Merge 216da0b7 (automated)

1 files changed, 1057 insertions, 0 deletions

diff --git a/runsc/fsgofer/fsgofer.go b/runsc/fsgofer/fsgofer.go
new file mode 100644
index 000000000..2cf50290a
--- /dev/null
+++ b/runsc/fsgofer/fsgofer.go
@@ -0,0 +1,1057 @@
+// Copyright 2018 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 fsgofer implements p9.File giving access to local files using
+// a simple mapping from a path prefix that is added to the path requested
+// by the sandbox. Ex:
+//
+//   prefix: "/docker/imgs/alpine"
+//   app path: /bin/ls => /docker/imgs/alpine/bin/ls
+package fsgofer
+
+import (
+	"fmt"
+	"io"
+	"math"
+	"os"
+	"path"
+	"path/filepath"
+	"runtime"
+	"sync"
+	"syscall"
+
+	"golang.org/x/sys/unix"
+	"gvisor.googlesource.com/gvisor/pkg/abi/linux"
+	"gvisor.googlesource.com/gvisor/pkg/fd"
+	"gvisor.googlesource.com/gvisor/pkg/log"
+	"gvisor.googlesource.com/gvisor/pkg/p9"
+	"gvisor.googlesource.com/gvisor/runsc/specutils"
+)
+
+const (
+	// invalidMode is set to a value that doesn't match any other valid
+	// modes to ensure an unopened/closed file fails all mode checks.
+	invalidMode = p9.OpenFlags(math.MaxUint32)
+
+	openFlags = syscall.O_NOFOLLOW | syscall.O_CLOEXEC
+)
+
+type fileType int
+
+const (
+	regular fileType = iota
+	directory
+	symlink
+	unknown
+)
+
+// String implements fmt.Stringer.
+func (f fileType) String() string {
+	switch f {
+	case regular:
+		return "regular"
+	case directory:
+		return "directory"
+	case symlink:
+		return "symlink"
+	}
+	return "unknown"
+}
+
+// ControlSocketAddr generates an abstract unix socket name for the given id.
+func ControlSocketAddr(id string) string {
+	return fmt.Sprintf("\x00runsc-gofer.%s", id)
+}
+
+// Config sets configuration options for each attach point.
+type Config struct {
+	// ROMount is set to true if this is a readonly mount.
+	ROMount bool
+
+	// PanicOnWrite panics on attempts to write to RO mounts.
+	PanicOnWrite bool
+}
+
+type attachPoint struct {
+	prefix string
+	conf   Config
+
+	// attachedMu protects attached.
+	attachedMu sync.Mutex
+	attached   bool
+
+	// deviceMu protects devices and nextDevice.
+	deviceMu sync.Mutex
+
+	// nextDevice is the next device id that will be allocated.
+	nextDevice uint8
+
+	// devices is a map from actual host devices to "small" integers that
+	// can be combined with host inode to form a unique virtual inode id.
+	devices map[uint64]uint8
+}
+
+// NewAttachPoint creates a new attacher that gives local file
+// access to all files under 'prefix'. 'prefix' must be an absolute path.
+func NewAttachPoint(prefix string, c Config) (p9.Attacher, error) {
+	// Sanity check the prefix.
+	if !filepath.IsAbs(prefix) {
+		return nil, fmt.Errorf("attach point prefix must be absolute %q", prefix)
+	}
+	return &attachPoint{
+		prefix:  prefix,
+		conf:    c,
+		devices: make(map[uint64]uint8),
+	}, nil
+}
+
+// Attach implements p9.Attacher.
+func (a *attachPoint) Attach() (p9.File, error) {
+	// dirFD (1st argument) is ignored because 'prefix' is always absolute.
+	stat, err := statAt(-1, a.prefix)
+	if err != nil {
+		return nil, fmt.Errorf("stat file %q, err: %v", a.prefix, err)
+	}
+	mode := syscall.O_RDWR
+	if a.conf.ROMount || stat.Mode&syscall.S_IFDIR != 0 {
+		mode = syscall.O_RDONLY
+	}
+
+	// Open the root directory.
+	f, err := fd.Open(a.prefix, openFlags|mode, 0)
+	if err != nil {
+		return nil, fmt.Errorf("unable to open file %q, err: %v", a.prefix, err)
+	}
+
+	a.attachedMu.Lock()
+	defer a.attachedMu.Unlock()
+	if a.attached {
+		f.Close()
+		return nil, fmt.Errorf("attach point already attached, prefix: %s", a.prefix)
+	}
+	a.attached = true
+
+	return newLocalFile(a, f, a.prefix, stat)
+}
+
+// makeQID returns a unique QID for the given stat buffer.
