VFS2 gofer client

Updates #1198

Opening host pipes (by spinning in fdpipe) and host sockets is not yet
complete, and will be done in a future CL.

Major differences from VFS1 gofer client (sentry/fs/gofer), with varying levels
of backportability:

- "Cache policies" are replaced by InteropMode, which control the behavior of
  timestamps in addition to caching. Under InteropModeExclusive (analogous to
  cacheAll) and InteropModeWritethrough (analogous to cacheAllWritethrough),
  client timestamps are *not* written back to the server (it is not possible in
  9P or Linux for clients to set ctime, so writing back client-authoritative
  timestamps results in incoherence between atime/mtime and ctime). Under
  InteropModeShared (analogous to cacheRemoteRevalidating), client timestamps
  are not used at all (remote filesystem clocks are authoritative). cacheNone
  is translated to InteropModeShared + new option
  filesystemOptions.specialRegularFiles.

- Under InteropModeShared, "unstable attribute" reloading for permission
  checks, lookup, and revalidation are fused, which is feasible in VFS2 since
  gofer.filesystem controls path resolution. This results in a ~33% reduction
  in RPCs for filesystem operations compared to cacheRemoteRevalidating. For
  example, consider stat("/foo/bar/baz") where "/foo/bar/baz" fails
  revalidation, resulting in the instantiation of a new dentry:

  VFS1 RPCs:
  getattr("/")                          // fs.MountNamespace.FindLink() => fs.Inode.CheckPermission() => gofer.inodeOperations.check() => gofer.inodeOperations.UnstableAttr()
  walkgetattr("/", "foo") = fid1        // fs.Dirent.walk() => gofer.session.Revalidate() => gofer.cachePolicy.Revalidate()
  clunk(fid1)
  getattr("/foo")                       // CheckPermission
  walkgetattr("/foo", "bar") = fid2     // Revalidate
  clunk(fid2)
  getattr("/foo/bar")                   // CheckPermission
  walkgetattr("/foo/bar", "baz") = fid3 // Revalidate
  clunk(fid3)
  walkgetattr("/foo/bar", "baz") = fid4 // fs.Dirent.walk() => gofer.inodeOperations.Lookup
  getattr("/foo/bar/baz")               // linux.stat() => gofer.inodeOperations.UnstableAttr()

  VFS2 RPCs:
  getattr("/")                          // gofer.filesystem.walkExistingLocked()
  walkgetattr("/", "foo") = fid1        // gofer.filesystem.stepExistingLocked()
  clunk(fid1)
                                        // No getattr: walkgetattr already updated metadata for permission check
  walkgetattr("/foo", "bar") = fid2
  clunk(fid2)
  walkgetattr("/foo/bar", "baz") = fid3
                                        // No clunk: fid3 used for new gofer.dentry
                                        // No getattr: walkgetattr already updated metadata for stat()

- gofer.filesystem.unlinkAt() does not require instantiation of a dentry that
  represents the file to be deleted. Updates #898.

- gofer.regularFileFD.OnClose() skips Tflushf for regular files under
  InteropModeExclusive, as it's nonsensical to request a remote file flush
  without flushing locally-buffered writes to that remote file first.

- Symlink targets are cached when InteropModeShared is not in effect.

- p9.QID.Path (which is already required to be unique for each file within a
  server, and is accordingly already synthesized from device/inode numbers in
  all known gofers) is used as-is for inode numbers, rather than being mapped
  along with attr.RDev in the client to yet another synthetic inode number.

- Relevant parts of fsutil.CachingInodeOperations are inlined directly into
  gofer package code. This avoids having to duplicate part of its functionality
  in fsutil.HostMappable.

