) error {
+		if !opts.ForSyntheticMountpoint {
+			// Can't create non-synthetic files in synthetic directories.
+			return syserror.EPERM
+		}
+		parent.createSyntheticDirectoryLocked(name, opts.Mode, creds.EffectiveKUID, creds.EffectiveKGID)
+		parent.incLinks()
+		return nil
 	})
 }
 
@@ -600,7 +675,7 @@ func (fs *filesystem) MknodAt(ctx context.Context, rp *vfs.ResolvingPath, opts v
 		creds := rp.Credentials()
 		_, err := parent.file.mknod(ctx, name, (p9.FileMode)(opts.Mode), opts.DevMajor, opts.DevMinor, (p9.UID)(creds.EffectiveKUID), (p9.GID)(creds.EffectiveKGID))
 		return err
-	})
+	}, nil)
 }
 
 // OpenAt implements vfs.FilesystemImpl.OpenAt.
@@ -620,7 +695,7 @@ func (fs *filesystem) OpenAt(ctx context.Context, rp *vfs.ResolvingPath, opts vf
 	defer fs.renameMuRUnlockAndCheckCaching(&ds)
 
 	start := rp.Start().Impl().(*dentry)
-	if fs.opts.interop == InteropModeShared {
+	if !start.cachedMetadataAuthoritative() {
 		// Get updated metadata for start as required by fs.stepLocked().
 		if err := start.updateFromGetattr(ctx); err != nil {
 			return nil, err
@@ -643,6 +718,10 @@ afterTrailingSymlink:
 	parent.dirMu.Lock()
 	child, err := fs.stepLocked(ctx, rp, parent, &ds)
 	if err == syserror.ENOENT && mayCreate {
+		if parent.isSynthetic() {
+			parent.dirMu.Unlock()
+			return nil, syserror.EPERM
+		}
 		fd, err := parent.createAndOpenChildLocked(ctx, rp, &opts)
 		parent.dirMu.Unlock()
 		return fd, err
@@ -702,8 +781,10 @@ func (d *dentry) openLocked(ctx context.Context, rp *vfs.ResolvingPath, opts *vf
 		if opts.Flags&linux.O_DIRECT != 0 {
 			return nil, syserror.EINVAL
 		}
-		if err := d.ensureSharedHandle(ctx, ats&vfs.MayRead != 0, false /* write */, false /* trunc */); err != nil {
-			return nil, err
+		if !d.isSynthetic() {
+			if err := d.ensureSharedHandle(ctx, ats&vfs.MayRead != 0, false /* write */, false /* trunc */); err != nil {
+				return nil, err
+			}
 		}
 		fd := &directoryFD{}
 		if err := fd.vfsfd.Init(fd, opts.Flags, mnt, &d.vfsd, &vfs.FileDescriptionOptions{}); err != nil {
@@ -733,6 +814,7 @@ func (d *dentry) openLocked(ctx context.Context, rp *vfs.ResolvingPath, opts *vf
 }
 
 // Preconditions: d.fs.renameMu must be locked. d.dirMu must be locked.
+// !d.isSynthetic().
 func (d *dentry) createAndOpenChildLocked(ctx context.Context, rp *vfs.ResolvingPath, opts *vfs.OpenOptions) (*vfs.FileDescription, error) {
 	if err := d.checkPermissions(rp.Credentials(), vfs.MayWrite); err != nil {
 		return nil, err
@@ -811,7 +893,7 @@ func (d *dentry) createAndOpenChildLocked(ctx context.Context, rp *vfs.Resolving
 	child.refs = 1
 	// Insert the dentry into the tree.
 	d.cacheNewChildLocked(child, name)
-	if d.fs.opts.interop != InteropModeShared {
+	if d.cachedMetadataAuthoritative() {
 		d.touchCMtime()
 		d.dirents = nil
 	}
@@ -888,7 +970,7 @@ func (fs *filesystem) RenameAt(ctx context.Context, rp *vfs.ResolvingPath, oldPa
 	defer mnt.EndWrite()
 