+func (a *attachPoint) makeQID(stat syscall.Stat_t) p9.QID {
+	a.deviceMu.Lock()
+	defer a.deviceMu.Unlock()
+
+	// First map the host device id to a unique 8-bit integer.
+	dev, ok := a.devices[stat.Dev]
+	if !ok {
+		a.devices[stat.Dev] = a.nextDevice
+		dev = a.nextDevice
+		a.nextDevice++
+		if a.nextDevice < dev {
+			panic(fmt.Sprintf("device id overflow! map: %+v", a.devices))
+		}
+	}
+
+	// Construct a "virtual" inode id with the uint8 device number in the
+	// first 8 bits, and the rest of the bits from the host inode id.
+	maskedIno := stat.Ino & 0x00ffffffffffffff
+	if maskedIno != stat.Ino {
+		log.Warningf("first 8 bytes of host inode id %x will be truncated to construct virtual inode id", stat.Ino)
+	}
+	ino := uint64(dev)<<56 | maskedIno
+	log.Debugf("host inode %x on device %x mapped to virtual inode %x", stat.Ino, stat.Dev, ino)
+
+	return p9.QID{
+		Type: p9.FileMode(stat.Mode).QIDType(),
+		Path: ino,
+	}
+}
+
+// localFile implements p9.File wrapping a local file. The underlying file
+// is opened during Walk() and stored in 'file' to be used with other
+// operations. The file is opened as readonly, unless it's a symlink or there is
+// no read access, which requires O_PATH. 'file' is dup'ed when Walk(nil) is
+// called to clone the file. This reduces the number of walks that need to be
+// done by the host file system when files are reused.
+//
+// The file may be reopened if the requested mode in Open() is not a subset of
+// current mode. Consequently, 'file' could have a mode wider than requested and
+// must be verified before read/write operations. Before the file is opened and
+// after it's closed, 'mode' is set to an invalid value to prevent an unopened
+// file from being used.
+//
+// The reason that the file is not opened initially as read-write is for better
+// performance with 'overlay2' storage driver. overlay2 eagerly copies the
+// entire file up when it's opened in write mode, and would perform badly when
+type localFile struct {
+	p9.DefaultWalkGetAttr
+
+	// attachPoint is the attachPoint that serves this localFile.
+	attachPoint *attachPoint
+
+	// hostPath will be safely updated by the Renamed hook.
+	hostPath string
+
+	// file is opened when localFile is created and it's never nil. It may be
+	// reopened if the Open() mode is wider than the mode the file was originally
+	// opened with.
+	file *fd.FD
+
+	// mode is the mode in which the file was opened. Set to invalidMode
+	// if localFile isn't opened.
+	mode p9.OpenFlags
+
+	// ft is the fileType for this file.
+	ft fileType
+
+	// readDirMu protects against concurrent Readdir calls.
+	readDirMu sync.Mutex
+
+	// lastDirentOffset is the last offset returned by Readdir(). If another call
+	// to Readdir is made at the same offset, the file doesn't need to be
+	// repositioned. This is an important optimization because the caller must
+	// always make one extra call to detect EOF (empty result, no error).
+	lastDirentOffset uint64
+}
+
+func openAnyFileFromParent(parent *localFile, name string) (*fd.FD, string, error) {
+	path := path.Join(parent.hostPath, name)
+	f, err := openAnyFile(path, func(mode int) (*fd.FD, error) {
+		return fd.OpenAt(parent.file, name, openFlags|mode, 0)
+	})
+	return f, path, err
+}
+
+// openAnyFile attempts to open the file in O_RDONLY and if it fails fallsback
+// to O_PATH. 'path' is used for logging messages only. 'fn' is what does the
+// actual file open and is customizable by the caller.
+func openAnyFile(path string, fn func(mode int) (*fd.FD, error)) (*fd.FD, error) {
+	// Attempt to open file in the following mode in order:
+	//   1. RDONLY | NONBLOCK: for all files, works for directories and ro mounts too.
+	//      Use non-blocking to prevent getting stuck inside open(2) for FIFOs. This option
+	//      has no effect on regular files.
+	//   2. PATH: for symlinks
+	modes := []int{syscall.O_RDONLY | syscall.O_NONBLOCK, unix.O_PATH}
+
+	var err error
+	var file *fd.FD
+	for i, mode := range modes {
+		file, err = fn(mode)
+		if err == nil {
+			// openat succeeded, we're done.
+			break
+		}
+		switch e := extractErrno(err); e {
+		case syscall.ENOENT:
+			// File doesn't exist, no point in retrying.
+			return nil, e
+		}
+		// openat failed. Try again with next mode, preserving 'err' in case this
+		// was the last attempt.
