Check in gVisor.

PiperOrigin-RevId: 194583126
Change-Id: Ica1d8821a90f74e7e745962d71801c598c652463

1 files changed, 937 insertions, 0 deletions

diff --git a/runsc/fsgofer/fsgofer.go b/runsc/fsgofer/fsgofer.go
new file mode 100644
index 000000000..5ddc75a9d
--- /dev/null
+++ b/runsc/fsgofer/fsgofer.go
@@ -0,0 +1,937 @@
+// Copyright 2018 Google Inc.
+//
+// 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"
+	"strings"
+	"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"
+)
+
+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
+)
+
+// 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"
+}
+
+// Config sets configuration options for each attach point.
+type Config struct {
+	// ROMount is set to true if this is a readonly mount.
+	ROMount bool
+
+	// LazyOpenForWrite makes the underlying file to be opened in RDONLY
+	// mode initially and be reopened in case write access is desired.
+	// This is done to workaround the behavior in 'overlay2' that
+	// copies the entire file up eagerly when it's opened in write mode
+	// even if the file is never actually written to.
+	LazyOpenForWrite bool
+}
+
+type attachPoint struct {
+	prefix string
+	conf   Config
+}
+
+// NewAttachPoint creates a new attacher that gives local file
+// access to all files under 'prefix'.
+func NewAttachPoint(prefix string, c Config) p9.Attacher {
+	return &attachPoint{prefix: prefix, conf: c}
+}
+
+// Attach implements p9.Attacher.
+func (a *attachPoint) Attach(appPath string) (p9.File, error) {
+	if !path.IsAbs(appPath) {
+		return nil, fmt.Errorf("invalid path %q", appPath)
+	}
+
+	root := filepath.Join(a.prefix, appPath)
+	f, err := os.OpenFile(root, openFlags|syscall.O_RDONLY, 0)
+	if err != nil {
+		return nil, fmt.Errorf("unable to open file %q, err: %v", root, err)
+	}
+	stat, err := stat(int(f.Fd()))
+	if err != nil {
+		return nil, fmt.Errorf("failed to stat file %q, err: %v", root, err)
+	}
+	return newLocalFile(a.conf, f, root, stat)
+}
+
+func makeQID(stat syscall.Stat_t) p9.QID {
+	return p9.QID{
+		Type: p9.FileMode(stat.Mode).QIDType(),
+		Path: stat.Ino,
+	}
+}
+
+func isNameValid(name string) bool {
+	if name == "" || name == "." || name == ".." {
+		log.Warningf("Invalid name: %s", name)
+		return false
+	}
+	if strings.IndexByte(name, '/') >= 0 {
+		log.Warningf("Invalid name: %s", name)
+		return false
+	}
+	return true
+}
+
+// localFile implements p9.File wrapping a local file. The underlying file
+// is opened during Walk() and stored in 'controlFile' to be used with other
+// operations. The mode in which the file is opened varies depending on the
+// configuration (see below). 'controlFile' is dup'ed when Walk(nil) is called
+// to clone the file.
+//
+// 'openedFile' is assigned when Open() is called. If requested open mode is
+// a subset of controlFile's mode, it's possible to use the same file. If mode
+// is not a subset, then another file is opened. Consequently, 'openedFile'
+// 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.
+//
+// localFile has 2 modes of operation based on the configuration:
+//
+// ** conf.lazyRWOpen == false **
+// This is the preferred mode. 'controlFile' is opened in RW mode in Walk()
+// and used across all functions. The file is never reopened as the mode will
+// always be a super set of the requested open mode. This reduces the number of
+// syscalls required per operation and makes it resilient to renames anywhere
+// in the path to the file.
+//
+// ** conf.lazyRWOpen == true **
+// This mode is used for better performance with 'overlay2' storage driver.
+// overlay2 eagerly copies the entire file up when it's opened in write mode
+// which makes the mode above perform badly when serveral of files are opened
+// for read (esp. startup). In this mode, 'controlFile' is opened as readonly
+// (or O_PATH for symlinks). Reopening the file is required if write mode
+// is requested in Open().
+type localFile struct {
+	p9.DefaultWalkGetAttr
+
+	// mu protects 'hostPath' when file is renamed.
+	mu sync.Mutex
+
+	// TODO: hostPath is not safe to use as path needs to be walked
+	// everytime (and can change underneath us). Remove all usages.
+	hostPath string
+
+	// controlFile is opened when localFile is created and it's never nil.
+	controlFile *os.File
+
+	// openedFile is nil until localFile is opened. It may point to controlFile
+	// or be a new file struct. See struct comment for more details.