 	oldParent := oldParentVD.Dentry().Impl().(*dentry)
-	if fs.opts.interop == InteropModeShared {
+	if !oldParent.cachedMetadataAuthoritative() {
 		if err := oldParent.updateFromGetattr(ctx); err != nil {
 			return err
 		}
@@ -933,35 +1015,22 @@ func (fs *filesystem) RenameAt(ctx context.Context, rp *vfs.ResolvingPath, oldPa
 	if newParent.isDeleted() {
 		return syserror.ENOENT
 	}
-	replaced := newParent.children[newName]
-	// This is similar to unlinkAt, except:
-	//
-	// - If a dentry exists for the file to be replaced, we revalidate it
-	// unconditionally (instead of only if PrepareRenameDentry fails) for
-	// simplicity.
-	//
-	// - If rp.MustBeDir(), then we need a dentry representing the replaced
-	// file regardless to confirm that it's a directory.
-	if replaced != nil || rp.MustBeDir() {
-		replaced, err = fs.getChildLocked(ctx, rp.VirtualFilesystem(), newParent, newName, &ds)
-		if err != nil {
-			return err
-		}
-		if replaced != nil {
-			if replaced.isDir() {
-				if !renamed.isDir() {
-					return syserror.EISDIR
-				}
-			} else {
-				if rp.MustBeDir() || renamed.isDir() {
-					return syserror.ENOTDIR
-				}
-			}
-		}
+	replaced, err := fs.getChildLocked(ctx, rp.VirtualFilesystem(), newParent, newName, &ds)
+	if err != nil {
+		return err
 	}
 	var replacedVFSD *vfs.Dentry
 	if replaced != nil {
 		replacedVFSD = &replaced.vfsd
+		if replaced.isDir() {
+			if !renamed.isDir() {
+				return syserror.EISDIR
+			}
+		} else {
+			if rp.MustBeDir() || renamed.isDir() {
+				return syserror.ENOTDIR
+			}
+		}
 	}
 