+		log.Debugf("Attempt %d to open file failed, mode: %#x, path: %q, err: %v", i, openFlags|mode, path, err)
+	}
+	if err != nil {
+		// All attempts to open file have failed, return the last error.
+		log.Debugf("Failed to open file, path: %q, err: %v", path, err)
+		return nil, extractErrno(err)
+	}
+
+	return file, nil
+}
+
+func getSupportedFileType(stat syscall.Stat_t) (fileType, error) {
+	var ft fileType
+	switch stat.Mode & syscall.S_IFMT {
+	case syscall.S_IFREG:
+		ft = regular
+	case syscall.S_IFDIR:
+		ft = directory
+	case syscall.S_IFLNK:
+		ft = symlink
+	default:
+		return unknown, syscall.EPERM
+	}
+	return ft, nil
+}
+
+func newLocalFile(a *attachPoint, file *fd.FD, path string, stat syscall.Stat_t) (*localFile, error) {
+	ft, err := getSupportedFileType(stat)
+	if err != nil {
+		return nil, err
+	}
+
+	return &localFile{
+		attachPoint: a,
+		hostPath:    path,
+		file:        file,
+		mode:        invalidMode,
+		ft:          ft,
+	}, nil
+}
+
+// newFDMaybe creates a fd.FD from a file, dup'ing the FD and setting it as
+// non-blocking. If anything fails, returns nil. It's better to have a file
+// without host FD, than to fail the operation.
+func newFDMaybe(file *fd.FD) *fd.FD {
+	dupFD, err := syscall.Dup(file.FD())
+	// Technically, the runtime may call the finalizer on file as soon as
+	// FD() returns.
+	runtime.KeepAlive(file)
+	if err != nil {
+		return nil
+	}
+	dup := fd.New(dupFD)
+
+	// fd is blocking; non-blocking is required.
+	if err := syscall.SetNonblock(dup.FD(), true); err != nil {
+		dup.Close()
+		return nil
+	}
+	return dup
+}
+
+func stat(fd int) (syscall.Stat_t, error) {
+	var stat syscall.Stat_t
+	if err := syscall.Fstat(fd, &stat); err != nil {
+		return syscall.Stat_t{}, err
+	}
+	return stat, nil
+}
+
+func fchown(fd int, uid p9.UID, gid p9.GID) error {
+	return syscall.Fchownat(fd, "", int(uid), int(gid), linux.AT_EMPTY_PATH|unix.AT_SYMLINK_NOFOLLOW)
+}
+
+// Open implements p9.File.
+func (l *localFile) Open(mode p9.OpenFlags) (*fd.FD, p9.QID, uint32, error) {
+	if l.isOpen() {
+		panic(fmt.Sprintf("attempting to open already opened file: %q", l.hostPath))
+	}
+
+	// Check if control file can be used or if a new open must be created.
+	var newFile *fd.FD
+	if mode == p9.ReadOnly {
+		log.Debugf("Open reusing control file, mode: %v, %q", mode, l.hostPath)
+		newFile = l.file
+	} else {
+		// Ideally reopen would call name_to_handle_at (with empty name) and
+		// open_by_handle_at to reopen the file without using 'hostPath'. However,
+		// name_to_handle_at and open_by_handle_at aren't supported by overlay2.
+		log.Debugf("Open reopening file, mode: %v, %q", mode, l.hostPath)
+		var err error
+		newFile, err = fd.Open(l.hostPath, openFlags|mode.OSFlags(), 0)
+		if err != nil {
+			return nil, p9.QID{}, 0, extractErrno(err)
+		}
+	}
+
+	stat, err := stat(newFile.FD())
+	if err != nil {
+		if newFile != l.file {
+			newFile.Close()
+		}
+		return nil, p9.QID{}, 0, extractErrno(err)
+	}
+
+	var fd *fd.FD
+	if stat.Mode&syscall.S_IFMT == syscall.S_IFREG {
+		// Donate FD for regular files only.
+		fd = newFDMaybe(newFile)
+	}
+
+	// Close old file in case a new one was created.
+	if newFile != l.file {
+		if err := l.file.Close(); err != nil {
+			log.Warningf("Error closing file %q: %v", l.hostPath, err)
+		}
+		l.file = newFile
+	}
+	l.mode = mode
+	return fd, l.attachPoint.makeQID(stat), 0, nil
+}
+
+// Create implements p9.File.
