path: root/pkg/sentry/fsimpl/tmpfs/tmpfs.go

// 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 tmpfs provides an in-memory filesystem whose contents are
// application-mutable, consistent with Linux's tmpfs.
//
// Lock order:
//
// filesystem.mu
//   inode.mu
//     regularFileFD.offMu
//       *** "memmap.Mappable locks" below this point
//       regularFile.mapsMu
//         *** "memmap.Mappable locks taken by Translate" below this point
//         regularFile.dataMu
//     directory.iterMu
package tmpfs

import (
	"fmt"
	"math"
	"strconv"
	"strings"
	"sync/atomic"

	"gvisor.dev/gvisor/pkg/abi/linux"
	"gvisor.dev/gvisor/pkg/context"
	fslock "gvisor.dev/gvisor/pkg/sentry/fs/lock"
	"gvisor.dev/gvisor/pkg/sentry/kernel/auth"
	"gvisor.dev/gvisor/pkg/sentry/kernel/time"
	"gvisor.dev/gvisor/pkg/sentry/pgalloc"
	"gvisor.dev/gvisor/pkg/sentry/vfs"
	"gvisor.dev/gvisor/pkg/sentry/vfs/memxattr"
	"gvisor.dev/gvisor/pkg/sync"
	"gvisor.dev/gvisor/pkg/syserror"
	"gvisor.dev/gvisor/pkg/usermem"
)

// Name is the default filesystem name.
const Name = "tmpfs"

// FilesystemType implements vfs.FilesystemType.
//
// +stateify savable
type FilesystemType struct{}

// filesystem implements vfs.FilesystemImpl.
//
// +stateify savable
type filesystem struct {
	vfsfs vfs.Filesystem

	// mfp is used to allocate memory that stores regular file contents. mfp is
	// immutable.
	mfp pgalloc.MemoryFileProvider

	// clock is a realtime clock used to set timestamps in file operations.
	clock time.Clock

	// devMinor is the filesystem's minor device number. devMinor is immutable.
	devMinor uint32

	// mu serializes changes to the Dentry tree.
	mu sync.RWMutex `state:"nosave"`

	nextInoMinusOne uint64 // accessed using atomic memory operations

	root *dentry
}

// Name implements vfs.FilesystemType.Name.
func (FilesystemType) Name() string {
	return Name
}

// Release implements vfs.FilesystemType.Release.
func (FilesystemType) Release(ctx context.Context) {}

// FilesystemOpts is used to pass configuration data to tmpfs.
//
// +stateify savable
type FilesystemOpts struct {
	// RootFileType is the FileType of the filesystem root. Valid values
	// are: S_IFDIR, S_IFREG, and S_IFLNK. Defaults to S_IFDIR.
	RootFileType uint16

	// RootSymlinkTarget is the target of the root symlink. Only valid if
	// RootFileType == S_IFLNK.
	RootSymlinkTarget string

	// FilesystemType allows setting a different FilesystemType for this
	// tmpfs filesystem. This allows tmpfs to "impersonate" other
	// filesystems, like ramdiskfs and cgroupfs.
	FilesystemType vfs.FilesystemType
}

// GetFilesystem implements vfs.FilesystemType.GetFilesystem.
func (fstype FilesystemType) GetFilesystem(ctx context.Context, vfsObj *vfs.VirtualFilesystem, creds *auth.Credentials, _ string, opts vfs.GetFilesystemOptions) (*vfs.Filesystem, *vfs.Dentry, error) {
	mfp := pgalloc.MemoryFileProviderFromContext(ctx)
	if mfp == nil {
		panic("MemoryFileProviderFromContext returned nil")
	}

	rootFileType := uint16(linux.S_IFDIR)
	newFSType := vfs.FilesystemType(&fstype)
	tmpfsOpts, ok := opts.InternalData.(FilesystemOpts)
	if ok {
		if tmpfsOpts.RootFileType != 0 {
			rootFileType = tmpfsOpts.RootFileType
		}
		if tmpfsOpts.FilesystemType != nil {
			newFSType = tmpfsOpts.FilesystemType
		}
	}