+	openedFile *os.File
+
+	// mode is the mode in which the file was opened. Set to invalidMode
+	// if localFile isn't opened.
+	mode p9.OpenFlags
+
+	ft fileType
+
+	conf Config
+
+	// readDirMu protects against concurrent Readdir calls.
+	readDirMu sync.Mutex
+}
+
+func openAnyFile(parent *localFile, name string) (*os.File, string, error) {
+	// Attempt to open file in the following mode in order:
+	//   1. RDWR: for files with rw mounts and LazyOpenForWrite disabled
+	//   2. RDONLY: for directories, ro mounts or LazyOpenForWrite enabled
+	//   3. PATH: for symlinks
+	modes := []int{syscall.O_RDWR, syscall.O_RDONLY, unix.O_PATH}
+	symlinkIdx := len(modes) - 1
+
+	startIdx := 0
+	if parent.conf.ROMount || parent.conf.LazyOpenForWrite {
+		// Skip attempt to open in RDWR based on configuration.
+		startIdx = 1
+	}
+
+	var err error
+	var fd int
+	for i := startIdx; i < len(modes); i++ {
+		fd, err = syscall.Openat(parent.controlFD(), name, openFlags|modes[i], 0)
+		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
+		case syscall.ELOOP:
+			if i < symlinkIdx {
+				// File was opened with O_NOFOLLOW, so this error can only happen when
+				// trying ot open a symlink. Jump straight to flags compatible with symlink.
+				i = symlinkIdx - 1
+			}
+		}
+		// 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: %s/%s, err: %v", i, openFlags|modes[i], parent.controlFile.Name(), name, err)
+	}
+	if err != nil {
+		// All attempts to open file have failed, return the last error.
+		log.Debugf("Failed to open file, path: %s/%s, err: %v", parent.controlFile.Name(), name, err)
+		return nil, "", extractErrno(err)
+	}
+
+	parent.mu.Lock()
+	defer parent.mu.Unlock()
+	newPath := path.Join(parent.hostPath, name)
+
+	return os.NewFile(uintptr(fd), newPath), newPath, nil
+}
+
+func newLocalFile(conf Config, file *os.File, path string, stat syscall.Stat_t) (*localFile, 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 nil, syscall.EINVAL
+	}
+	return &localFile{
+		hostPath:    path,
+		controlFile: file,
+		conf:        conf,
+		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 *os.File) *fd.FD {
+	fd, err := fd.NewFromFile(file)
+	if err != nil {
+		return nil
+	}
+
+	// fd is blocking; non-blocking is required.
+	if err := syscall.SetNonblock(fd.FD(), true); err != nil {
+		fd.Close()
+		return nil
+	}
+	return fd
+}
+
+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)
+}
+
+func (l *localFile) controlFD() int {
+	return int(l.controlFile.Fd())
+}
+
+func (l *localFile) openedFD() int {
+	if l.openedFile == nil {
+		panic(fmt.Sprintf("trying to use an unopened file: %q", l.controlFile.Name()))
+	}
+	return int(l.openedFile.Fd())
+}
+
+// Open implements p9.File.
+func (l *localFile) Open(mode p9.OpenFlags) (*fd.FD, p9.QID, uint32, error) {
+	if l.openedFile != nil {
+		panic(fmt.Sprintf("attempting to open already opened file: %q", l.controlFile.Name()))
+	}
+
+	// Check if control file can be used or if a new open must be created.