+func (l *localFile) Create(name string, mode p9.OpenFlags, perm p9.FileMode, uid p9.UID, gid p9.GID) (*fd.FD, p9.File, p9.QID, uint32, error) {
+	conf := l.attachPoint.conf
+	if conf.ROMount {
+		if conf.PanicOnWrite {
+			panic("attempt to write to RO mount")
+		}
+		return nil, nil, p9.QID{}, 0, syscall.EBADF
+	}
+
+	// 'file' may be used for other operations (e.g. Walk), so read access is
+	// always added to flags. Note that resulting file might have a wider mode
+	// than needed for each particular case.
+	flags := openFlags | syscall.O_CREAT | syscall.O_EXCL
+	if mode == p9.WriteOnly {
+		flags |= syscall.O_RDWR
+	} else {
+		flags |= mode.OSFlags()
+	}
+
+	child, err := fd.OpenAt(l.file, name, flags, uint32(perm.Permissions()))
+	if err != nil {
+		return nil, nil, p9.QID{}, 0, extractErrno(err)
+	}
+	cu := specutils.MakeCleanup(func() {
+		child.Close()
+		// Best effort attempt to remove the file in case of failure.
+		if err := syscall.Unlinkat(l.file.FD(), name); err != nil {
+			log.Warningf("error unlinking file %q after failure: %v", path.Join(l.hostPath, name), err)
+		}
+	})
+	defer cu.Clean()
+
+	if err := fchown(child.FD(), uid, gid); err != nil {
+		return nil, nil, p9.QID{}, 0, extractErrno(err)
+	}
+	stat, err := stat(child.FD())
+	if err != nil {
+		return nil, nil, p9.QID{}, 0, extractErrno(err)
+	}
+
+	c := &localFile{
+		attachPoint: l.attachPoint,
+		hostPath:    path.Join(l.hostPath, name),
+		file:        child,
+		mode:        mode,
+	}
+
+	cu.Release()
+	return newFDMaybe(c.file), c, l.attachPoint.makeQID(stat), 0, nil
+}
+
+// Mkdir implements p9.File.
+func (l *localFile) Mkdir(name string, perm p9.FileMode, uid p9.UID, gid p9.GID) (p9.QID, error) {
+	conf := l.attachPoint.conf
+	if conf.ROMount {
+		if conf.PanicOnWrite {
+			panic("attempt to write to RO mount")
+		}
+		return p9.QID{}, syscall.EBADF
+	}
+
+	if err := syscall.Mkdirat(l.file.FD(), name, uint32(perm.Permissions())); err != nil {
+		return p9.QID{}, extractErrno(err)
+	}
+	cu := specutils.MakeCleanup(func() {
+		// Best effort attempt to remove the dir in case of failure.
+		if err := unix.Unlinkat(l.file.FD(), name, unix.AT_REMOVEDIR); err != nil {
+			log.Warningf("error unlinking dir %q after failure: %v", path.Join(l.hostPath, name), err)
+		}
+	})
+	defer cu.Clean()
+
+	// Open directory to change ownership and stat it.
+	flags := syscall.O_DIRECTORY | syscall.O_RDONLY | openFlags
+	f, err := fd.OpenAt(l.file, name, flags, 0)
+	if err != nil {
+		return p9.QID{}, extractErrno(err)
+	}
+	defer f.Close()
+
+	if err := fchown(f.FD(), uid, gid); err != nil {
+		return p9.QID{}, extractErrno(err)
+	}
+	stat, err := stat(f.FD())
+	if err != nil {
+		return p9.QID{}, extractErrno(err)
+	}
+
+	cu.Release()
+	return l.attachPoint.makeQID(stat), nil
+}
+
+// Walk implements p9.File.
+func (l *localFile) Walk(names []string) ([]p9.QID, p9.File, error) {
+	// Duplicate current file if 'names' is empty.
+	if len(names) == 0 {
+		newFile, err := openAnyFile(l.hostPath, func(mode int) (*fd.FD, error) {
+			return fd.Open(l.hostPath, openFlags|mode, 0)
+		})
+		if err != nil {
+			return nil, nil, extractErrno(err)
+		}
+
+		stat, err := stat(newFile.FD())
+		if err != nil {
+			newFile.Close()
+			return nil, nil, extractErrno(err)
+		}
+
+		c := &localFile{
+			attachPoint: l.attachPoint,
+			hostPath:    l.hostPath,
+			file:        newFile,
+			mode:        invalidMode,
+		}
+		return []p9.QID{l.attachPoint.makeQID(stat)}, c, nil
+	}
+
+	var qids []p9.QID
+	last := l
+	for _, name := range names {
+		f, path, err := openAnyFileFromParent(last, name)
+		if last != l {
+			last.Close()
+		}
+		if err != nil {
+			return nil, nil, extractErrno(err)
+		}
+		stat, err := stat(f.FD())
+		if err != nil {
+			f.Close()
+			return nil, nil, extractErrno(err)
+		}
+		c, err := newLocalFile(last.attachPoint, f, path, stat)
+		if err != nil {
+			f.Close()
+			return nil, nil, extractErrno(err)
+		}
+
+		qids = append(qids, l.attachPoint.makeQID(stat))
+		last = c
+	}
+	return qids, last, nil
+}
+
+// StatFS implements p9.File.
