// 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"
	"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/lock"
	"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.
type FilesystemType struct{}

// filesystem implements vfs.FilesystemImpl.
type filesystem struct {
	vfsfs vfs.Filesystem

	// memFile is used to allocate pages to for regular files.
	memFile *pgalloc.MemoryFile

	// 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

	nextInoMinusOne uint64 // accessed using atomic memory operations
}

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

// FilesystemOpts is used to pass configuration data to tmpfs.
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) {
	memFileProvider := pgalloc.MemoryFileProviderFromContext(ctx)
	if memFileProvider == 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
		}
	}

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

	var root *dentry
	switch rootFileType {
	case linux.S_IFREG:
		root = fs.newDentry(fs.newRegularFile(creds, 0777))
	case linux.S_IFLNK:
		root = fs.newDentry(fs.newSymlink(creds, tmpfsOpts.RootSymlinkTarget))
	case linux.S_IFDIR:
		root = &fs.newDirectory(creds, 01777).dentry
	default:
		fs.vfsfs.DecRef()
		return nil, nil, fmt.Errorf("invalid tmpfs root file type: %#o", rootFileType)
	}
	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() {
	fs.vfsfs.VirtualFilesystem().PutAnonBlockDevMinor(fs.devMinor)
}

// dentry implements vfs.DentryImpl.
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() {
	d.inode.decRef()
}

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

	// refs is a reference count. refs is accessed using atomic memory
	// operations.
	//
	// A reference is held on all inodes that are reachable in the filesystem
	// tree. For non-directories (which may have multiple hard links), this
	// means that a reference is dropped when nlink reaches 0. For directories,
	// nlink never reaches 0 due to the "." entry; instead,
	// filesystem.RmdirAt() drops the reference.
	refs int64

	// 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
	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

	// Advisory file locks, which lock at the inode level.
	locks lock.FileLocks

	impl interface{} // immutable
}

const maxLinks = math.MaxUint32

func (i *inode) init(impl interface{}, fs *filesystem, creds *auth.Credentials, mode linux.FileMode) {
	if mode.FileType() == 0 {
		panic("file type is required in FileMode")
	}
	i.fs = fs
	i.refs = 1
	i.mode = uint32(mode)
	i.uid = uint32(creds.EffectiveKUID)
	i.gid = uint32(creds.EffectiveKGID)
	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
}

// 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.
//
// Preconditions: filesystem.mu must be locked for writing. i.nlink != 0.
func (i *inode) decLinksLocked() {
	if i.nlink == 0 {
		panic("tmpfs.inode.decLinksLocked() called with no existing links")
	}
	atomic.AddUint32(&i.nlink, ^uint32(0))
}

func (i *inode) incRef() {
	if atomic.AddInt64(&i.refs, 1) <= 1 {
		panic("tmpfs.inode.incRef() called without holding a reference")
	}
}

func (i *inode) tryIncRef() bool {
	for {
		refs := atomic.LoadInt64(&i.refs)
		if refs == 0 {
			return false
		}
		if atomic.CompareAndSwapInt64(&i.refs, refs, refs+1) {
			return true
		}
	}
}

func (i *inode) decRef() {
	if refs := atomic.AddInt64(&i.refs, -1); refs == 0 {
		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)
		}
	} else if refs < 0 {
		panic("tmpfs.inode.decRef() called without holding a reference")
	}
}

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, stat *linux.Statx) 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_CTIME|linux.STATX_SIZE) != 0 {
		return syserror.EPERM
	}
	mode := linux.FileMode(atomic.LoadUint32(&i.mode))
	if err := vfs.CheckSetStat(ctx, creds, stat, 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
}

// TODO(gvisor.dev/issue/1480): support file locking for file types other than regular.
func (i *inode) lockBSD(uid fslock.UniqueID, t fslock.LockType, block fslock.Blocker) error {
	switch i.impl.(type) {
	case *regularFile:
		return i.locks.LockBSD(uid, t, block)
	}
	return syserror.EBADF
}

// TODO(gvisor.dev/issue/1480): support file locking for file types other than regular.
func (i *inode) unlockBSD(uid fslock.UniqueID) error {
	switch i.impl.(type) {
	case *regularFile:
		i.locks.UnlockBSD(uid)
		return nil
	}
	return syserror.EBADF
}