PiperOrigin-RevId: 293190213

1 files changed, 1147 insertions, 0 deletions

diff --git a/pkg/sentry/fsimpl/gofer/gofer.go b/pkg/sentry/fsimpl/gofer/gofer.go
new file mode 100644
index 000000000..d0552bd99
--- /dev/null
+++ b/pkg/sentry/fsimpl/gofer/gofer.go
@@ -0,0 +1,1147 @@
+// Copyright 2019 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 gofer provides a filesystem implementation that is backed by a 9p
+// server, interchangably referred to as "gofers" throughout this package.
+//
+// Lock order:
+//   regularFileFD/directoryFD.mu
+//     filesystem.renameMu
+//       dentry.dirMu
+//         filesystem.syncMu
+//         dentry.metadataMu
+//           *** "memmap.Mappable locks" below this point
+//           dentry.mapsMu
+//             *** "memmap.Mappable locks taken by Translate" below this point
+//             dentry.handleMu
+//               dentry.dataMu
+//
+// Locking dentry.dirMu in multiple dentries requires holding
+// filesystem.renameMu for writing.
+package gofer
+
+import (
+	"fmt"
+	"strconv"
+	"sync"
+	"sync/atomic"
+	"syscall"
+
+	"gvisor.dev/gvisor/pkg/abi/linux"
+	"gvisor.dev/gvisor/pkg/context"
+	"gvisor.dev/gvisor/pkg/log"
+	"gvisor.dev/gvisor/pkg/p9"
+	"gvisor.dev/gvisor/pkg/sentry/fs/fsutil"
+	"gvisor.dev/gvisor/pkg/sentry/kernel/auth"
+	"gvisor.dev/gvisor/pkg/sentry/memmap"
+	"gvisor.dev/gvisor/pkg/sentry/pgalloc"
+	"gvisor.dev/gvisor/pkg/sentry/vfs"
+	"gvisor.dev/gvisor/pkg/syserror"
+	"gvisor.dev/gvisor/pkg/unet"
+	"gvisor.dev/gvisor/pkg/usermem"
+)
+
+// FilesystemType implements vfs.FilesystemType.
+type FilesystemType struct{}
+
+// filesystem implements vfs.FilesystemImpl.
+type filesystem struct {
+	vfsfs vfs.Filesystem
+
+	// mfp is used to allocate memory that caches regular file contents. mfp is
+	// immutable.
+	mfp pgalloc.MemoryFileProvider
+
+	// Immutable options.
+	opts filesystemOptions
+
+	// client is the client used by this filesystem. client is immutable.
+	client *p9.Client
+
+	// uid and gid are the effective KUID and KGID of the filesystem's creator,
+	// and are used as the owner and group for files that don't specify one.
+	// uid and gid are immutable.
+	uid auth.KUID
+	gid auth.KGID
+
+	// renameMu serves two purposes:
+	//
+	// - It synchronizes path resolution with renaming initiated by this
+	// client.
+	//
+	// - It is held by path resolution to ensure that reachable dentries remain
+	// valid. A dentry is reachable by path resolution if it has a non-zero
+	// reference count (such that it is usable as vfs.ResolvingPath.Start() or
+	// is reachable from its children), or if it is a child dentry (such that
+	// it is reachable from its parent).
+	renameMu sync.RWMutex
+
+	// cachedDentries contains all dentries with 0 references. (Due to race
+	// conditions, it may also contain dentries with non-zero references.)
+	// cachedDentriesLen is the number of dentries in cachedDentries. These
+	// fields are protected by renameMu.
+	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{}
+}
+
+type filesystemOptions struct {
+	// "Standard" 9P options.
+	fd      int
+	aname   string
+	interop InteropMode // derived from the "cache" mount option
+	msize   uint32
+	version string
+
+	// maxCachedDentries is the maximum number of dentries with 0 references
+	// retained by the client.
+	maxCachedDentries uint64
+
+	// If forcePageCache is true, host FDs may not be used for application
+	// memory mappings even if available; instead, the client must perform its
+	// own caching of regular file pages. This is primarily useful for testing.
+	forcePageCache bool
+
+	// If limitHostFDTranslation is true, apply maxFillRange() constraints to
+	// host FD mappings returned by dentry.(memmap.Mappable).Translate(). This
+	// makes memory accounting behavior more consistent between cases where
+	// host FDs are / are not available, but may increase the frequency of
+	// sentry-handled page faults on files for which a host FD is available.
+	limitHostFDTranslation bool
+
+	// If overlayfsStaleRead is true, O_RDONLY host FDs provided by the remote
+	// filesystem may not be coherent with writable host FDs opened later, so
+	// mappings of the former must be replaced by mappings of the latter. This
+	// is usually only the case when the remote filesystem is an overlayfs
+	// mount on Linux < 4.19.
+	overlayfsStaleRead bool
+
+	// If regularFilesUseSpecialFileFD is true, application FDs representing
+	// regular files will use distinct file handles for each FD, in the same
+	// way that application FDs representing "special files" such as sockets
+	// do. Note that this disables client caching and mmap for regular files.
+	regularFilesUseSpecialFileFD bool
+}
+
+// InteropMode controls the client's interaction with other remote filesystem
+// users.
+type InteropMode uint32
+
+const (
+	// InteropModeExclusive is appropriate when the filesystem client is the
+	// only user of the remote filesystem.
+	//
+	// - The client may cache arbitrary filesystem state (file data, metadata,
+	// filesystem structure, etc.).
+	//