+func (l *localFile) StatFS() (p9.FSStat, error) {
+	var s syscall.Statfs_t
+	if err := syscall.Fstatfs(l.file.FD(), &s); err != nil {
+		return p9.FSStat{}, extractErrno(err)
+	}
+
+	// Populate with what's available.
+	return p9.FSStat{
+		Type:            uint32(s.Type),
+		BlockSize:       uint32(s.Bsize),
+		Blocks:          s.Blocks,
+		BlocksFree:      s.Bfree,
+		BlocksAvailable: s.Bavail,
+		Files:           s.Files,
+		FilesFree:       s.Ffree,
+		NameLength:      uint32(s.Namelen),
+	}, nil
+}
+
+// FSync implements p9.File.
+func (l *localFile) FSync() error {
+	if !l.isOpen() {
+		return syscall.EBADF
+	}
+	if err := syscall.Fsync(l.file.FD()); err != nil {
+		return extractErrno(err)
+	}
+	return nil
+}
+
+// GetAttr implements p9.File.
+func (l *localFile) GetAttr(_ p9.AttrMask) (p9.QID, p9.AttrMask, p9.Attr, error) {
+	stat, err := stat(l.file.FD())
+	if err != nil {
+		return p9.QID{}, p9.AttrMask{}, p9.Attr{}, extractErrno(err)
+	}
+
+	attr := p9.Attr{
+		Mode:             p9.FileMode(stat.Mode),
+		UID:              p9.UID(stat.Uid),
+		GID:              p9.GID(stat.Gid),
+		NLink:            stat.Nlink,
+		RDev:             stat.Rdev,
+		Size:             uint64(stat.Size),
+		BlockSize:        uint64(stat.Blksize),
+		Blocks:           uint64(stat.Blocks),
+		ATimeSeconds:     uint64(stat.Atim.Sec),
+		ATimeNanoSeconds: uint64(stat.Atim.Nsec),
+		MTimeSeconds:     uint64(stat.Mtim.Sec),
+		MTimeNanoSeconds: uint64(stat.Mtim.Nsec),
+		CTimeSeconds:     uint64(stat.Ctim.Sec),
+		CTimeNanoSeconds: uint64(stat.Ctim.Nsec),
+	}
+	valid := p9.AttrMask{
+		Mode:   true,
+		UID:    true,
+		GID:    true,
+		NLink:  true,
+		RDev:   true,
+		Size:   true,
+		Blocks: true,
+		ATime:  true,
+		MTime:  true,
+		CTime:  true,
+	}
+
+	return l.attachPoint.makeQID(stat), valid, attr, nil
+}
+
+// SetAttr implements p9.File. Due to mismatch in file API, options
+// cannot be changed atomicaly and user may see partial changes when
+// an error happens.
+func (l *localFile) SetAttr(valid p9.SetAttrMask, attr p9.SetAttr) error {
+	conf := l.attachPoint.conf
+	if conf.ROMount {
+		if conf.PanicOnWrite {
+			panic("attempt to write to RO mount")
+		}
+		return syscall.EBADF
+	}
+
+	allowed := p9.SetAttrMask{
+		Permissions:        true,
+		UID:                true,
+		GID:                true,
+		Size:               true,
+		ATime:              true,
+		MTime:              true,
+		ATimeNotSystemTime: true,
+		MTimeNotSystemTime: true,
+	}
+
+	if valid.Empty() {
+		// Nothing to do.
+		return nil
+	}
+
+	// Handle all the sanity checks up front so that the client gets a
+	// consistent result that is not attribute dependent.
+	if !valid.IsSubsetOf(allowed) {
+		log.Warningf("SetAttr() failed for %q, mask: %v", l.hostPath, valid)
+		return syscall.EPERM
+	}
+
+	// Check if it's possible to use cached file, or if another one needs to be
+	// opened for write.
+	f := l.file
+	if l.ft == regular && l.mode != p9.WriteOnly && l.mode != p9.ReadWrite {
+		var err error
+		f, err = fd.Open(l.hostPath, openFlags|syscall.O_WRONLY, 0)
+		if err != nil {
+			return extractErrno(err)
+		}
+		defer f.Close()
+	}
+
+	// The semantics are to either return an error if no changes were made,
+	// or no error if *all* changes were made. Well, this can be impossible
+	// if the filesystem rejects at least one of the changes, especially
+	// since some operations are not easy to undo atomically.