// TODO(gvisor.dev/issue/1480): support file locking for file types other than regular.
func (i *inode) lockPOSIX(uid fslock.UniqueID, t fslock.LockType, rng fslock.LockRange, block fslock.Blocker) error {
	switch i.impl.(type) {
	case *regularFile:
		return i.locks.LockPOSIX(uid, t, rng, block)
	}
	return syserror.EBADF
}

// TODO(gvisor.dev/issue/1480): support file locking for file types other than regular.
func (i *inode) unlockPOSIX(uid fslock.UniqueID, rng fslock.LockRange) error {
	switch i.impl.(type) {
	case *regularFile:
		i.locks.UnlockPOSIX(uid, rng)
		return nil
	}
	return syserror.EBADF
}

// 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 *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 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() and holds
// inode.mu.
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.checkPermissions(creds, vfs.MayRead); err != nil {
		return "", err
	}
	if !strings.HasPrefix(opts.Name, linux.XATTR_USER_PREFIX) {
		return "", syserror.EOPNOTSUPP
	}
	if !i.userXattrSupported() {
		return "", syserror.ENODATA
	}
	return i.xattrs.Getxattr(opts)
}

func (i *inode) setxattr(creds *auth.Credentials, opts *vfs.SetxattrOptions) error {
	if err := i.checkPermissions(creds, vfs.MayWrite); err != nil {
		return err
	}
	if !strings.HasPrefix(opts.Name, linux.XATTR_USER_PREFIX) {
		return syserror.EOPNOTSUPP
	}
	if !i.userXattrSupported() {
		return syserror.EPERM
	}
	return i.xattrs.Setxattr(opts)
}

func (i *inode) removexattr(creds *auth.Credentials, name string) error {
	if err := i.checkPermissions(creds, vfs.MayWrite); err != nil {
		return err
	}
	if !strings.HasPrefix(name, linux.XATTR_USER_PREFIX) {
		return syserror.EOPNOTSUPP
	}
	if !i.userXattrSupported() {
		return syserror.EPERM
	}
	return i.xattrs.Removexattr(name)
}

// Extended attributes in the user.* namespace are only supported for regular
// files and directories.
func (i *inode) userXattrSupported() bool {
	filetype := linux.S_IFMT & atomic.LoadUint32(&i.mode)
	return filetype == linux.S_IFREG || filetype == linux.S_IFDIR
}

// fileDescription is embedded by tmpfs 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) inode() *inode {
	return fd.vfsfd.Dentry().Impl().(*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)
	return fd.inode().setStat(ctx, creds, &opts.Stat)
}

// 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 {
	return fd.inode().setxattr(auth.CredentialsFromContext(ctx), &opts)
}

// Removexattr implements vfs.FileDescriptionImpl.Removexattr.
func (fd *fileDescription) Removexattr(ctx context.Context, name string) error {
	return fd.inode().removexattr(auth.CredentialsFromContext(ctx), name)
}

// NewMemfd creates a new tmpfs regular file and file description that can back
// an anonymous fd created by memfd_create.
func NewMemfd(mount *vfs.Mount, creds *auth.Credentials, allowSeals bool, name string) (*vfs.FileDescription, error) {
	fs, ok := mount.Filesystem().Impl().(*filesystem)
	if !ok {
		panic("NewMemfd() called with non-tmpfs mount")
	}

	// Per Linux, mm/shmem.c:__shmem_file_setup(), memfd inodes are set up with
	// S_IRWXUGO.
	mode := linux.FileMode(0777)
	inode := fs.newRegularFile(creds, mode)
	rf := inode.impl.(*regularFile)
	if allowSeals {
		rf.seals = 0
	}

	d := fs.newDentry(inode)
	defer d.DecRef()
	d.name = name

	// Per Linux, mm/shmem.c:__shmem_file_setup(), memfd files are set up with
	// FMODE_READ | FMODE_WRITE.
	var fd regularFileFD
	flags := uint32(linux.O_RDWR)
	if err := fd.vfsfd.Init(&fd, flags, mount, &d.vfsd, &vfs.FileDescriptionOptions{}); err != nil {
		return nil, err
	}
	return &fd.vfsfd, nil
}