 	if oldParent == newParent && oldName == newName {
@@ -972,27 +1041,47 @@ func (fs *filesystem) RenameAt(ctx context.Context, rp *vfs.ResolvingPath, oldPa
 	if err := vfsObj.PrepareRenameDentry(mntns, &renamed.vfsd, replacedVFSD); err != nil {
 		return err
 	}
-	if err := renamed.file.rename(ctx, newParent.file, newName); err != nil {
-		vfsObj.AbortRenameDentry(&renamed.vfsd, replacedVFSD)
-		return err
+
+	// Update the remote filesystem.
+	if !renamed.isSynthetic() {
+		if err := renamed.file.rename(ctx, newParent.file, newName); err != nil {
+			vfsObj.AbortRenameDentry(&renamed.vfsd, replacedVFSD)
+			return err
+		}
+	} else if replaced != nil && !replaced.isSynthetic() {
+		// We are replacing an existing real file with a synthetic one, so we
+		// need to unlink the former.
+		flags := uint32(0)
+		if replaced.isDir() {
+			flags = linux.AT_REMOVEDIR
+		}
+		if err := newParent.file.unlinkAt(ctx, newName, flags); err != nil {
+			vfsObj.AbortRenameDentry(&renamed.vfsd, replacedVFSD)
+			return err
+		}
 	}
-	if fs.opts.interop != InteropModeShared {
-		oldParent.cacheNegativeChildLocked(oldName)
-		oldParent.dirents = nil
-		newParent.dirents = nil
-		if renamed.isDir() {
-			oldParent.decLinks()
-			newParent.incLinks()
+
+	// Update the dentry tree.
+	vfsObj.CommitRenameReplaceDentry(&renamed.vfsd, replacedVFSD)
+	if replaced != nil {
+		replaced.setDeleted()
+		if replaced.isSynthetic() {
+			newParent.syntheticChildren--
+			replaced.decRefLocked()
 		}
-		oldParent.touchCMtime()
-		newParent.touchCMtime()
-		renamed.touchCtime()
-	} else {
-		delete(oldParent.children, oldName)
+		ds = appendDentry(ds, replaced)
 	}
+	oldParent.cacheNegativeLookupLocked(oldName)
+	// We don't use newParent.cacheNewChildLocked() since we don't want to mess
+	// with reference counts and queue oldParent for checkCachingLocked if the
+	// parent isn't actually changing.
 	if oldParent != newParent {
-		appendDentry(ds, oldParent)
+		ds = appendDentry(ds, oldParent)
 		newParent.IncRef()
+		if renamed.isSynthetic() {
+			oldParent.syntheticChildren--
+			newParent.syntheticChildren++
+		}
 	}
 	renamed.parent = newParent
 	renamed.name = newName
@@ -1000,11 +1089,25 @@ func (fs *filesystem) RenameAt(ctx context.Context, rp *vfs.ResolvingPath, oldPa
 		newParent.children = make(map[string]*dentry)
 	}
 	newParent.children[newName] = renamed
-	if replaced != nil {
-		replaced.setDeleted()
-		appendDentry(ds, replaced)
+
+	// Update metadata.
+	if renamed.cachedMetadataAuthoritative() {
+		renamed.touchCtime()
+	}
+	if oldParent.cachedMetadataAuthoritative() {
+		oldParent.dirents = nil
+		oldParent.touchCMtime()
+		if renamed.isDir() {
+			oldParent.decLinks()
+		}
+	}
+	if newParent.cachedMetadataAuthoritative() {
+		newParent.dirents = nil
+		newParent.touchCMtime()
+		if renamed.isDir() {
+			newParent.incLinks()
+		}
 	}
-	vfsObj.CommitRenameReplaceDentry(&renamed.vfsd, replacedVFSD)
 	return nil
 }
 
@@ -1051,6 +1154,10 @@ func (fs *filesystem) StatFSAt(ctx context.Context, rp *vfs.ResolvingPath) (linu
 	if err != nil {
 		return linux.Statfs{}, err
 	}
+	// If d is synthetic, invoke statfs on the first ancestor of d that isn't.
+	for d.isSynthetic() {
+		d = d.parent
+	}
 	fsstat, err := d.file.statFS(ctx)
 	if err != nil {
 		return linux.Statfs{}, err
@@ -1080,7 +1187,7 @@ func (fs *filesystem) SymlinkAt(ctx context.Context, rp *vfs.ResolvingPath, targ
 		creds := rp.Credentials()
 		_, err := parent.file.symlink(ctx, target, name, (p9.UID)(creds.EffectiveKUID), (p9.GID)(creds.EffectiveKGID))
 		return err
-	})
+	}, nil)
 }
 
 // UnlinkAt implements vfs.FilesystemImpl.UnlinkAt.
diff --git a/pkg/sentry/fsimpl/gofer/gofer.go b/pkg/sentry/fsimpl/gofer/gofer.go
index 293df2545..8b4e91d17 100644
--- a/pkg/sentry/fsimpl/gofer/gofer.go
+++ b/pkg/sentry/fsimpl/gofer/gofer.go
@@ -27,8 +27,9 @@
 //             dentry.handleMu
 //               dentry.dataMu
 //
-// Locking dentry.dirMu in multiple dentries requires holding
-// filesystem.renameMu for writing.
+// Locking dentry.dirMu in multiple dentries requires that either ancestor
+// dentries are locked before descendant dentries, or that filesystem.renameMu
+// is locked for writing.
 package gofer
 
 import (
@@ -102,11 +103,12 @@ type filesystem struct {
 	cachedDentries    dentryList
 	cachedDentriesLen uint64
 