+	//
+	// This could be made better if SetAttr actually returned the changes
+	// it did make, so the client can at least know what has changed. So
+	// we at least attempt to make all of the changes and return a generic
+	// error if any of them fails, which at least doesn't bias any change
+	// over another.
+	var err error
+	if valid.Permissions {
+		if cerr := syscall.Fchmod(f.FD(), uint32(attr.Permissions)); cerr != nil {
+			log.Debugf("SetAttr fchmod failed %q, err: %v", l.hostPath, cerr)
+			err = extractErrno(cerr)
+		}
+	}
+
+	if valid.Size {
+		if terr := syscall.Ftruncate(f.FD(), int64(attr.Size)); terr != nil {
+			log.Debugf("SetAttr ftruncate failed %q, err: %v", l.hostPath, terr)
+			err = extractErrno(terr)
+		}
+	}
+
+	if valid.ATime || valid.MTime {
+		utimes := [2]syscall.Timespec{
+			{Sec: 0, Nsec: linux.UTIME_OMIT},
+			{Sec: 0, Nsec: linux.UTIME_OMIT},
+		}
+		if valid.ATime {
+			if valid.ATimeNotSystemTime {
+				utimes[0].Sec = int64(attr.ATimeSeconds)
+				utimes[0].Nsec = int64(attr.ATimeNanoSeconds)
+			} else {
+				utimes[0].Nsec = linux.UTIME_NOW
+			}
+		}
+		if valid.MTime {
+			if valid.MTimeNotSystemTime {
+				utimes[1].Sec = int64(attr.MTimeSeconds)
+				utimes[1].Nsec = int64(attr.MTimeNanoSeconds)
+			} else {
+				utimes[1].Nsec = linux.UTIME_NOW
+			}
+		}
+
+		if l.ft == symlink {
+			// utimensat operates different that other syscalls. To operate on a
+			// symlink it *requires* AT_SYMLINK_NOFOLLOW with dirFD and a non-empty
+			// name.
+			parent, err := syscall.Open(path.Dir(l.hostPath), openFlags|unix.O_PATH, 0)
+			if err != nil {
+				return extractErrno(err)
+			}
+			defer syscall.Close(parent)
+
+			if terr := utimensat(parent, path.Base(l.hostPath), utimes, linux.AT_SYMLINK_NOFOLLOW); terr != nil {
+				log.Debugf("SetAttr utimens failed %q, err: %v", l.hostPath, terr)
+				err = extractErrno(terr)
+			}
+		} else {
+			// Directories and regular files can operate directly on the fd
+			// using empty name.
+			if terr := utimensat(f.FD(), "", utimes, 0); terr != nil {
+				log.Debugf("SetAttr utimens failed %q, err: %v", l.hostPath, terr)
+				err = extractErrno(terr)
+			}
+		}
+	}
+
+	if valid.UID || valid.GID {
+		uid := -1
+		if valid.UID {
+			uid = int(attr.UID)
+		}
+		gid := -1
+		if valid.GID {
+			gid = int(attr.GID)
+		}
+		if oerr := syscall.Fchownat(f.FD(), "", uid, gid, linux.AT_EMPTY_PATH|linux.AT_SYMLINK_NOFOLLOW); oerr != nil {
+			log.Debugf("SetAttr fchownat failed %q, err: %v", l.hostPath, oerr)
+			err = extractErrno(oerr)
+		}
+	}
+
+	return err
+}
+
+// Allocate implements p9.File.
+func (l *localFile) Allocate(mode p9.AllocateMode, offset, length uint64) error {
+	if !l.isOpen() {
+		return syscall.EBADF
+	}
+
+	if err := syscall.Fallocate(l.file.FD(), mode.ToLinux(), int64(offset), int64(length)); err != nil {
+		return extractErrno(err)
+	}
+	return nil
+}
+
+// Rename implements p9.File; this should never be called.
+func (l *localFile) Rename(p9.File, string) error {
+	panic("rename called directly")
+}
+
+// RenameAt implements p9.File.RenameAt.
+func (l *localFile) RenameAt(oldName string, directory p9.File, newName string) error {
+	conf := l.attachPoint.conf
+	if conf.ROMount {
+		if conf.PanicOnWrite {
+			panic("attempt to write to RO mount")
+		}
+		return syscall.EBADF
+	}
+
+	newParent := directory.(*localFile)
+	if err := renameat(l.file.FD(), oldName, newParent.file.FD(), newName); err != nil {
+		return extractErrno(err)
+	}
+	return nil
+}
+
+// ReadAt implements p9.File.