	mopts := vfs.GenericParseMountOptions(opts.Data)
	rootMode := linux.FileMode(0777)
	if rootFileType == linux.S_IFDIR {
		rootMode = 01777
	}
	modeStr, ok := mopts["mode"]
	if ok {
		delete(mopts, "mode")
		mode, err := strconv.ParseUint(modeStr, 8, 32)
		if err != nil {
			ctx.Warningf("tmpfs.FilesystemType.GetFilesystem: invalid mode: %q", modeStr)
			return nil, nil, syserror.EINVAL
		}
		rootMode = linux.FileMode(mode & 07777)
	}
	rootKUID := creds.EffectiveKUID
	uidStr, ok := mopts["uid"]
	if ok {
		delete(mopts, "uid")
		uid, err := strconv.ParseUint(uidStr, 10, 32)
		if err != nil {
			ctx.Warningf("tmpfs.FilesystemType.GetFilesystem: invalid uid: %q", uidStr)
			return nil, nil, syserror.EINVAL
		}
		kuid := creds.UserNamespace.MapToKUID(auth.UID(uid))
		if !kuid.Ok() {
			ctx.Warningf("tmpfs.FilesystemType.GetFilesystem: unmapped uid: %d", uid)
			return nil, nil, syserror.EINVAL
		}
		rootKUID = kuid
	}
	rootKGID := creds.EffectiveKGID
	gidStr, ok := mopts["gid"]
	if ok {
		delete(mopts, "gid")
		gid, err := strconv.ParseUint(gidStr, 10, 32)
		if err != nil {
			ctx.Warningf("tmpfs.FilesystemType.GetFilesystem: invalid gid: %q", gidStr)
			return nil, nil, syserror.EINVAL
		}
		kgid := creds.UserNamespace.MapToKGID(auth.GID(gid))
		if !kgid.Ok() {
			ctx.Warningf("tmpfs.FilesystemType.GetFilesystem: unmapped gid: %d", gid)
			return nil, nil, syserror.EINVAL
		}
		rootKGID = kgid
	}
	if len(mopts) != 0 {
		ctx.Warningf("tmpfs.FilesystemType.GetFilesystem: unknown options: %v", mopts)
		return nil, nil, syserror.EINVAL
	}

	devMinor, err := vfsObj.GetAnonBlockDevMinor()
	if err != nil {
		return nil, nil, err
	}
	clock := time.RealtimeClockFromContext(ctx)
	fs := filesystem{
		mfp:      mfp,
		clock:    clock,
		devMinor: devMinor,
	}
	fs.vfsfs.Init(vfsObj, newFSType, &fs)

	var root *dentry
	switch rootFileType {
	case linux.S_IFREG:
		root = fs.newDentry(fs.newRegularFile(rootKUID, rootKGID, rootMode))
	case linux.S_IFLNK:
		root = fs.newDentry(fs.newSymlink(rootKUID, rootKGID, rootMode, tmpfsOpts.RootSymlinkTarget))
	case linux.S_IFDIR:
		root = &fs.newDirectory(rootKUID, rootKGID, rootMode).dentry
	default:
		fs.vfsfs.DecRef(ctx)
		return nil, nil, fmt.Errorf("invalid tmpfs root file type: %#o", rootFileType)
	}
	fs.root = root
	return &fs.vfsfs, &root.vfsd, nil
}

// NewFilesystem returns a new tmpfs filesystem.
func NewFilesystem(ctx context.Context, vfsObj *vfs.VirtualFilesystem, creds *auth.Credentials) (*vfs.Filesystem, *vfs.Dentry, error) {
	return FilesystemType{}.GetFilesystem(ctx, vfsObj, creds, "", vfs.GetFilesystemOptions{})
}

// Release implements vfs.FilesystemImpl.Release.
func (fs *filesystem) Release(ctx context.Context) {
	fs.vfsfs.VirtualFilesystem().PutAnonBlockDevMinor(fs.devMinor)
	fs.mu.Lock()
	if fs.root.inode.isDir() {
		fs.root.releaseChildrenLocked(ctx)
	}
	fs.mu.Unlock()
}

// releaseChildrenLocked is called on the mount point by filesystem.Release() to
// destroy all objects in the mount. It performs a depth-first walk of the
// filesystem and "unlinks" everything by decrementing link counts
// appropriately. There should be no open file descriptors when this is called,
// so each inode should only have one outstanding reference that is removed once
// its link count hits zero.
//
// Note that we do not update filesystem state precisely while tearing down (for
// instance, the child maps are ignored)--we only care to remove all remaining
// references so that every filesystem object gets destroyed. Also note that we
// do not need to trigger DecRef on the mount point itself or any child mount;
// these are taken care of by the destructor of the enclosing MountNamespace.
//
// Precondition: filesystem.mu is held.
func (d *dentry) releaseChildrenLocked(ctx context.Context) {
	dir := d.inode.impl.(*directory)
	for _, child := range dir.childMap {
		if child.inode.isDir() {
			child.releaseChildrenLocked(ctx)
			child.inode.decLinksLocked(ctx) // link for child/.
			dir.inode.decLinksLocked(ctx)   // link for child/..
		}
		child.inode.decLinksLocked(ctx) // link for child
	}
}