-	// dentries contains all dentries in this filesystem. specialFileFDs
-	// contains all open specialFileFDs. These fields are protected by syncMu.
-	syncMu         sync.Mutex
-	dentries       map[*dentry]struct{}
-	specialFileFDs map[*specialFileFD]struct{}
+	// syncableDentries contains all dentries in this filesystem for which
+	// !dentry.file.isNil(). specialFileFDs contains all open specialFileFDs.
+	// These fields are protected by syncMu.
+	syncMu           sync.Mutex
+	syncableDentries map[*dentry]struct{}
+	specialFileFDs   map[*specialFileFD]struct{}
 }
 
 type filesystemOptions struct {
@@ -187,7 +189,8 @@ const (
 	// InteropModeShared is appropriate when there are users of the remote
 	// filesystem that may mutate its state other than the client.
 	//
-	// - The client must verify cached filesystem state before using it.
+	// - The client must verify ("revalidate") cached filesystem state before
+	// using it.
 	//
 	// - Client changes to filesystem state must be sent to the remote
 	// filesystem synchronously.
@@ -376,14 +379,14 @@ func (fstype FilesystemType) GetFilesystem(ctx context.Context, vfsObj *vfs.Virt
 
 	// Construct the filesystem object.
 	fs := &filesystem{
-		mfp:            mfp,
-		opts:           fsopts,
-		uid:            creds.EffectiveKUID,
-		gid:            creds.EffectiveKGID,
-		client:         client,
-		clock:          ktime.RealtimeClockFromContext(ctx),
-		dentries:       make(map[*dentry]struct{}),
-		specialFileFDs: make(map[*specialFileFD]struct{}),
+		mfp:              mfp,
+		opts:             fsopts,
+		uid:              creds.EffectiveKUID,
+		gid:              creds.EffectiveKGID,
+		client:           client,
+		clock:            ktime.RealtimeClockFromContext(ctx),
+		syncableDentries: make(map[*dentry]struct{}),
+		specialFileFDs:   make(map[*specialFileFD]struct{}),
 	}
 	fs.vfsfs.Init(vfsObj, &fstype, fs)
 
@@ -409,7 +412,7 @@ func (fs *filesystem) Release() {
 	mf := fs.mfp.MemoryFile()
 
 	fs.syncMu.Lock()
-	for d := range fs.dentries {
+	for d := range fs.syncableDentries {
 		d.handleMu.Lock()
 		d.dataMu.Lock()
 		if d.handleWritable {
@@ -444,9 +447,11 @@ type dentry struct {
 	vfsd vfs.Dentry
 
 	// refs is the reference count. Each dentry holds a reference on its
-	// parent, even if disowned. refs is accessed using atomic memory
-	// operations. When refs reaches 0, the dentry may be added to the cache or
-	// destroyed. If refs==-1 the dentry has already been destroyed.
+	// parent, even if disowned. An additional reference is held on all
+	// synthetic dentries until they are unlinked or invalidated. When refs
+	// reaches 0, the dentry may be added to the cache or destroyed. If refs ==
+	// -1, the dentry has already been destroyed. refs is accessed using atomic
+	// memory operations.
 	refs int64
 
 	// fs is the owning filesystem. fs is immutable.
@@ -465,6 +470,12 @@ type dentry struct {
 	// We don't support hard links, so each dentry maps 1:1 to an inode.
 