+func (l *localFile) ReadAt(p []byte, offset uint64) (int, error) {
+	if l.mode != p9.ReadOnly && l.mode != p9.ReadWrite {
+		return 0, syscall.EBADF
+	}
+	if !l.isOpen() {
+		return 0, syscall.EBADF
+	}
+
+	r, err := l.file.ReadAt(p, int64(offset))
+	switch err {
+	case nil, io.EOF:
+		return r, nil
+	default:
+		return r, extractErrno(err)
+	}
+}
+
+// WriteAt implements p9.File.
+func (l *localFile) WriteAt(p []byte, offset uint64) (int, error) {
+	if l.mode != p9.WriteOnly && l.mode != p9.ReadWrite {
+		return 0, syscall.EBADF
+	}
+	if !l.isOpen() {
+		return 0, syscall.EBADF
+	}
+
+	w, err := l.file.WriteAt(p, int64(offset))
+	if err != nil {
+		return w, extractErrno(err)
+	}
+	return w, nil
+}
+
+// Symlink implements p9.File.
+func (l *localFile) Symlink(target, newName string, uid p9.UID, gid p9.GID) (p9.QID, error) {
+	conf := l.attachPoint.conf
+	if conf.ROMount {
+		if conf.PanicOnWrite {
+			panic("attempt to write to RO mount")
+		}
+		return p9.QID{}, syscall.EBADF
+	}
+
+	if err := unix.Symlinkat(target, l.file.FD(), newName); err != nil {
+		return p9.QID{}, extractErrno(err)
+	}
+	cu := specutils.MakeCleanup(func() {
+		// Best effort attempt to remove the symlink in case of failure.
+		if err := syscall.Unlinkat(l.file.FD(), newName); err != nil {
+			log.Warningf("error unlinking file %q after failure: %v", path.Join(l.hostPath, newName), err)
+		}
+	})
+	defer cu.Clean()
+
+	// Open symlink to change ownership and stat it.
+	f, err := fd.OpenAt(l.file, newName, unix.O_PATH|openFlags, 0)
+	if err != nil {
+		return p9.QID{}, extractErrno(err)
+	}
+	defer f.Close()
+
+	if err := fchown(f.FD(), uid, gid); err != nil {
+		return p9.QID{}, extractErrno(err)
+	}
+	stat, err := stat(f.FD())
+	if err != nil {
+		return p9.QID{}, extractErrno(err)
+	}
+
+	cu.Release()
+	return l.attachPoint.makeQID(stat), nil
+}
+
+// Link implements p9.File.
+func (l *localFile) Link(target p9.File, newName string) error {
+	conf := l.attachPoint.conf
+	if conf.ROMount {
+		if conf.PanicOnWrite {
+			panic("attempt to write to RO mount")
+		}
+		return syscall.EBADF
+	}
+
+	targetFile := target.(*localFile)
+	if err := unix.Linkat(targetFile.file.FD(), "", l.file.FD(), newName, linux.AT_EMPTY_PATH); err != nil {
+		return extractErrno(err)
+	}
+	return nil
+}
+
+// Mknod implements p9.File.
+//
+// Not implemented.
+func (*localFile) Mknod(_ string, _ p9.FileMode, _ uint32, _ uint32, _ p9.UID, _ p9.GID) (p9.QID, error) {
+	// From mknod(2) man page:
+	// "EPERM: [...] if the filesystem containing pathname does not support
+	// the type of node requested."
+	return p9.QID{}, syscall.EPERM
+}
+
+// UnlinkAt implements p9.File.
+func (l *localFile) UnlinkAt(name string, flags uint32) error {
+	conf := l.attachPoint.conf
+	if conf.ROMount {
+		if conf.PanicOnWrite {
+			panic("attempt to write to RO mount")
+		}
+		return syscall.EBADF
+	}
+
+	if err := unix.Unlinkat(l.file.FD(), name, int(flags)); err != nil {
+		return extractErrno(err)
+	}
+	return nil
+}
+
+// Readdir implements p9.File.
+func (l *localFile) Readdir(offset uint64, count uint32) ([]p9.Dirent, error) {
+	if l.mode != p9.ReadOnly && l.mode != p9.ReadWrite {
+		return nil, syscall.EBADF
+	}
+	if !l.isOpen() {
+		return nil, syscall.EBADF
+	}
+
+	// Readdirnames is a cursor over directories, so seek back to 0 to ensure it's
+	// reading all directory contents. Take a lock because this operation is
+	// stateful.
+	l.readDirMu.Lock()
+	defer l.readDirMu.Unlock()
+
+	skip := uint64(0)
+
+	// Check if the file is at the correct position already. If not, seek to the
+	// beginning and read the entire directory again.