// immutable
var globalStatfs = linux.Statfs{
	Type:         linux.TMPFS_MAGIC,
	BlockSize:    usermem.PageSize,
	FragmentSize: usermem.PageSize,
	NameLength:   linux.NAME_MAX,

	// tmpfs currently does not support configurable size limits. In Linux,
	// such a tmpfs mount will return f_blocks == f_bfree == f_bavail == 0 from
	// statfs(2). However, many applications treat this as having a size limit
	// of 0. To work around this, claim to have a very large but non-zero size,
	// chosen to ensure that BlockSize * Blocks does not overflow int64 (which
	// applications may also handle incorrectly).
	// TODO(b/29637826): allow configuring a tmpfs size and enforce it.
	Blocks:          math.MaxInt64 / usermem.PageSize,
	BlocksFree:      math.MaxInt64 / usermem.PageSize,
	BlocksAvailable: math.MaxInt64 / usermem.PageSize,
}

// dentry implements vfs.DentryImpl.
//
// +stateify savable
type dentry struct {
	vfsd vfs.Dentry

	// parent is this dentry's parent directory. Each referenced dentry holds a
	// reference on parent.dentry. If this dentry is a filesystem root, parent
	// is nil. parent is protected by filesystem.mu.
	parent *dentry

	// name is the name of this dentry in its parent. If this dentry is a
	// filesystem root, name is the empty string. name is protected by
	// filesystem.mu.
	name string

	// dentryEntry (ugh) links dentries into their parent directory.childList.
	dentryEntry

	// inode is the inode represented by this dentry. Multiple Dentries may
	// share a single non-directory inode (with hard links). inode is
	// immutable.
	//
	// tmpfs doesn't count references on dentries; because the dentry tree is
	// the sole source of truth, it is by definition always consistent with the
	// state of the filesystem. However, it does count references on inodes,
	// because inode resources are released when all references are dropped.
	// dentry therefore forwards reference counting directly to inode.
	inode *inode
}

func (fs *filesystem) newDentry(inode *inode) *dentry {
	d := &dentry{
		inode: inode,
	}
	d.vfsd.Init(d)
	return d
}

// IncRef implements vfs.DentryImpl.IncRef.
func (d *dentry) IncRef() {
	d.inode.incRef()
}

// TryIncRef implements vfs.DentryImpl.TryIncRef.
func (d *dentry) TryIncRef() bool {
	return d.inode.tryIncRef()
}

// DecRef implements vfs.DentryImpl.DecRef.
func (d *dentry) DecRef(ctx context.Context) {
	d.inode.decRef(ctx)
}

// InotifyWithParent implements vfs.DentryImpl.InotifyWithParent.
func (d *dentry) InotifyWithParent(ctx context.Context, events, cookie uint32, et vfs.EventType) {
	if d.inode.isDir() {
		events |= linux.IN_ISDIR
	}

	// tmpfs never calls VFS.InvalidateDentry(), so d.vfsd.IsDead() indicates
	// that d was deleted.
	deleted := d.vfsd.IsDead()

	d.inode.fs.mu.RLock()
	// The ordering below is important, Linux always notifies the parent first.
	if d.parent != nil {
		d.parent.inode.watches.Notify(ctx, d.name, events, cookie, et, deleted)
	}
	d.inode.watches.Notify(ctx, "", events, cookie, et, deleted)
	d.inode.fs.mu.RUnlock()
}

// Watches implements vfs.DentryImpl.Watches.
func (d *dentry) Watches() *vfs.Watches {
	return &d.inode.watches
}

// OnZeroWatches implements vfs.Dentry.OnZeroWatches.
func (d *dentry) OnZeroWatches(context.Context) {}

// inode represents a filesystem object.
//
// +stateify savable
type inode struct {
	// fs is the owning filesystem. fs is immutable.
	fs *filesystem