 	// file is the unopened p9.File that backs this dentry. file is immutable.
+	//
+	// If file.isNil(), this dentry represents a synthetic file, i.e. a file
+	// that does not exist on the remote filesystem. As of this writing, this
+	// is only possible for a directory created with
+	// MkdirOptions.ForSyntheticMountpoint == true.
+	// TODO(gvisor.dev/issue/1476): Support synthetic sockets (and pipes).
 	file p9file
 
 	// If deleted is non-zero, the file represented by this dentry has been
@@ -484,15 +495,21 @@ type dentry struct {
 	// - Mappings of child filenames to dentries representing those children.
 	//
 	// - Mappings of child filenames that are known not to exist to nil
-	// dentries (only if InteropModeShared is not in effect).
+	// dentries (only if InteropModeShared is not in effect and the directory
+	// is not synthetic).
 	//
 	// children is protected by dirMu.
 	children map[string]*dentry
 
-	// If this dentry represents a directory, InteropModeShared is not in
-	// effect, and dirents is not nil, it is a cache of all entries in the
-	// directory, in the order they were returned by the server. dirents is
-	// protected by dirMu.
+	// If this dentry represents a directory, syntheticChildren is the number
+	// of child dentries for which dentry.isSynthetic() == true.
+	// syntheticChildren is protected by dirMu.
+	syntheticChildren int
+
+	// If this dentry represents a directory,
+	// dentry.cachedMetadataAuthoritative() == true, and dirents is not nil, it
+	// is a cache of all entries in the directory, in the order they were
+	// returned by the server. dirents is protected by dirMu.
 	dirents []vfs.Dirent
 
 	// Cached metadata; protected by metadataMu and accessed using atomic
@@ -589,6 +606,8 @@ func dentryAttrMask() p9.AttrMask {
 // initially has no references, but is not cached; it is the caller's
 // responsibility to set the dentry's reference count and/or call
 // dentry.checkCachingLocked() as appropriate.
+//
+// Preconditions: !file.isNil().
 func (fs *filesystem) newDentry(ctx context.Context, file p9file, qid p9.QID, mask p9.AttrMask, attr *p9.Attr) (*dentry, error) {
 	if !mask.Mode {
 		ctx.Warningf("can't create gofer.dentry without file type")
@@ -612,10 +631,10 @@ func (fs *filesystem) newDentry(ctx context.Context, file p9file, qid p9.QID, ma
 		},
 	}
 	d.pf.dentry = d
-	if mask.UID {
+	if mask.UID && attr.UID != auth.NoID {
 		d.uid = uint32(attr.UID)
 	}
-	if mask.GID {
+	if mask.GID && attr.GID != auth.NoID {
 		d.gid = uint32(attr.GID)
 	}
 	if mask.Size {
@@ -642,11 +661,19 @@ func (fs *filesystem) newDentry(ctx context.Context, file p9file, qid p9.QID, ma
 	d.vfsd.Init(d)
 
 	fs.syncMu.Lock()
-	fs.dentries[d] = struct{}{}
+	fs.syncableDentries[d] = struct{}{}
 	fs.syncMu.Unlock()
 	return d, nil
 }
 
+func (d *dentry) isSynthetic() bool {
+	return d.file.isNil()
+}
+
+func (d *dentry) cachedMetadataAuthoritative() bool {
+	return d.fs.opts.interop != InteropModeShared || d.isSynthetic()
+}
+
 // updateFromP9Attrs is called to update d's metadata after an update from the
 // remote filesystem.
 func (d *dentry) updateFromP9Attrs(mask p9.AttrMask, attr *p9.Attr) {
@@ -691,6 +718,7 @@ func (d *dentry) updateFromP9Attrs(mask p9.AttrMask, attr *p9.Attr) {
 	d.metadataMu.Unlock()
 }
 