+	// - Client changes to filesystem state may be sent to the remote
+	// filesystem asynchronously, except when server permission checks are
+	// necessary.
+	//
+	// - File timestamps are based on client clocks. This ensures that users of
+	// the client observe timestamps that are coherent with their own clocks
+	// and consistent with Linux's semantics. However, since it is not always
+	// possible for clients to set arbitrary atimes and mtimes, and never
+	// possible for clients to set arbitrary ctimes, file timestamp changes are
+	// stored in the client only and never sent to the remote filesystem.
+	InteropModeExclusive InteropMode = iota
+
+	// InteropModeWritethrough is appropriate when there are read-only users of
+	// the remote filesystem that expect to observe changes made by the
+	// filesystem client.
+	//
+	// - The client may cache arbitrary filesystem state.
+	//
+	// - Client changes to filesystem state must be sent to the remote
+	// filesystem synchronously.
+	//
+	// - File timestamps are based on client clocks. As a corollary, access
+	// timestamp changes from other remote filesystem users will not be visible
+	// to the client.
+	InteropModeWritethrough
+
+	// 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.
+	//
+	// - Client changes to filesystem state must be sent to the remote
+	// filesystem synchronously.
+	//
+	// - File timestamps are based on server clocks. This is necessary to
+	// ensure that timestamp changes are synchronized between remote filesystem
+	// users.
+	//
+	// Note that the correctness of InteropModeShared depends on the server
+	// correctly implementing 9P fids (i.e. each fid immutably represents a
+	// single filesystem object), even in the presence of remote filesystem
+	// mutations from other users. If this is violated, the behavior of the
+	// client is undefined.
+	InteropModeShared
+)
+
+// 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) {
+	mfp := pgalloc.MemoryFileProviderFromContext(ctx)
+	if mfp == nil {
+		ctx.Warningf("gofer.FilesystemType.GetFilesystem: context does not provide a pgalloc.MemoryFileProvider")
+		return nil, nil, syserror.EINVAL
+	}
+
+	mopts := vfs.GenericParseMountOptions(opts.Data)
+	var fsopts filesystemOptions
+
+	// Check that the transport is "fd".
+	trans, ok := mopts["trans"]
+	if !ok {
+		ctx.Warningf("gofer.FilesystemType.GetFilesystem: transport must be specified as 'trans=fd'")
+		return nil, nil, syserror.EINVAL
+	}
+	delete(mopts, "trans")
+	if trans != "fd" {
+		ctx.Warningf("gofer.FilesystemType.GetFilesystem: unsupported transport: trans=%s", trans)
+		return nil, nil, syserror.EINVAL
+	}
+
+	// Check that read and write FDs are provided and identical.
+	rfdstr, ok := mopts["rfdno"]
+	if !ok {
+		ctx.Warningf("gofer.FilesystemType.GetFilesystem: read FD must be specified as 'rfdno=<file descriptor>")
+		return nil, nil, syserror.EINVAL
+	}
+	delete(mopts, "rfdno")
+	rfd, err := strconv.Atoi(rfdstr)
+	if err != nil {
+		ctx.Warningf("gofer.FilesystemType.GetFilesystem: invalid read FD: rfdno=%s", rfdstr)
+		return nil, nil, syserror.EINVAL
+	}
+	wfdstr, ok := mopts["wfdno"]
+	if !ok {
+		ctx.Warningf("gofer.FilesystemType.GetFilesystem: write FD must be specified as 'wfdno=<file descriptor>")
+		return nil, nil, syserror.EINVAL
+	}
+	delete(mopts, "wfdno")
+	wfd, err := strconv.Atoi(wfdstr)
+	if err != nil {
+		ctx.Warningf("gofer.FilesystemType.GetFilesystem: invalid write FD: wfdno=%s", wfdstr)
+		return nil, nil, syserror.EINVAL
+	}
+	if rfd != wfd {
+		ctx.Warningf("gofer.FilesystemType.GetFilesystem: read FD (%d) and write FD (%d) must be equal", rfd, wfd)
+		return nil, nil, syserror.EINVAL
+	}
+	fsopts.fd = rfd
+
+	// Get the attach name.
+	fsopts.aname = "/"
+	if aname, ok := mopts["aname"]; ok {
+		delete(mopts, "aname")
+		fsopts.aname = aname
+	}
+
+	// Parse the cache policy. For historical reasons, this defaults to the
+	// least generally-applicable option, InteropModeExclusive.
+	fsopts.interop = InteropModeExclusive
+	if cache, ok := mopts["cache"]; ok {
+		delete(mopts, "cache")
+		switch cache {
+		case "fscache":
+			fsopts.interop = InteropModeExclusive
+		case "fscache_writethrough":
+			fsopts.interop = InteropModeWritethrough
+		case "none":
+			fsopts.regularFilesUseSpecialFileFD = true
+			fallthrough
+		case "remote_revalidating":
+			fsopts.interop = InteropModeShared
+		default:
+			ctx.Warningf("gofer.FilesystemType.GetFilesystem: invalid cache policy: cache=%s", cache)
+			return nil, nil, syserror.EINVAL
+		}
+	}
+
+	// Parse the 9P message size.
+	fsopts.msize = 1024 * 1024 // 1M, tested to give good enough performance up to 64M
+	if msizestr, ok := mopts["msize"]; ok {
+		delete(mopts, "msize")
+		msize, err := strconv.ParseUint(msizestr, 10, 32)
+		if err != nil {
+			ctx.Warningf("gofer.FilesystemType.GetFilesystem: invalid message size: msize=%s", msizestr)
+			return nil, nil, syserror.EINVAL
+		}
+		fsopts.msize = uint32(msize)
+	}
+
+	// Parse the 9P protocol version.
+	fsopts.version = p9.HighestVersionString()
+	if version, ok := mopts["version"]; ok {