	// A reference is held on all inodes as long as they are reachable in the
	// filesystem tree, i.e. nlink is nonzero. This reference is dropped when
	// nlink reaches 0.
	refs inodeRefs

	// xattrs implements extended attributes.
	//
	// TODO(b/148380782): Support xattrs other than user.*
	xattrs memxattr.SimpleExtendedAttributes

	// Inode metadata. Writing multiple fields atomically requires holding
	// mu, othewise atomic operations can be used.
	mu    sync.Mutex `state:"nosave"`
	mode  uint32     // file type and mode
	nlink uint32     // protected by filesystem.mu instead of inode.mu
	uid   uint32     // auth.KUID, but stored as raw uint32 for sync/atomic
	gid   uint32     // auth.KGID, but ...
	ino   uint64     // immutable

	// Linux's tmpfs has no concept of btime.
	atime int64 // nanoseconds
	ctime int64 // nanoseconds
	mtime int64 // nanoseconds

	locks vfs.FileLocks

	// Inotify watches for this inode.
	watches vfs.Watches

	impl interface{} // immutable
}

const maxLinks = math.MaxUint32

func (i *inode) init(impl interface{}, fs *filesystem, kuid auth.KUID, kgid auth.KGID, mode linux.FileMode) {
	if mode.FileType() == 0 {
		panic("file type is required in FileMode")
	}
	i.fs = fs
	i.mode = uint32(mode)
	i.uid = uint32(kuid)
	i.gid = uint32(kgid)
	i.ino = atomic.AddUint64(&fs.nextInoMinusOne, 1)
	// Tmpfs creation sets atime, ctime, and mtime to current time.
	now := fs.clock.Now().Nanoseconds()
	i.atime = now
	i.ctime = now
	i.mtime = now
	// i.nlink initialized by caller
	i.impl = impl
	i.refs.EnableLeakCheck()
}

// incLinksLocked increments i's link count.
//
// Preconditions:
// * filesystem.mu must be locked for writing.
// * i.nlink != 0.
// * i.nlink < maxLinks.
func (i *inode) incLinksLocked() {
	if i.nlink == 0 {
		panic("tmpfs.inode.incLinksLocked() called with no existing links")
	}
	if i.nlink == maxLinks {
		panic("tmpfs.inode.incLinksLocked() called with maximum link count")
	}
	atomic.AddUint32(&i.nlink, 1)
}

// decLinksLocked decrements i's link count. If the link count reaches 0, we
// remove a reference on i as well.
//
// Preconditions:
// * filesystem.mu must be locked for writing.
// * i.nlink != 0.
func (i *inode) decLinksLocked(ctx context.Context) {
	if i.nlink == 0 {
		panic("tmpfs.inode.decLinksLocked() called with no existing links")
	}
	if atomic.AddUint32(&i.nlink, ^uint32(0)) == 0 {
		i.decRef(ctx)
	}
}

func (i *inode) incRef() {
	i.refs.IncRef()
}

func (i *inode) tryIncRef() bool {
	return i.refs.TryIncRef()
}

func (i *inode) decRef(ctx context.Context) {
	i.refs.DecRef(func() {
		i.watches.HandleDeletion(ctx)
		if regFile, ok := i.impl.(*regularFile); ok {
			// Release memory used by regFile to store data. Since regFile is
			// no longer usable, we don't need to grab any locks or update any
			// metadata.
			regFile.data.DropAll(regFile.memFile)
		}
	})
}

func (i *inode) checkPermissions(creds *auth.Credentials, ats vfs.AccessTypes) error {
	mode := linux.FileMode(atomic.LoadUint32(&i.mode))
	return vfs.GenericCheckPermissions(creds, ats, mode, auth.KUID(atomic.LoadUint32(&i.uid)), auth.KGID(atomic.LoadUint32(&i.gid)))
}