+// Preconditions: !d.isSynthetic()
 func (d *dentry) updateFromGetattr(ctx context.Context) error {
 	// Use d.handle.file, which represents a 9P fid that has been opened, in
 	// preference to d.file, which represents a 9P fid that has not. This may
@@ -758,7 +786,7 @@ func (d *dentry) setStat(ctx context.Context, creds *auth.Credentials, stat *lin
 	defer mnt.EndWrite()
 	setLocalAtime := false
 	setLocalMtime := false
-	if d.fs.opts.interop != InteropModeShared {
+	if d.cachedMetadataAuthoritative() {
 		// Timestamp updates will be handled locally.
 		setLocalAtime = stat.Mask&linux.STATX_ATIME != 0
 		setLocalMtime = stat.Mask&linux.STATX_MTIME != 0
@@ -771,35 +799,37 @@ func (d *dentry) setStat(ctx context.Context, creds *auth.Credentials, stat *lin
 	}
 	d.metadataMu.Lock()
 	defer d.metadataMu.Unlock()
-	if stat.Mask != 0 {
-		if err := d.file.setAttr(ctx, p9.SetAttrMask{
-			Permissions:        stat.Mask&linux.STATX_MODE != 0,
-			UID:                stat.Mask&linux.STATX_UID != 0,
-			GID:                stat.Mask&linux.STATX_GID != 0,
-			Size:               stat.Mask&linux.STATX_SIZE != 0,
-			ATime:              stat.Mask&linux.STATX_ATIME != 0,
-			MTime:              stat.Mask&linux.STATX_MTIME != 0,
-			ATimeNotSystemTime: stat.Atime.Nsec != linux.UTIME_NOW,
-			MTimeNotSystemTime: stat.Mtime.Nsec != linux.UTIME_NOW,
-		}, p9.SetAttr{
-			Permissions:      p9.FileMode(stat.Mode),
-			UID:              p9.UID(stat.UID),
-			GID:              p9.GID(stat.GID),
-			Size:             stat.Size,
-			ATimeSeconds:     uint64(stat.Atime.Sec),
-			ATimeNanoSeconds: uint64(stat.Atime.Nsec),
-			MTimeSeconds:     uint64(stat.Mtime.Sec),
-			MTimeNanoSeconds: uint64(stat.Mtime.Nsec),
-		}); err != nil {
-			return err
+	if !d.isSynthetic() {
+		if stat.Mask != 0 {
+			if err := d.file.setAttr(ctx, p9.SetAttrMask{
+				Permissions:        stat.Mask&linux.STATX_MODE != 0,
+				UID:                stat.Mask&linux.STATX_UID != 0,
+				GID:                stat.Mask&linux.STATX_GID != 0,
+				Size:               stat.Mask&linux.STATX_SIZE != 0,
+				ATime:              stat.Mask&linux.STATX_ATIME != 0,
+				MTime:              stat.Mask&linux.STATX_MTIME != 0,
+				ATimeNotSystemTime: stat.Atime.Nsec != linux.UTIME_NOW,
+				MTimeNotSystemTime: stat.Mtime.Nsec != linux.UTIME_NOW,
+			}, p9.SetAttr{
+				Permissions:      p9.FileMode(stat.Mode),
+				UID:              p9.UID(stat.UID),
+				GID:              p9.GID(stat.GID),
+				Size:             stat.Size,
+				ATimeSeconds:     uint64(stat.Atime.Sec),
+				ATimeNanoSeconds: uint64(stat.Atime.Nsec),
+				MTimeSeconds:     uint64(stat.Mtime.Sec),
+				MTimeNanoSeconds: uint64(stat.Mtime.Nsec),
+			}); err != nil {
+				return err
+			}
+		}
+		if d.fs.opts.interop == InteropModeShared {
+			// There's no point to updating d's metadata in this case since
+			// it'll be overwritten by revalidation before the next time it's
+			// used anyway. (InteropModeShared inhibits client caching of
+			// regular file data, so there's no cache to truncate either.)
+			return nil
 		}
-	}
-	if d.fs.opts.interop == InteropModeShared {
-		// There's no point to updating d's metadata in this case since it'll
-		// be overwritten by revalidation before the next time it's used
-		// anyway. (InteropModeShared inhibits client caching of regular file
-		// data, so there's no cache to truncate either.)
-		return nil
 	}
 	now := d.fs.clock.Now().Nanoseconds()
 	if stat.Mask&linux.STATX_MODE != 0 {
@@ -897,6 +927,15 @@ func (d *dentry) DecRef() {
 	}
 }
 