+		delete(mopts, "version")
+		fsopts.version = version
+	}
+
+	// Parse the dentry cache limit.
+	fsopts.maxCachedDentries = 1000
+	if str, ok := mopts["dentry_cache_limit"]; ok {
+		delete(mopts, "dentry_cache_limit")
+		maxCachedDentries, err := strconv.ParseUint(str, 10, 64)
+		if err != nil {
+			ctx.Warningf("gofer.FilesystemType.GetFilesystem: invalid dentry cache limit: dentry_cache_limit=%s", str)
+			return nil, nil, syserror.EINVAL
+		}
+		fsopts.maxCachedDentries = maxCachedDentries
+	}
+
+	// Handle simple flags.
+	if _, ok := mopts["force_page_cache"]; ok {
+		delete(mopts, "force_page_cache")
+		fsopts.forcePageCache = true
+	}
+	if _, ok := mopts["limit_host_fd_translation"]; ok {
+		delete(mopts, "limit_host_fd_translation")
+		fsopts.limitHostFDTranslation = true
+	}
+	if _, ok := mopts["overlayfs_stale_read"]; ok {
+		delete(mopts, "overlayfs_stale_read")
+		fsopts.overlayfsStaleRead = true
+	}
+	// fsopts.regularFilesUseSpecialFileFD can only be enabled by specifying
+	// "cache=none".
+
+	// Check for unparsed options.
+	if len(mopts) != 0 {
+		ctx.Warningf("gofer.FilesystemType.GetFilesystem: unknown options: %v", mopts)
+		return nil, nil, syserror.EINVAL
+	}
+
+	// Establish a connection with the server.
+	conn, err := unet.NewSocket(fsopts.fd)
+	if err != nil {
+		return nil, nil, err
+	}
+
+	// Perform version negotiation with the server.
+	ctx.UninterruptibleSleepStart(false)
+	client, err := p9.NewClient(conn, fsopts.msize, fsopts.version)
+	ctx.UninterruptibleSleepFinish(false)
+	if err != nil {
+		conn.Close()
+		return nil, nil, err
+	}
+	// Ownership of conn has been transferred to client.
+
+	// Perform attach to obtain the filesystem root.
+	ctx.UninterruptibleSleepStart(false)
+	attached, err := client.Attach(fsopts.aname)
+	ctx.UninterruptibleSleepFinish(false)
+	if err != nil {
+		client.Close()
+		return nil, nil, err
+	}
+	attachFile := p9file{attached}
+	qid, attrMask, attr, err := attachFile.getAttr(ctx, dentryAttrMask())
+	if err != nil {
+		attachFile.close(ctx)
+		client.Close()
+		return nil, nil, err
+	}
+
+	// Construct the filesystem object.
+	fs := &filesystem{
+		mfp:            mfp,
+		opts:           fsopts,
+		uid:            creds.EffectiveKUID,
+		gid:            creds.EffectiveKGID,
+		client:         client,
+		dentries:       make(map[*dentry]struct{}),
+		specialFileFDs: make(map[*specialFileFD]struct{}),
+	}
+	fs.vfsfs.Init(vfsObj, fs)
+
+	// Construct the root dentry.
+	root, err := fs.newDentry(ctx, attachFile, qid, attrMask, &attr)
+	if err != nil {
+		attachFile.close(ctx)
+		fs.vfsfs.DecRef()
+		return nil, nil, err
+	}
+	// Set the root's reference count to 2. One reference is returned to the
+	// caller, and the other is deliberately leaked to prevent the root from
+	// being "cached" and subsequently evicted. Its resources will still be
+	// cleaned up by fs.Release().
+	root.refs = 2
+
+	return &fs.vfsfs, &root.vfsd, nil
+}
+
+// Release implements vfs.FilesystemImpl.Release.
+func (fs *filesystem) Release() {
+	ctx := context.Background()
+	mf := fs.mfp.MemoryFile()
+
+	fs.syncMu.Lock()
+	for d := range fs.dentries {
+		d.handleMu.Lock()
+		d.dataMu.Lock()
+		if d.handleWritable {
+			// Write dirty cached data to the remote file.
+			if err := fsutil.SyncDirtyAll(ctx, &d.cache, &d.dirty, d.size, fs.mfp.MemoryFile(), d.handle.writeFromBlocksAt); err != nil {
+				log.Warningf("gofer.filesystem.Release: failed to flush dentry: %v", err)
+			}
+			// TODO(jamieliu): Do we need to flushf/fsync d?
+		}
+		// Discard cached pages.
+		d.cache.DropAll(mf)
+		d.dirty.RemoveAll()
+		d.dataMu.Unlock()
+		// Close the host fd if one exists.
+		if d.handle.fd >= 0 {
+			syscall.Close(int(d.handle.fd))
+			d.handle.fd = -1
+		}
+		d.handleMu.Unlock()
+	}
+	// There can't be any specialFileFDs still using fs, since each such
+	// FileDescription would hold a reference on a Mount holding a reference on
+	// fs.
+	fs.syncMu.Unlock()
+
+	// Close the connection to the server. This implicitly clunks all fids.
+	fs.client.Close()
+}
+
+// dentry implements vfs.DentryImpl.
+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.
+	refs int64
+
+	// fs is the owning filesystem. fs is immutable.
+	fs *filesystem
+
+	// 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.
+	file p9file
+
+	// If deleted is non-zero, the file represented by this dentry has been
+	// deleted. deleted is accessed using atomic memory operations.
+	deleted uint32
+
+	// If cached is true, dentryEntry links dentry into
+	// filesystem.cachedDentries. cached and dentryEntry are protected by
+	// filesystem.renameMu.
+	cached bool
+	dentryEntry
+
+	dirMu sync.Mutex
+
+	// If this dentry represents a directory, and InteropModeShared is not in
+	// effect, negativeChildren is a set of child names in this directory that
+	// are known not to exist. negativeChildren is protected by dirMu.
+	negativeChildren map[string]struct{}
+
+	// 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.
+	dirents []vfs.Dirent
+
+	// Cached metadata; protected by metadataMu and accessed using atomic