// Go won't inline this function, and returning linux.Statx (which is quite
// big) means spending a lot of time in runtime.duffcopy(), so instead it's an
// output parameter.
//
// Note that Linux does not guarantee to return consistent data (in the case of
// a concurrent modification), so we do not require holding inode.mu.
func (i *inode) statTo(stat *linux.Statx) {
	stat.Mask = linux.STATX_TYPE | linux.STATX_MODE | linux.STATX_NLINK |
		linux.STATX_UID | linux.STATX_GID | linux.STATX_INO | linux.STATX_SIZE |
		linux.STATX_BLOCKS | linux.STATX_ATIME | linux.STATX_CTIME |
		linux.STATX_MTIME
	stat.Blksize = usermem.PageSize
	stat.Nlink = atomic.LoadUint32(&i.nlink)
	stat.UID = atomic.LoadUint32(&i.uid)
	stat.GID = atomic.LoadUint32(&i.gid)
	stat.Mode = uint16(atomic.LoadUint32(&i.mode))
	stat.Ino = i.ino
	stat.Atime = linux.NsecToStatxTimestamp(i.atime)
	stat.Ctime = linux.NsecToStatxTimestamp(i.ctime)
	stat.Mtime = linux.NsecToStatxTimestamp(i.mtime)
	stat.DevMajor = linux.UNNAMED_MAJOR
	stat.DevMinor = i.fs.devMinor
	switch impl := i.impl.(type) {
	case *regularFile:
		stat.Mask |= linux.STATX_SIZE | linux.STATX_BLOCKS
		stat.Size = uint64(atomic.LoadUint64(&impl.size))
		// TODO(jamieliu): This should be impl.data.Span() / 512, but this is
		// too expensive to compute here. Cache it in regularFile.
		stat.Blocks = allocatedBlocksForSize(stat.Size)
	case *directory:
		// "20" is mm/shmem.c:BOGO_DIRENT_SIZE.
		stat.Size = 20 * (2 + uint64(atomic.LoadInt64(&impl.numChildren)))
		// stat.Blocks is 0.
	case *symlink:
		stat.Size = uint64(len(impl.target))
		// stat.Blocks is 0.
	case *namedPipe, *socketFile:
		// stat.Size and stat.Blocks are 0.
	case *deviceFile:
		// stat.Size and stat.Blocks are 0.
		stat.RdevMajor = impl.major
		stat.RdevMinor = impl.minor
	default:
		panic(fmt.Sprintf("unknown inode type: %T", i.impl))
	}
}

func (i *inode) setStat(ctx context.Context, creds *auth.Credentials, opts *vfs.SetStatOptions) error {
	stat := &opts.Stat
	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_CTIME|linux.STATX_SIZE) != 0 {
		return syserror.EPERM
	}
	mode := linux.FileMode(atomic.LoadUint32(&i.mode))
	if err := vfs.CheckSetStat(ctx, creds, opts, mode, auth.KUID(atomic.LoadUint32(&i.uid)), auth.KGID(atomic.LoadUint32(&i.gid))); err != nil {
		return err
	}
	i.mu.Lock()
	defer i.mu.Unlock()
	var (
		needsMtimeBump bool
		needsCtimeBump bool
	)
	mask := stat.Mask
	if mask&linux.STATX_MODE != 0 {
		ft := atomic.LoadUint32(&i.mode) & linux.S_IFMT
		atomic.StoreUint32(&i.mode, ft|uint32(stat.Mode&^linux.S_IFMT))
		needsCtimeBump = true
	}
	if mask&linux.STATX_UID != 0 {
		atomic.StoreUint32(&i.uid, stat.UID)
		needsCtimeBump = true
	}
	if mask&linux.STATX_GID != 0 {
		atomic.StoreUint32(&i.gid, stat.GID)
		needsCtimeBump = true
	}
	if mask&linux.STATX_SIZE != 0 {
		switch impl := i.impl.(type) {
		case *regularFile:
			updated, err := impl.truncateLocked(stat.Size)
			if err != nil {
				return err
			}
			if updated {
				needsMtimeBump = true
				needsCtimeBump = true
			}
		case *directory:
			return syserror.EISDIR
		default:
			return syserror.EINVAL
		}
	}
	now := i.fs.clock.Now().Nanoseconds()
	if mask&linux.STATX_ATIME != 0 {
		if stat.Atime.Nsec == linux.UTIME_NOW {
			atomic.StoreInt64(&i.atime, now)
		} else {
			atomic.StoreInt64(&i.atime, stat.Atime.ToNsecCapped())
		}
		needsCtimeBump = true
	}
	if mask&linux.STATX_MTIME != 0 {
		if stat.Mtime.Nsec == linux.UTIME_NOW {
			atomic.StoreInt64(&i.mtime, now)
		} else {
			atomic.StoreInt64(&i.mtime, stat.Mtime.ToNsecCapped())
		}
		needsCtimeBump = true
		// Ignore the mtime bump, since we just set it ourselves.
		needsMtimeBump = false
	}
	if mask&linux.STATX_CTIME != 0 {
		if stat.Ctime.Nsec == linux.UTIME_NOW {
			atomic.StoreInt64(&i.ctime, now)
		} else {
			atomic.StoreInt64(&i.ctime, stat.Ctime.ToNsecCapped())
		}
		// Ignore the ctime bump, since we just set it ourselves.
		needsCtimeBump = false
	}
	if needsMtimeBump {
		atomic.StoreInt64(&i.mtime, now)
	}
	if needsCtimeBump {
		atomic.StoreInt64(&i.ctime, now)
	}