+	if l.lastDirentOffset != offset {
+		if _, err := syscall.Seek(l.file.FD(), 0, 0); err != nil {
+			return nil, extractErrno(err)
+		}
+		skip = offset
+	}
+
+	dirents, err := l.readDirent(l.file.FD(), offset, count, skip)
+	if err == nil {
+		// On success, remember the offset that was returned at the current
+		// position.
+		l.lastDirentOffset = offset + uint64(len(dirents))
+	} else {
+		// On failure, the state is unknown, force call to seek() next time.
+		l.lastDirentOffset = math.MaxUint64
+	}
+	return dirents, err
+}
+
+func (l *localFile) readDirent(f int, offset uint64, count uint32, skip uint64) ([]p9.Dirent, error) {
+	// Limit 'count' to cap the slice size that is returned.
+	const maxCount = 100000
+	if count > maxCount {
+		count = maxCount
+	}
+
+	dirents := make([]p9.Dirent, 0, count)
+
+	// Pre-allocate buffers that will be reused to get partial results.
+	direntsBuf := make([]byte, 8192)
+	names := make([]string, 0, 100)
+
+	end := offset + uint64(count)
+	for offset < end {
+		dirSize, err := syscall.ReadDirent(f, direntsBuf)
+		if err != nil {
+			return dirents, err
+		}
+		if dirSize <= 0 {
+			return dirents, nil
+		}
+
+		names := names[:0]
+		_, _, names = syscall.ParseDirent(direntsBuf[:dirSize], -1, names)
+
+		// Skip over entries that the caller is not interested in.
+		if skip > 0 {
+			if skip > uint64(len(names)) {
+				skip -= uint64(len(names))
+				names = names[:0]
+			} else {
+				names = names[skip:]
+				skip = 0
+			}
+		}
+		for _, name := range names {
+			stat, err := statAt(l.file.FD(), name)
+			if err != nil {
+				log.Warningf("Readdir is skipping file with failed stat %q, err: %v", l.hostPath, err)
+				continue
+			}
+			qid := l.attachPoint.makeQID(stat)
+			offset++
+			dirents = append(dirents, p9.Dirent{
+				QID:    qid,
+				Type:   qid.Type,
+				Name:   name,
+				Offset: offset,
+			})
+		}
+	}
+	return dirents, nil
+}
+
+// Readlink implements p9.File.
+func (l *localFile) Readlink() (string, error) {
+	// Shamelessly stolen from os.Readlink (added upper bound limit to buffer).
+	const limit = 1024 * 1024
+	for len := 128; len < limit; len *= 2 {
+		b := make([]byte, len)
+		n, err := unix.Readlinkat(l.file.FD(), "", b)
+		if err != nil {
+			return "", extractErrno(err)
+		}
+		if n < len {
+			return string(b[:n]), nil
+		}
+	}
+	return "", syscall.ENOMEM
+}
+
+// Flush implements p9.File.
+func (l *localFile) Flush() error {
+	return nil
+}
+
+// Connect implements p9.File.
+func (l *localFile) Connect(p9.ConnectFlags) (*fd.FD, error) {
+	return nil, syscall.ECONNREFUSED
+}
+
+// Close implements p9.File.
+func (l *localFile) Close() error {
+	l.mode = invalidMode
+	err := l.file.Close()
+	l.file = nil
+	return err
+}
+
+func (l *localFile) isOpen() bool {
+	return l.mode != invalidMode
+}
+
+// Renamed implements p9.Renamed.
+func (l *localFile) Renamed(newDir p9.File, newName string) {
+	l.hostPath = path.Join(newDir.(*localFile).hostPath, newName)
+}
+
+// extractErrno tries to determine the errno.
+func extractErrno(err error) syscall.Errno {
+	if err == nil {
+		// This should never happen. The likely result will be that
+		// some user gets the frustrating "error: SUCCESS" message.
+		log.Warningf("extractErrno called with nil error!")
+		return 0
+	}
+
+	switch err {
+	case os.ErrNotExist:
+		return syscall.ENOENT
+	case os.ErrExist:
+		return syscall.EEXIST
+	case os.ErrPermission:
+		return syscall.EACCES
+	case os.ErrInvalid:
+		return syscall.EINVAL
+	}
+
+	// See if it's an errno or a common wrapped error.
+	switch e := err.(type) {
+	case syscall.Errno:
+		return e
+	case *os.PathError:
+		return extractErrno(e.Err)
+	case *os.LinkError:
+		return extractErrno(e.Err)
+	case *os.SyscallError:
+		return extractErrno(e.Err)
+	}
+
+	// Fall back to EIO.
+	log.Debugf("Unknown error: %v, defaulting to EIO", err)
+	return syscall.EIO
+}