+	var newFile *os.File
+	if mode == p9.ReadOnly || !l.conf.LazyOpenForWrite {
+		log.Debugf("Open reusing control file, mode: %v, %q", mode, l.controlFile.Name())
+		newFile = l.controlFile
+	} 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.controlFile.Name())
+		var err error
+
+		l.mu.Lock()
+		newFile, err = os.OpenFile(l.hostPath, openFlags|mode.OSFlags(), 0)
+		if err != nil {
+			l.mu.Unlock()
+			return nil, p9.QID{}, 0, extractErrno(err)
+		}
+		l.mu.Unlock()
+	}
+
+	stat, err := stat(int(newFile.Fd()))
+	if err != nil {
+		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)
+	}
+
+	// Set fields on success
+	l.openedFile = newFile
+	l.mode = mode
+	return fd, 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) {
+	if l.conf.ROMount {
+		return nil, nil, p9.QID{}, 0, syscall.EBADF
+	}
+	if !isNameValid(name) {
+		return nil, nil, p9.QID{}, 0, syscall.EINVAL
+	}
+
+	// Use a single file for both 'controlFile' and 'openedFile'. Mode must include read for control
+	// and whichever else was requested by caller. 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()
+	}
+
+	fd, err := syscall.Openat(l.controlFD(), name, flags, uint32(perm.Permissions()))
+	if err != nil {
+		return nil, nil, p9.QID{}, 0, extractErrno(err)
+	}
+	if err := fchown(fd, uid, gid); err != nil {
+		syscall.Close(fd)
+		return nil, nil, p9.QID{}, 0, extractErrno(err)
+	}
+	stat, err := stat(fd)
+	if err != nil {
+		syscall.Close(fd)
+		return nil, nil, p9.QID{}, 0, extractErrno(err)
+	}
+
+	l.mu.Lock()
+	defer l.mu.Unlock()
+
+	cPath := path.Join(l.hostPath, name)
+	f := os.NewFile(uintptr(fd), cPath)
+	c := &localFile{
+		hostPath:    cPath,
+		controlFile: f,
+		openedFile:  f,
+		mode:        mode,
+		conf:        l.conf,
+	}
+	return newFDMaybe(c.openedFile), c, 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) {
+	if l.conf.ROMount {
+		return p9.QID{}, syscall.EBADF
+	}
+
+	if !isNameValid(name) {
+		return p9.QID{}, syscall.EINVAL
+	}
+
+	if err := syscall.Mkdirat(l.controlFD(), name, uint32(perm.Permissions())); err != nil {
+		return p9.QID{}, extractErrno(err)
+	}
+
+	// Open directory to change ownership and stat it.
+	flags := syscall.O_DIRECTORY | syscall.O_RDONLY | openFlags
+	fd, err := syscall.Openat(l.controlFD(), name, flags, 0)
+	if err != nil {
+		return p9.QID{}, extractErrno(err)
+	}
+	defer syscall.Close(fd)
+
+	if err := fchown(fd, uid, gid); err != nil {
+		return p9.QID{}, extractErrno(err)
+	}
+	stat, err := stat(fd)
+	if err != nil {
+		return p9.QID{}, extractErrno(err)
+	}
+	return 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 {
+		newFd, err := syscall.Dup(l.controlFD())
+		if err != nil {
+			return nil, nil, extractErrno(err)
+		}
+		stat, err := stat(newFd)
+		if err != nil {
+			syscall.Close(newFd)
+			return nil, nil, extractErrno(err)
+		}
+
+		l.mu.Lock()
+		defer l.mu.Unlock()
+
+		c := &localFile{
+			hostPath:    l.hostPath,
+			controlFile: os.NewFile(uintptr(newFd), l.hostPath),
+			mode:        invalidMode,
+			conf:        l.conf,
+		}
+		return []p9.QID{makeQID(stat)}, c, nil
+	}
+
+	var qids []p9.QID
+	last := l
+	for _, name := range names {
+		if !isNameValid(name) {
+			return nil, nil, syscall.EINVAL
+		}
+
+		f, path, err := openAnyFile(last, name)
+		if err != nil {
+			return nil, nil, extractErrno(err)
+		}
+		stat, err := stat(int(f.Fd()))
+		if err != nil {
+			return nil, nil, extractErrno(err)
+		}
+		c, err := newLocalFile(last.conf, f, path, stat)
+		if err != nil {
+			return nil, nil, extractErrno(err)
+		}
+
+		qids = append(qids, 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.controlFD(), &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.openedFile == nil {
+		return syscall.EBADF
+	}
+	if err := l.openedFile.Sync(); 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.controlFD())
+	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 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 {
+	if l.conf.ROMount {
+		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.controlFile.Name(), valid)
+		return syscall.EPERM
+	}
+
+	fd := l.controlFD()
+	if l.conf.LazyOpenForWrite && l.ft == regular {
+		// Regular files are opened in RO mode when lazy open is set.
+		// Thus it needs to be reopened here for write.
+		f, err := os.OpenFile(l.hostPath, openFlags|os.O_WRONLY, 0)
+		if err != nil {
+			return extractErrno(err)
+		}
+		defer f.Close()
+		fd = int(f.Fd())
+	}
+
+	// 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(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(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{
+			syscall.Timespec{Sec: 0, Nsec: linux.UTIME_OMIT},
+			syscall.Timespec{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.
+			f, err := os.OpenFile(path.Dir(l.hostPath), openFlags|unix.O_PATH, 0)
+			if err != nil {
+				return extractErrno(err)
+			}
+			defer f.Close()
+
+			if terr := utimensat(int(f.Fd()), 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(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(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
+}
+
+// Remove implements p9.File.
+//
+// This is deprecated in favor of UnlinkAt.