	return nil
}

// allocatedBlocksForSize returns the number of 512B blocks needed to
// accommodate the given size in bytes, as appropriate for struct
// stat::st_blocks and struct statx::stx_blocks. (Note that this 512B block
// size is independent of the "preferred block size for I/O", struct
// stat::st_blksize and struct statx::stx_blksize.)
func allocatedBlocksForSize(size uint64) uint64 {
	return (size + 511) / 512
}

func (i *inode) direntType() uint8 {
	switch impl := i.impl.(type) {
	case *regularFile:
		return linux.DT_REG
	case *directory:
		return linux.DT_DIR
	case *symlink:
		return linux.DT_LNK
	case *socketFile:
		return linux.DT_SOCK
	case *namedPipe:
		return linux.DT_FIFO
	case *deviceFile:
		switch impl.kind {
		case vfs.BlockDevice:
			return linux.DT_BLK
		case vfs.CharDevice:
			return linux.DT_CHR
		default:
			panic(fmt.Sprintf("unknown vfs.DeviceKind: %v", impl.kind))
		}
	default:
		panic(fmt.Sprintf("unknown inode type: %T", i.impl))
	}
}

func (i *inode) isDir() bool {
	return linux.FileMode(i.mode).FileType() == linux.S_IFDIR
}

func (i *inode) touchAtime(mnt *vfs.Mount) {
	if mnt.Flags.NoATime {
		return
	}
	if err := mnt.CheckBeginWrite(); err != nil {
		return
	}
	now := i.fs.clock.Now().Nanoseconds()
	i.mu.Lock()
	atomic.StoreInt64(&i.atime, now)
	i.mu.Unlock()
	mnt.EndWrite()
}

// Preconditions: The caller has called vfs.Mount.CheckBeginWrite().
func (i *inode) touchCtime() {
	now := i.fs.clock.Now().Nanoseconds()
	i.mu.Lock()
	atomic.StoreInt64(&i.ctime, now)
	i.mu.Unlock()
}

// Preconditions: The caller has called vfs.Mount.CheckBeginWrite().
func (i *inode) touchCMtime() {
	now := i.fs.clock.Now().Nanoseconds()
	i.mu.Lock()
	atomic.StoreInt64(&i.mtime, now)
	atomic.StoreInt64(&i.ctime, now)
	i.mu.Unlock()
}

// Preconditions:
// * The caller has called vfs.Mount.CheckBeginWrite().
// * inode.mu must be locked.
func (i *inode) touchCMtimeLocked() {
	now := i.fs.clock.Now().Nanoseconds()
	atomic.StoreInt64(&i.mtime, now)
	atomic.StoreInt64(&i.ctime, now)
}

func (i *inode) listXattr(size uint64) ([]string, error) {
	return i.xattrs.ListXattr(size)
}

func (i *inode) getXattr(creds *auth.Credentials, opts *vfs.GetXattrOptions) (string, error) {
	if err := i.checkXattrPermissions(creds, opts.Name, vfs.MayRead); err != nil {
		return "", err
	}
	return i.xattrs.GetXattr(opts)
}

func (i *inode) setXattr(creds *auth.Credentials, opts *vfs.SetXattrOptions) error {
	if err := i.checkXattrPermissions(creds, opts.Name, vfs.MayWrite); err != nil {
		return err
	}
	return i.xattrs.SetXattr(opts)
}

func (i *inode) removeXattr(creds *auth.Credentials, name string) error {
	if err := i.checkXattrPermissions(creds, name, vfs.MayWrite); err != nil {
		return err
	}
	return i.xattrs.RemoveXattr(name)
}