+// decRefLocked decrements d's reference count without calling
+// d.checkCachingLocked, even if d's reference count reaches 0; callers are
+// responsible for ensuring that d.checkCachingLocked will be called later.
+func (d *dentry) decRefLocked() {
+	if refs := atomic.AddInt64(&d.refs, -1); refs < 0 {
+		panic("gofer.dentry.decRefLocked() called without holding a reference")
+	}
+}
+
 // checkCachingLocked should be called after d's reference count becomes 0 or it
 // becomes disowned.
 //
@@ -1013,11 +1052,11 @@ func (d *dentry) destroyLocked() {
 	if !d.file.isNil() {
 		d.file.close(ctx)
 		d.file = p9file{}
+		// Remove d from the set of syncable dentries.
+		d.fs.syncMu.Lock()
+		delete(d.fs.syncableDentries, d)
+		d.fs.syncMu.Unlock()
 	}
-	// Remove d from the set of all dentries.
-	d.fs.syncMu.Lock()
-	delete(d.fs.dentries, d)
-	d.fs.syncMu.Unlock()
 	// Drop the reference held by d on its parent without recursively locking
 	// d.fs.renameMu.
 	if d.parent != nil {
@@ -1040,6 +1079,9 @@ func (d *dentry) setDeleted() {
 // We only support xattrs prefixed with "user." (see b/148380782). Currently,
 // there is no need to expose any other xattrs through a gofer.
 func (d *dentry) listxattr(ctx context.Context, creds *auth.Credentials, size uint64) ([]string, error) {
+	if d.file.isNil() {
+		return nil, nil
+	}
 	xattrMap, err := d.file.listXattr(ctx, size)
 	if err != nil {
 		return nil, err
@@ -1054,6 +1096,9 @@ func (d *dentry) listxattr(ctx context.Context, creds *auth.Credentials, size ui
 }
 
 func (d *dentry) getxattr(ctx context.Context, creds *auth.Credentials, opts *vfs.GetxattrOptions) (string, error) {
+	if d.file.isNil() {
+		return "", syserror.ENODATA
+	}
 	if err := d.checkPermissions(creds, vfs.MayRead); err != nil {
 		return "", err
 	}
@@ -1064,6 +1109,9 @@ func (d *dentry) getxattr(ctx context.Context, creds *auth.Credentials, opts *vf
 }
 
 func (d *dentry) setxattr(ctx context.Context, creds *auth.Credentials, opts *vfs.SetxattrOptions) error {
+	if d.file.isNil() {
+		return syserror.EPERM
+	}
 	if err := d.checkPermissions(creds, vfs.MayWrite); err != nil {
 		return err
 	}
@@ -1074,6 +1122,9 @@ func (d *dentry) setxattr(ctx context.Context, creds *auth.Credentials, opts *vf
 }
 
 func (d *dentry) removexattr(ctx context.Context, creds *auth.Credentials, name string) error {
+	if d.file.isNil() {
+		return syserror.EPERM
+	}
 	if err := d.checkPermissions(creds, vfs.MayWrite); err != nil {
 		return err
 	}
@@ -1083,7 +1134,7 @@ func (d *dentry) removexattr(ctx context.Context, creds *auth.Credentials, name
 	return d.file.removeXattr(ctx, name)
 }
 