+func (*localFile) Remove() error {
+	return syscall.ENOSYS
+}
+
+// Rename implements p9.File.
+func (l *localFile) Rename(directory p9.File, name string) error {
+	if l.conf.ROMount {
+		return syscall.EBADF
+	}
+	if !isNameValid(name) {
+		return syscall.EINVAL
+	}
+
+	l.mu.Lock()
+	defer l.mu.Unlock()
+
+	// TODO: change to renameat(2)
+	parent := directory.(*localFile)
+	newPath := path.Join(parent.hostPath, name)
+	if err := os.Rename(l.hostPath, newPath); err != nil {
+		return extractErrno(err)
+	}
+
+	// Update path on success.
+	// TODO: this doesn't cover cases where any of the
+	// parents have been renamed.
+	l.hostPath = newPath
+	return nil
+}
+
+// RenameAt implements p9.File.RenameAt.
+//
+// Code still uses [deprecated] Rename().
+func (*localFile) RenameAt(_ string, _ p9.File, _ string) error {
+	return syscall.ENOSYS
+}
+
+// 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.openedFile == nil {
+		return 0, syscall.EBADF
+	}
+
+	r, err := l.openedFile.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.openedFile == nil {
+		return 0, syscall.EBADF
+	}
+
+	w, err := l.openedFile.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) {
+	if l.conf.ROMount {
+		return p9.QID{}, syscall.EBADF
+	}
+	if !isNameValid(newName) {
+		return p9.QID{}, syscall.EINVAL
+	}
+
+	if err := unix.Symlinkat(target, l.controlFD(), newName); err != nil {
+		return p9.QID{}, extractErrno(err)
+	}
+
+	// Open symlink to change ownership and stat it.
+	fd, err := syscall.Openat(l.controlFD(), newName, unix.O_PATH|openFlags, 0)
+	if err != nil {
+		return p9.QID{}, extractErrno(err)
+	}
+	defer syscall.Close(fd)
+
+	if err := fchown(fd, uid, gid); err != nil {
+		return p9.QID{}, extractErrno(err)
+	}
+	stat, err := stat(fd)
+	if err != nil {
+		return p9.QID{}, extractErrno(err)
+	}
+	return makeQID(stat), nil
+}
+
+// Link implements p9.File.
+func (l *localFile) Link(target p9.File, newName string) error {
+	if l.conf.ROMount {
+		return syscall.EBADF
+	}
+	if !isNameValid(newName) {
+		return syscall.EINVAL
+	}
+
+	targetFile := target.(*localFile)
+	if err := unix.Linkat(targetFile.controlFD(), "", l.controlFD(), 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) {
+	return p9.QID{}, syscall.ENOSYS
+}
+
+// UnlinkAt implements p9.File.
+func (l *localFile) UnlinkAt(name string, flags uint32) error {
+	if l.conf.ROMount {
+		return syscall.EBADF
+	}
+	if !isNameValid(name) {
+		return syscall.EINVAL
+	}
+	if err := unix.Unlinkat(l.controlFD(), 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.openedFile == nil {
+		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()
+	if _, err := l.openedFile.Seek(0, 0); err != nil {
+		l.readDirMu.Unlock()
+		return nil, extractErrno(err)
+	}
+	names, err := l.openedFile.Readdirnames(-1)
+	if err != nil {
+		l.readDirMu.Unlock()
+		return nil, extractErrno(err)
+	}
+	l.readDirMu.Unlock()
+
+	var dirents []p9.Dirent
+	for i := int(offset); i >= 0 && i < len(names); i++ {
+		stat, err := statAt(l.openedFD(), names[i])
+		if err != nil {
+			continue
+		}
+		qid := makeQID(stat)
+		dirents = append(dirents, p9.Dirent{
+			QID:    qid,
+			Type:   qid.Type,
+			Name:   names[i],
+			Offset: uint64(i + 1),
+		})
+	}
+	return dirents, nil
+}
+
+// Readlink implements p9.File.
+func (l *localFile) Readlink() (string, error) {
+	// Shamelessly stolen from os.Readlink (added upper bound limit to buffer).
+	for len := 128; len < 1024*1024; len *= 2 {
+		b := make([]byte, len)
+		n, err := unix.Readlinkat(l.controlFD(), "", 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 {
+	err := l.controlFile.Close()
+
+	// Close only once in case opened and control files point to
+	// the same os.File struct.
+	if l.openedFile != nil && l.openedFile != l.controlFile {
+		err = l.openedFile.Close()
+	}
+
+	l.openedFile = nil
+	l.controlFile = nil
+	l.mode = invalidMode
+	return err
+}
+
+// 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 frustration "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
+}