// Copyright 2018 Google LLC
//
// 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"
	"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 := os.O_RDWR
	if a.conf.ROMount || stat.Mode&syscall.S_IFDIR != 0 {
		mode = os.O_RDONLY
	}

	// Open the root directory.
	f, err := os.OpenFile(a.prefix, mode|openFlags, 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 'controlFile' to be used with other
// operations. The control file is opened as readonly, unless it's a symlink
// which requires O_PATH. 'controlFile' 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.
//
// '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.
//
// The reason that the control file is never opened 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
// multiple files are being opened for read-only (esp. startup).
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

	// 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 is the fileType for this file.
	ft fileType

	// 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. 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 fd int
	for i, mode := range modes {
		fd, err = syscall.Openat(parent.controlFD(), name, openFlags|mode, 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
		}
		// 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|mode, 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)
	}

	newPath := path.Join(parent.hostPath, name)
	return os.NewFile(uintptr(fd), newPath), newPath, 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 *os.File, path string, stat syscall.Stat_t) (*localFile, error) {
	ft, err := getSupportedFileType(stat)
	if err != nil {
		return nil, err
	}

	return &localFile{
		attachPoint: a,
		hostPath:    path,
		controlFile: 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 *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 {
		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

		newFile, err = os.OpenFile(l.hostPath, openFlags|mode.OSFlags(), 0)
		if err != nil {
			return nil, p9.QID{}, 0, extractErrno(err)
		}
	}

	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, 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
	}

	// 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)
	}
	cu := specutils.MakeCleanup(func() {
		syscall.Close(fd)
		// Best effort attempt to remove the file in case of failure.
		if err := syscall.Unlinkat(l.controlFD(), name); err != nil {
			log.Warningf("error unlinking file %q after failure: %v", path.Join(l.hostPath, name), err)
		}
	})
	defer cu.Clean()

	if err := fchown(fd, uid, gid); err != nil {
		return nil, nil, p9.QID{}, 0, extractErrno(err)
	}
	stat, err := stat(fd)
	if err != nil {
		return nil, nil, p9.QID{}, 0, extractErrno(err)
	}

	cPath := path.Join(l.hostPath, name)
	f := os.NewFile(uintptr(fd), cPath)
	c := &localFile{
		attachPoint: l.attachPoint,
		hostPath:    cPath,
		controlFile: f,
		openedFile:  f,
		mode:        mode,
	}

	cu.Release()
	return newFDMaybe(c.openedFile), 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.controlFD(), 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.controlFD(), 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
	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)
	}

	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 {
		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)
		}

		c := &localFile{
			attachPoint: l.attachPoint,
			hostPath:    l.hostPath,
			controlFile: os.NewFile(uintptr(newFd), l.hostPath),
			mode:        invalidMode,
		}
		return []p9.QID{l.attachPoint.makeQID(stat)}, c, nil
	}

	var qids []p9.QID
	last := l
	for _, name := range names {
		f, path, err := openAnyFile(last, name)
		if err != nil {
			return nil, nil, extractErrno(err)
		}
		stat, err := stat(int(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.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 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.controlFile.Name(), valid)
		return syscall.EPERM
	}

	fd := l.controlFD()
	if l.ft == regular {
		// Regular files are opened in RO mode, 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{
			{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.
			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
}

// 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.controlFD(), oldName, newParent.controlFD(), 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.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) {
	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.controlFD(), 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.controlFD(), 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.
	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)
	}

	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.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 {
	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.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 := l.attachPoint.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
}

// 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
}