+	// memory operations unless otherwise specified.
+	metadataMu sync.Mutex
+	ino        uint64 // immutable
+	mode       uint32 // type is immutable, perms are mutable
+	uid        uint32 // auth.KUID, but stored as raw uint32 for sync/atomic
+	gid        uint32 // auth.KGID, but ...
+	blockSize  uint32 // 0 if unknown
+	// Timestamps, all nsecs from the Unix epoch.
+	atime int64
+	mtime int64
+	ctime int64
+	btime int64
+	// File size, protected by both metadataMu and dataMu (i.e. both must be
+	// locked to mutate it).
+	size uint64
+
+	mapsMu sync.Mutex
+
+	// If this dentry represents a regular file, mappings tracks mappings of
+	// the file into memmap.MappingSpaces. mappings is protected by mapsMu.
+	mappings memmap.MappingSet
+
+	// If this dentry represents a regular file or directory:
+	//
+	// - handle is the I/O handle used by all regularFileFDs/directoryFDs
+	// representing this dentry.
+	//
+	// - handleReadable is true if handle is readable.
+	//
+	// - handleWritable is true if handle is writable.
+	//
+	// Invariants:
+	//
+	// - If handleReadable == handleWritable == false, then handle.file == nil
+	// (i.e. there is no open handle). Conversely, if handleReadable ||
+	// handleWritable == true, then handle.file != nil (i.e. there is an open
+	// handle).
+	//
+	// - handleReadable and handleWritable cannot transition from true to false
+	// (i.e. handles may not be downgraded).
+	//
+	// These fields are protected by handleMu.
+	handleMu       sync.RWMutex
+	handle         handle
+	handleReadable bool
+	handleWritable bool
+
+	dataMu sync.RWMutex
+
+	// If this dentry represents a regular file that is client-cached, cache
+	// maps offsets into the cached file to offsets into
+	// filesystem.mfp.MemoryFile() that store the file's data. cache is
+	// protected by dataMu.
+	cache fsutil.FileRangeSet
+
+	// If this dentry represents a regular file that is client-cached, dirty
+	// tracks dirty segments in cache. dirty is protected by dataMu.
+	dirty fsutil.DirtySet
+
+	// pf implements platform.File for mappings of handle.fd.
+	pf dentryPlatformFile
+
+	// If this dentry represents a symbolic link, InteropModeShared is not in
+	// effect, and haveTarget is true, target is the symlink target. haveTarget
+	// and target are protected by dataMu.
+	haveTarget bool
+	target     string
+}
+
+// dentryAttrMask returns a p9.AttrMask enabling all attributes used by the
+// gofer client.
+func dentryAttrMask() p9.AttrMask {
+	return p9.AttrMask{
+		Mode:  true,
+		UID:   true,
+		GID:   true,
+		ATime: true,
+		MTime: true,
+		CTime: true,
+		Size:  true,
+		BTime: true,
+	}
+}
+
+// newDentry creates a new dentry representing the given file. The dentry
+// 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.
+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")
+		return nil, syserror.EIO
+	}
+	if attr.Mode.FileType() == p9.ModeRegular && !mask.Size {
+		ctx.Warningf("can't create regular file gofer.dentry without file size")
+		return nil, syserror.EIO
+	}
+
+	d := &dentry{
+		fs:        fs,
+		file:      file,
+		ino:       qid.Path,
+		mode:      uint32(attr.Mode),
+		uid:       uint32(fs.uid),
+		gid:       uint32(fs.gid),
+		blockSize: usermem.PageSize,
+		handle: handle{
+			fd: -1,
+		},
+	}
+	d.pf.dentry = d
+	if mask.UID {
+		d.uid = uint32(attr.UID)
+	}
+	if mask.GID {
+		d.gid = uint32(attr.GID)
+	}
+	if mask.Size {
+		d.size = attr.Size
+	}
+	if attr.BlockSize != 0 {
+		d.blockSize = uint32(attr.BlockSize)
+	}
+	if mask.ATime {
+		d.atime = dentryTimestampFromP9(attr.ATimeSeconds, attr.ATimeNanoSeconds)
+	}
+	if mask.MTime {
+		d.mtime = dentryTimestampFromP9(attr.MTimeSeconds, attr.MTimeNanoSeconds)
+	}
+	if mask.CTime {
+		d.ctime = dentryTimestampFromP9(attr.CTimeSeconds, attr.CTimeNanoSeconds)
+	}
+	if mask.BTime {
+		d.btime = dentryTimestampFromP9(attr.BTimeSeconds, attr.BTimeNanoSeconds)
+	}
+	d.vfsd.Init(d)
+
+	fs.syncMu.Lock()
+	fs.dentries[d] = struct{}{}
+	fs.syncMu.Unlock()
+	return d, nil
+}
+
+// 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) {
+	d.metadataMu.Lock()
+	if mask.Mode {
+		if got, want := uint32(attr.Mode.FileType()), d.fileType(); got != want {
+			d.metadataMu.Unlock()
+			panic(fmt.Sprintf("gofer.dentry file type changed from %#o to %#o", want, got))
+		}
+		atomic.StoreUint32(&d.mode, uint32(attr.Mode))
+	}
+	if mask.UID {
+		atomic.StoreUint32(&d.uid, uint32(attr.UID))
+	}
+	if mask.GID {
+		atomic.StoreUint32(&d.gid, uint32(attr.GID))
+	}
+	// There is no P9_GETATTR_* bit for I/O block size.
+	if attr.BlockSize != 0 {
+		atomic.StoreUint32(&d.blockSize, uint32(attr.BlockSize))
+	}
+	if mask.ATime {
+		atomic.StoreInt64(&d.atime, dentryTimestampFromP9(attr.ATimeSeconds, attr.ATimeNanoSeconds))
+	}
+	if mask.MTime {
+		atomic.StoreInt64(&d.mtime, dentryTimestampFromP9(attr.MTimeSeconds, attr.MTimeNanoSeconds))
+	}
+	if mask.CTime {
+		atomic.StoreInt64(&d.ctime, dentryTimestampFromP9(attr.CTimeSeconds, attr.CTimeNanoSeconds))
+	}
+	if mask.BTime {
+		atomic.StoreInt64(&d.btime, dentryTimestampFromP9(attr.BTimeSeconds, attr.BTimeNanoSeconds))