-// Preconditions: d.isRegularFile() || d.isDirectory().
+// Preconditions: !d.file.isNil(). d.isRegularFile() || d.isDirectory().
 func (d *dentry) ensureSharedHandle(ctx context.Context, read, write, trunc bool) error {
 	// O_TRUNC unconditionally requires us to obtain a new handle (opened with
 	// O_TRUNC).
@@ -1213,7 +1264,7 @@ func (fd *fileDescription) dentry() *dentry {
 func (fd *fileDescription) Stat(ctx context.Context, opts vfs.StatOptions) (linux.Statx, error) {
 	d := fd.dentry()
 	const validMask = uint32(linux.STATX_MODE | linux.STATX_UID | linux.STATX_GID | linux.STATX_ATIME | linux.STATX_MTIME | linux.STATX_CTIME | linux.STATX_SIZE | linux.STATX_BLOCKS | linux.STATX_BTIME)
-	if d.fs.opts.interop == InteropModeShared && opts.Mask&(validMask) != 0 && opts.Sync != linux.AT_STATX_DONT_SYNC {
+	if !d.cachedMetadataAuthoritative() && opts.Mask&validMask != 0 && opts.Sync != linux.AT_STATX_DONT_SYNC {
 		// TODO(jamieliu): Use specialFileFD.handle.file for the getattr if
 		// available?
 		if err := d.updateFromGetattr(ctx); err != nil {
diff --git a/pkg/sentry/fsimpl/gofer/gofer_test.go b/pkg/sentry/fsimpl/gofer/gofer_test.go
index 4041fb252..adff39490 100644
--- a/pkg/sentry/fsimpl/gofer/gofer_test.go
+++ b/pkg/sentry/fsimpl/gofer/gofer_test.go
@@ -24,7 +24,7 @@ import (
 
 func TestDestroyIdempotent(t *testing.T) {
 	fs := filesystem{
-		dentries: make(map[*dentry]struct{}),
+		syncableDentries: make(map[*dentry]struct{}),
 		opts: filesystemOptions{
 			// Test relies on no dentry being held in the cache.
 			maxCachedDentries: 0,
diff --git a/pkg/sentry/vfs/filesystem.go b/pkg/sentry/vfs/filesystem.go
index 74577bc2f..20e5bb072 100644
--- a/pkg/sentry/vfs/filesystem.go
+++ b/pkg/sentry/vfs/filesystem.go
@@ -443,8 +443,7 @@ type FilesystemImpl interface {
 	// Errors:
 	//
 	// - If extended attributes are not supported by the filesystem,
-	// ListxattrAt returns nil. (See FileDescription.Listxattr for an
-	// explanation.)
+	// ListxattrAt returns ENOTSUP.
 	//
 	// - If the size of the list (including a NUL terminating byte after every
 	// entry) would exceed size, ERANGE may be returned. Note that
diff --git a/pkg/sentry/vfs/options.go b/pkg/sentry/vfs/options.go
index 534528ce6..022bac127 100644
--- a/pkg/sentry/vfs/options.go
+++ b/pkg/sentry/vfs/options.go
@@ -33,6 +33,25 @@ type GetDentryOptions struct {
 type MkdirOptions struct {
 	// Mode is the file mode bits for the created directory.
 	Mode linux.FileMode
+
+	// If ForSyntheticMountpoint is true, FilesystemImpl.MkdirAt() may create
+	// the given directory in memory only (as opposed to persistent storage).
+	// The created directory should be able to support the creation of
+	// subdirectories with ForSyntheticMountpoint == true. It does not need to
+	// support the creation of subdirectories with ForSyntheticMountpoint ==
+	// false, or files of other types.
+	//
+	// FilesystemImpls are permitted to ignore the ForSyntheticMountpoint
+	// option.
+	//
+	// The ForSyntheticMountpoint option exists because, unlike mount(2), the
+	// OCI Runtime Specification permits the specification of mount points that
+	// do not exist, under the expectation that container runtimes will create
+	// them. (More accurately, the OCI Runtime Specification completely fails
+	// to document this feature, but it's implemented by runc.)
+	// ForSyntheticMountpoint allows such mount points to be created even when
+	// the underlying persistent filesystem is immutable.
+	ForSyntheticMountpoint bool
 }
 
 // MknodOptions contains options to VirtualFilesystem.MknodAt() and