func (i *inode) checkXattrPermissions(creds *auth.Credentials, name string, ats vfs.AccessTypes) error {
	// We currently only support extended attributes in the user.* and
	// trusted.* namespaces. See b/148380782.
	if !strings.HasPrefix(name, linux.XATTR_USER_PREFIX) && !strings.HasPrefix(name, linux.XATTR_TRUSTED_PREFIX) {
		return syserror.EOPNOTSUPP
	}
	mode := linux.FileMode(atomic.LoadUint32(&i.mode))
	kuid := auth.KUID(atomic.LoadUint32(&i.uid))
	kgid := auth.KGID(atomic.LoadUint32(&i.gid))
	if err := vfs.GenericCheckPermissions(creds, ats, mode, kuid, kgid); err != nil {
		return err
	}
	return vfs.CheckXattrPermissions(creds, ats, mode, kuid, name)
}

// fileDescription is embedded by tmpfs implementations of
// vfs.FileDescriptionImpl.
//
// +stateify savable
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)
}

func (fd *fileDescription) inode() *inode {
	return fd.dentry().inode
}

// Stat implements vfs.FileDescriptionImpl.Stat.
func (fd *fileDescription) Stat(ctx context.Context, opts vfs.StatOptions) (linux.Statx, error) {
	var stat linux.Statx
	fd.inode().statTo(&stat)
	return stat, nil
}

// SetStat implements vfs.FileDescriptionImpl.SetStat.
func (fd *fileDescription) SetStat(ctx context.Context, opts vfs.SetStatOptions) error {
	creds := auth.CredentialsFromContext(ctx)
	d := fd.dentry()
	if err := d.inode.setStat(ctx, creds, &opts); err != nil {
		return err
	}

	if ev := vfs.InotifyEventFromStatMask(opts.Stat.Mask); ev != 0 {
		d.InotifyWithParent(ctx, ev, 0, vfs.InodeEvent)
	}
	return nil
}

// StatFS implements vfs.FileDescriptionImpl.StatFS.
func (fd *fileDescription) StatFS(ctx context.Context) (linux.Statfs, error) {
	return globalStatfs, nil
}

// ListXattr implements vfs.FileDescriptionImpl.ListXattr.
func (fd *fileDescription) ListXattr(ctx context.Context, size uint64) ([]string, error) {
	return fd.inode().listXattr(size)
}

// GetXattr implements vfs.FileDescriptionImpl.GetXattr.
func (fd *fileDescription) GetXattr(ctx context.Context, opts vfs.GetXattrOptions) (string, error) {
	return fd.inode().getXattr(auth.CredentialsFromContext(ctx), &opts)
}

// SetXattr implements vfs.FileDescriptionImpl.SetXattr.
func (fd *fileDescription) SetXattr(ctx context.Context, opts vfs.SetXattrOptions) error {
	d := fd.dentry()
	if err := d.inode.setXattr(auth.CredentialsFromContext(ctx), &opts); err != nil {
		return err
	}

	// Generate inotify events.
	d.InotifyWithParent(ctx, linux.IN_ATTRIB, 0, vfs.InodeEvent)
	return nil
}

// RemoveXattr implements vfs.FileDescriptionImpl.RemoveXattr.
func (fd *fileDescription) RemoveXattr(ctx context.Context, name string) error {
	d := fd.dentry()
	if err := d.inode.removeXattr(auth.CredentialsFromContext(ctx), name); err != nil {
		return err
	}

	// Generate inotify events.
	d.InotifyWithParent(ctx, linux.IN_ATTRIB, 0, vfs.InodeEvent)
	return nil
}

// LockPOSIX implements vfs.FileDescriptionImpl.LockPOSIX.
func (fd *fileDescription) LockPOSIX(ctx context.Context, uid fslock.UniqueID, t fslock.LockType, start, length uint64, whence int16, block fslock.Blocker) error {
	return fd.Locks().LockPOSIX(ctx, &fd.vfsfd, uid, t, start, length, whence, block)
}

// UnlockPOSIX implements vfs.FileDescriptionImpl.UnlockPOSIX.
func (fd *fileDescription) UnlockPOSIX(ctx context.Context, uid fslock.UniqueID, start, length uint64, whence int16) error {
	return fd.Locks().UnlockPOSIX(ctx, &fd.vfsfd, uid, start, length, whence)
}

// Sync implements vfs.FileDescriptionImpl.Sync. It does nothing because all
// filesystem state is in-memory.
func (*fileDescription) Sync(context.Context) error {
	return nil
}