+	}
+	if mask.Size {
+		d.dataMu.Lock()
+		atomic.StoreUint64(&d.size, attr.Size)
+		d.dataMu.Unlock()
+	}
+	d.metadataMu.Unlock()
+}
+
+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
+	// be significantly more efficient in some implementations.
+	var (
+		file            p9file
+		handleMuRLocked bool
+	)
+	d.handleMu.RLock()
+	if !d.handle.file.isNil() {
+		file = d.handle.file
+		handleMuRLocked = true
+	} else {
+		file = d.file
+		d.handleMu.RUnlock()
+	}
+	_, attrMask, attr, err := file.getAttr(ctx, dentryAttrMask())
+	if handleMuRLocked {
+		d.handleMu.RUnlock()
+	}
+	if err != nil {
+		return err
+	}
+	d.updateFromP9Attrs(attrMask, &attr)
+	return nil
+}
+
+func (d *dentry) fileType() uint32 {
+	return atomic.LoadUint32(&d.mode) & linux.S_IFMT
+}
+
+func (d *dentry) statTo(stat *linux.Statx) {
+	stat.Mask = linux.STATX_TYPE | linux.STATX_MODE | linux.STATX_NLINK | linux.STATX_UID | linux.STATX_GID | linux.STATX_ATIME | linux.STATX_MTIME | linux.STATX_CTIME | linux.STATX_INO | linux.STATX_SIZE | linux.STATX_BLOCKS | linux.STATX_BTIME
+	stat.Blksize = atomic.LoadUint32(&d.blockSize)
+	stat.Nlink = 1
+	if d.isDir() {
+		stat.Nlink = 2
+	}
+	stat.UID = atomic.LoadUint32(&d.uid)
+	stat.GID = atomic.LoadUint32(&d.gid)
+	stat.Mode = uint16(atomic.LoadUint32(&d.mode))
+	stat.Ino = d.ino
+	stat.Size = atomic.LoadUint64(&d.size)
+	// This is consistent with regularFileFD.Seek(), which treats regular files
+	// as having no holes.
+	stat.Blocks = (stat.Size + 511) / 512
+	stat.Atime = statxTimestampFromDentry(atomic.LoadInt64(&d.atime))
+	stat.Btime = statxTimestampFromDentry(atomic.LoadInt64(&d.btime))
+	stat.Ctime = statxTimestampFromDentry(atomic.LoadInt64(&d.ctime))
+	stat.Mtime = statxTimestampFromDentry(atomic.LoadInt64(&d.mtime))
+	// TODO(jamieliu): device number
+}
+
+func (d *dentry) setStat(ctx context.Context, creds *auth.Credentials, stat *linux.Statx, mnt *vfs.Mount) error {
+	if stat.Mask == 0 {
+		return nil
+	}
+	if stat.Mask&^(linux.STATX_MODE|linux.STATX_UID|linux.STATX_GID|linux.STATX_ATIME|linux.STATX_MTIME|linux.STATX_SIZE) != 0 {
+		return syserror.EPERM
+	}
+	if err := vfs.CheckSetStat(creds, stat, uint16(atomic.LoadUint32(&d.mode))&^linux.S_IFMT, auth.KUID(atomic.LoadUint32(&d.uid)), auth.KGID(atomic.LoadUint32(&d.gid))); err != nil {
+		return err
+	}
+	if err := mnt.CheckBeginWrite(); err != nil {
+		return err
+	}
+	defer mnt.EndWrite()
+	setLocalAtime := false
+	setLocalMtime := false
+	if d.fs.opts.interop != InteropModeShared {
+		// Timestamp updates will be handled locally.
+		setLocalAtime = stat.Mask&linux.STATX_ATIME != 0
+		setLocalMtime = stat.Mask&linux.STATX_MTIME != 0
+		stat.Mask &^= linux.STATX_ATIME | linux.STATX_MTIME
+		if !setLocalMtime && (stat.Mask&linux.STATX_SIZE != 0) {
+			// Truncate updates mtime.
+			setLocalMtime = true
+			stat.Mtime.Nsec = linux.UTIME_NOW
+		}
+	}
+	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.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, haveNow := nowFromContext(ctx)
+	if !haveNow {
+		ctx.Warningf("gofer.dentry.setStat: current time not available")
+	}
+	if stat.Mask&linux.STATX_MODE != 0 {
+		atomic.StoreUint32(&d.mode, d.fileType()|uint32(stat.Mode))
+	}
+	if stat.Mask&linux.STATX_UID != 0 {
+		atomic.StoreUint32(&d.uid, stat.UID)
+	}
+	if stat.Mask&linux.STATX_GID != 0 {
+		atomic.StoreUint32(&d.gid, stat.GID)
+	}
+	if setLocalAtime {
+		if stat.Atime.Nsec == linux.UTIME_NOW {
+			if haveNow {
+				atomic.StoreInt64(&d.atime, now)
+			}
+		} else {
+			atomic.StoreInt64(&d.atime, dentryTimestampFromStatx(stat.Atime))
+		}
+	}
+	if setLocalMtime {
+		if stat.Mtime.Nsec == linux.UTIME_NOW {
+			if haveNow {
+				atomic.StoreInt64(&d.mtime, now)
+			}
+		} else {
+			atomic.StoreInt64(&d.mtime, dentryTimestampFromStatx(stat.Mtime))
+		}
+	}
+	if haveNow {
+		atomic.StoreInt64(&d.ctime, now)
+	}
+	if stat.Mask&linux.STATX_SIZE != 0 {
+		d.dataMu.Lock()
+		oldSize := d.size
+		d.size = stat.Size
+		// d.dataMu must be unlocked to lock d.mapsMu and invalidate mappings
+		// below. This allows concurrent calls to Read/Translate/etc. These
+		// functions synchronize with truncation by refusing to use cache
+		// contents beyond the new d.size. (We are still holding d.metadataMu,
+		// so we can't race with Write or another truncate.)
+		d.dataMu.Unlock()
+		if d.size < oldSize {
+			oldpgend := pageRoundUp(oldSize)
+			newpgend := pageRoundUp(d.size)
+			if oldpgend != newpgend {
+				d.mapsMu.Lock()
+				d.mappings.Invalidate(memmap.MappableRange{newpgend, oldpgend}, memmap.InvalidateOpts{
+					// Compare Linux's mm/truncate.c:truncate_setsize() =>
+					// truncate_pagecache() =>
+					// mm/memory.c:unmap_mapping_range(evencows=1).
+					InvalidatePrivate: true,
+				})
+				d.mapsMu.Unlock()
+			}
+			// We are now guaranteed that there are no translations of
+			// truncated pages, and can remove them from the cache. Since
+			// truncated pages have been removed from the remote file, they
+			// should be dropped without being written back.
+			d.dataMu.Lock()
+			d.cache.Truncate(d.size, d.fs.mfp.MemoryFile())
+			d.dirty.KeepClean(memmap.MappableRange{d.size, oldpgend})
+			d.dataMu.Unlock()
+		}
+	}
+	return nil
+}
+
+func (d *dentry) checkPermissions(creds *auth.Credentials, ats vfs.AccessTypes, isDir bool) error {
+	return vfs.GenericCheckPermissions(creds, ats, isDir, uint16(atomic.LoadUint32(&d.mode))&0777, auth.KUID(atomic.LoadUint32(&d.uid)), auth.KGID(atomic.LoadUint32(&d.gid)))
+}
+
+// IncRef implements vfs.DentryImpl.IncRef.
+func (d *dentry) IncRef() {
+	// d.refs may be 0 if d.fs.renameMu is locked, which serializes against
+	// d.checkCachingLocked().
+	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.checkCachingLocked()
+		d.fs.renameMu.Unlock()
+	} else if refs < 0 {
+		panic("gofer.dentry.DecRef() called without holding a reference")
+	}
+}
+
+// checkCachingLocked should be called after d's reference count becomes 0 or it
+// becomes disowned.
+//
+// Preconditions: d.fs.renameMu must be locked for writing.
+func (d *dentry) checkCachingLocked() {
+	// Dentries with a non-zero 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.)
+	if atomic.LoadInt64(&d.refs) != 0 {
+		if d.cached {
+			d.fs.cachedDentries.Remove(d)
+			d.fs.cachedDentriesLen--
+			d.cached = false
+		}
+		return
+	}
+	// Non-child dentries with zero references are no longer reachable by path
+	// resolution and should be dropped immediately.
+	if d.vfsd.Parent() == nil || d.vfsd.IsDisowned() {
+		if d.cached {
+			d.fs.cachedDentries.Remove(d)
+			d.fs.cachedDentriesLen--
+			d.cached = false
+		}
+		d.destroyLocked()
+		return
+	}
+	// If d is already cached, just move it to the front of the LRU.
+	if d.cached {
+		d.fs.cachedDentries.Remove(d)
+		d.fs.cachedDentries.PushFront(d)
+		return
+	}
+	// Cache the dentry, then evict the least recently used cached dentry if
+	// the cache becomes over-full.
+	d.fs.cachedDentries.PushFront(d)
+	d.fs.cachedDentriesLen++
+	d.cached = true
+	if d.fs.cachedDentriesLen > d.fs.opts.maxCachedDentries {
+		victim := d.fs.cachedDentries.Back()
+		d.fs.cachedDentries.Remove(victim)
+		d.fs.cachedDentriesLen--
+		victim.cached = false
+		// victim.refs may have become non-zero from an earlier path
+		// resolution since it was inserted into fs.cachedDentries; see
+		// dentry.incRefLocked(). Either way, we brought
+		// fs.cachedDentriesLen back down to fs.opts.maxCachedDentries, so
+		// we don't loop.
+		if atomic.LoadInt64(&victim.refs) == 0 {
+			if victimParentVFSD := victim.vfsd.Parent(); victimParentVFSD != nil {
+				victimParent := victimParentVFSD.Impl().(*dentry)
+				victimParent.dirMu.Lock()
+				if !victim.vfsd.IsDisowned() {
+					// victim can't be a mount point (in any mount
+					// namespace), since VFS holds references on mount
+					// points.
+					d.fs.vfsfs.VirtualFilesystem().ForceDeleteDentry(&victim.vfsd)
+					// We're only deleting the dentry, not the file it
+					// represents, so we don't need to update
+					// victimParent.dirents etc.
+				}
+				victimParent.dirMu.Unlock()
+			}
+			victim.destroyLocked()
+		}
+	}
+}
+
+// Preconditions: d.fs.renameMu must be locked for writing. d.refs == 0. d is
+// not a child dentry.
+func (d *dentry) destroyLocked() {
+	ctx := context.Background()
+	d.handleMu.Lock()
+	if !d.handle.file.isNil() {
+		mf := d.fs.mfp.MemoryFile()
+		d.dataMu.Lock()
+		// Write dirty pages back to the remote filesystem.
+		if d.handleWritable {
+			if err := fsutil.SyncDirtyAll(ctx, &d.cache, &d.dirty, d.size, mf, d.handle.writeFromBlocksAt); err != nil {
+				log.Warningf("gofer.dentry.DecRef: failed to write dirty data back: %v", err)
+			}
+		}
+		// Discard cached data.
+		d.cache.DropAll(mf)
+		d.dirty.RemoveAll()
+		d.dataMu.Unlock()
+		// Clunk open fids and close open host FDs.
+		d.handle.close(ctx)
+	}
+	d.handleMu.Unlock()
+	d.file.close(ctx)
+	// 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.
+	if parentVFSD := d.vfsd.Parent(); parentVFSD != nil {
+		parent := parentVFSD.Impl().(*dentry)
+		// This is parent.DecRef() without recursive locking of d.fs.renameMu.
+		if refs := atomic.AddInt64(&parent.refs, -1); refs == 0 {
+			parent.checkCachingLocked()
+		} else if refs < 0 {
+			panic("gofer.dentry.DecRef() called without holding a reference")
+		}
+	}
+}
+
+func (d *dentry) isDeleted() bool {
+	return atomic.LoadUint32(&d.deleted) != 0
+}
+
+func (d *dentry) setDeleted() {
+	atomic.StoreUint32(&d.deleted, 1)
+}
+
+func (d *dentry) listxattr(ctx context.Context) ([]string, error) {
+	return nil, syserror.ENOTSUP
+}
+
+func (d *dentry) getxattr(ctx context.Context, name string) (string, error) {
+	// TODO(jamieliu): add vfs.GetxattrOptions.Size
+	return d.file.getXattr(ctx, name, linux.XATTR_SIZE_MAX)
+}
+
+func (d *dentry) setxattr(ctx context.Context, opts *vfs.SetxattrOptions) error {
+	return d.file.setXattr(ctx, opts.Name, opts.Value, opts.Flags)
+}
+
+func (d *dentry) removexattr(ctx context.Context, name string) error {
+	return syserror.ENOTSUP
+}
+
+// Preconditions: 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).
+	if !trunc {
+		d.handleMu.RLock()
+		if (!read || d.handleReadable) && (!write || d.handleWritable) {
+			// The current handle is sufficient.
+			d.handleMu.RUnlock()
+			return nil
+		}
+		d.handleMu.RUnlock()
+	}
+
+	haveOldFD := false
+	d.handleMu.Lock()
+	if (read && !d.handleReadable) || (write && !d.handleWritable) || trunc {
+		// Get a new handle.
+		wantReadable := d.handleReadable || read
+		wantWritable := d.handleWritable || write
+		h, err := openHandle(ctx, d.file, wantReadable, wantWritable, trunc)
+		if err != nil {
+			d.handleMu.Unlock()
+			return err
+		}
+		if !d.handle.file.isNil() {
+			// Check that old and new handles are compatible: If the old handle
+			// includes a host file descriptor but the new one does not, or
+			// vice versa, old and new memory mappings may be incoherent.
+			haveOldFD = d.handle.fd >= 0
+			haveNewFD := h.fd >= 0
+			if haveOldFD != haveNewFD {
+				d.handleMu.Unlock()
+				ctx.Warningf("gofer.dentry.ensureSharedHandle: can't change host FD availability from %v to %v across dentry handle upgrade", haveOldFD, haveNewFD)
+				h.close(ctx)
+				return syserror.EIO
+			}
+			if haveOldFD {
+				// We may have raced with callers of d.pf.FD() that are now
+				// using the old file descriptor, preventing us from safely
+				// closing it. We could handle this by invalidating existing
+				// memmap.Translations, but this is expensive. Instead, use
+				// dup2() to make the old file descriptor refer to the new file
+				// description, then close the new file descriptor (which is no
+				// longer needed). Racing callers may use the old or new file
+				// description, but this doesn't matter since they refer to the
+				// same file (unless d.fs.opts.overlayfsStaleRead is true,
+				// which we handle separately).
+				if err := syscall.Dup2(int(h.fd), int(d.handle.fd)); err != nil {
+					d.handleMu.Unlock()
+					ctx.Warningf("gofer.dentry.ensureSharedHandle: failed to dup fd %d to fd %d: %v", h.fd, d.handle.fd, err)
+					h.close(ctx)
+					return err
+				}
+				syscall.Close(int(h.fd))
+				h.fd = d.handle.fd
+				if d.fs.opts.overlayfsStaleRead {
+					// Replace sentry mappings of the old FD with mappings of
+					// the new FD, since the two are not necessarily coherent.
+					if err := d.pf.hostFileMapper.RegenerateMappings(int(h.fd)); err != nil {
+						d.handleMu.Unlock()
+						ctx.Warningf("gofer.dentry.ensureSharedHandle: failed to replace sentry mappings of old FD with mappings of new FD: %v", err)
+						h.close(ctx)
+						return err
+					}
+				}
+				// Clunk the old fid before making the new handle visible (by
+				// unlocking d.handleMu).
+				d.handle.file.close(ctx)
+			}
+		}
+		// Switch to the new handle.
+		d.handle = h
+		d.handleReadable = wantReadable
+		d.handleWritable = wantWritable
+	}
+	d.handleMu.Unlock()
+
+	if d.fs.opts.overlayfsStaleRead && haveOldFD {
+		// Invalidate application mappings that may be using the old FD; they
+		// will be replaced with mappings using the new FD after future calls
+		// to d.Translate(). This requires holding d.mapsMu, which precedes
+		// d.handleMu in the lock order.
+		d.mapsMu.Lock()
+		d.mappings.InvalidateAll(memmap.InvalidateOpts{})
+		d.mapsMu.Unlock()
+	}
+
+	return nil
+}
+
+// fileDescription is embedded by gofer implementations of
+// vfs.FileDescriptionImpl.
+type fileDescription struct {
+	vfsfd vfs.FileDescription
+	vfs.FileDescriptionDefaultImpl
+}
+
+func (fd *fileDescription) filesystem() *filesystem {
+	return fd.vfsfd.Mount().Filesystem().Impl().(*filesystem)
+}
+
+func (fd *fileDescription) dentry() *dentry {
+	return fd.vfsfd.Dentry().Impl().(*dentry)
+}
+
+// Stat implements vfs.FileDescriptionImpl.Stat.
+func (fd *fileDescription) Stat(ctx context.Context, opts vfs.StatOptions) (linux.Statx, error) {
+	d := fd.dentry()
+	if d.fs.opts.interop == InteropModeShared && opts.Mask&(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) != 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 {
+			return linux.Statx{}, err
+		}
+	}
+	var stat linux.Statx
+	d.statTo(&stat)
+	return stat, nil
+}
+
+// SetStat implements vfs.FileDescriptionImpl.SetStat.
+func (fd *fileDescription) SetStat(ctx context.Context, opts vfs.SetStatOptions) error {
+	return fd.dentry().setStat(ctx, auth.CredentialsFromContext(ctx), &opts.Stat, fd.vfsfd.Mount())
+}
+
+// Listxattr implements vfs.FileDescriptionImpl.Listxattr.
+func (fd *fileDescription) Listxattr(ctx context.Context) ([]string, error) {
+	return fd.dentry().listxattr(ctx)
+}
+
+// Getxattr implements vfs.FileDescriptionImpl.Getxattr.
+func (fd *fileDescription) Getxattr(ctx context.Context, name string) (string, error) {
+	return fd.dentry().getxattr(ctx, name)
+}
+
+// Setxattr implements vfs.FileDescriptionImpl.Setxattr.
+func (fd *fileDescription) Setxattr(ctx context.Context, opts vfs.SetxattrOptions) error {
+	return fd.dentry().setxattr(ctx, &opts)
+}
+
+// Removexattr implements vfs.FileDescriptionImpl.Removexattr.
+func (fd *fileDescription) Removexattr(ctx context.Context, name string) error {
+	return fd.dentry().removexattr(ctx, name)
+}