src/os/root.go - go.git - Git at Google

 // Copyright 2024 The Go Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.

 package os

 import (
 	"errors"
 	"internal/bytealg"
 	"internal/stringslite"
 	"internal/testlog"
 	"io/fs"
 	"runtime"
 	"slices"
 	"time"
 )

 // OpenInRoot opens the file name in the directory dir.
 // It is equivalent to OpenRoot(dir) followed by opening the file in the root.
 //
 // OpenInRoot returns an error if any component of the name
 // references a location outside of dir.
 //
 // See [Root] for details and limitations.
 func OpenInRoot(dir, name string) (*File, error) {
 	r, err := OpenRoot(dir)
 	if err != nil {
 		return nil, err
 	}
 	defer r.Close()
 	return r.Open(name)
 }

 // Root may be used to only access files within a single directory tree.
 //
 // Methods on Root can only access files and directories beneath a root directory.
 // If any component of a file name passed to a method of Root references a location
 // outside the root, the method returns an error.
 // File names may reference the directory itself (.).
 //
 // Methods on Root will follow symbolic links, but symbolic links may not
 // reference a location outside the root.
 // Symbolic links must not be absolute.
 //
 // Methods on Root do not prohibit traversal of filesystem boundaries,
 // Linux bind mounts, /proc special files, or access to Unix device files.
 //
 // Methods on Root are safe to be used from multiple goroutines simultaneously.
 //
 // On most platforms, creating a Root opens a file descriptor or handle referencing
 // the directory. If the directory is moved, methods on Root reference the original
 // directory in its new location.
 //
 // Root's behavior differs on some platforms:
 //
 //   - When GOOS=windows, file names may not reference Windows reserved device names
 //     such as NUL and COM1.
 //   - On Unix, [Root.Chmod], [Root.Chown], and [Root.Chtimes] are vulnerable to a race condition.
 //     If the target of the operation is changed from a regular file to a symlink
 //     while the operation is in progress, the operation may be performed on the link
 //     rather than the link target.
 //   - When GOOS=js, Root is vulnerable to TOCTOU (time-of-check-time-of-use)
 //     attacks in symlink validation, and cannot ensure that operations will not
 //     escape the root.
 //   - When GOOS=plan9 or GOOS=js, Root does not track directories across renames.
 //     On these platforms, a Root references a directory name, not a file descriptor.
 //   - WASI preview 1 (GOOS=wasip1) does not support [Root.Chmod].
 type Root struct {
 	root *root
 }

 const (
 	// Maximum number of symbolic links we will follow when resolving a file in a root.
 	// 8 is __POSIX_SYMLOOP_MAX (the minimum allowed value for SYMLOOP_MAX),
 	// and a common limit.
 	rootMaxSymlinks = 8
 )

 // OpenRoot opens the named directory.
 // It follows symbolic links in the directory name.
 // If there is an error, it will be of type [*PathError].
 func OpenRoot(name string) (*Root, error) {
 	testlog.Open(name)
 	return openRootNolog(name)
 }

 // Name returns the name of the directory presented to OpenRoot.
 //
 // It is safe to call Name after [Close].
 func (r *Root) Name() string {
 	return r.root.Name()
 }

 // Close closes the Root.
 // After Close is called, methods on Root return errors.
 func (r *Root) Close() error {
 	return r.root.Close()
 }

 // Open opens the named file in the root for reading.
 // See [Open] for more details.
 func (r *Root) Open(name string) (*File, error) {
 	return r.OpenFile(name, O_RDONLY, 0)
 }

 // Create creates or truncates the named file in the root.
 // See [Create] for more details.
 func (r *Root) Create(name string) (*File, error) {
 	return r.OpenFile(name, O_RDWR|O_CREATE|O_TRUNC, 0666)
 }

 // OpenFile opens the named file in the root.
 // See [OpenFile] for more details.
 //
 // If perm contains bits other than the nine least-significant bits (0o777),
 // OpenFile returns an error.
 func (r *Root) OpenFile(name string, flag int, perm FileMode) (*File, error) {
 	if perm&0o777 != perm {
 		return nil, &PathError{Op: "openat", Path: name, Err: errors.New("unsupported file mode")}
 	}
 	r.logOpen(name)
 	rf, err := rootOpenFileNolog(r, name, flag, perm)
 	if err != nil {
 		return nil, err
 	}
 	rf.appendMode = flag&O_APPEND != 0
 	return rf, nil
 }

 // OpenRoot opens the named directory in the root.
 // If there is an error, it will be of type [*PathError].
 func (r *Root) OpenRoot(name string) (*Root, error) {
 	r.logOpen(name)
 	return openRootInRoot(r, name)
 }

 // Chmod changes the mode of the named file in the root to mode.
 // See [Chmod] for more details.
 func (r *Root) Chmod(name string, mode FileMode) error {
 	return rootChmod(r, name, mode)
 }

 // Mkdir creates a new directory in the root
 // with the specified name and permission bits (before umask).
 // See [Mkdir] for more details.
 //
 // If perm contains bits other than the nine least-significant bits (0o777),
 // Mkdir returns an error.
 func (r *Root) Mkdir(name string, perm FileMode) error {
 	if perm&0o777 != perm {
 		return &PathError{Op: "mkdirat", Path: name, Err: errors.New("unsupported file mode")}
 	}
 	return rootMkdir(r, name, perm)
 }

 // MkdirAll creates a new directory in the root, along with any necessary parents.
 // See [MkdirAll] for more details.
 //
 // If perm contains bits other than the nine least-significant bits (0o777),
 // MkdirAll returns an error.
 func (r *Root) MkdirAll(name string, perm FileMode) error {
 	if perm&0o777 != perm {
 		return &PathError{Op: "mkdirat", Path: name, Err: errors.New("unsupported file mode")}
 	}
 	return rootMkdirAll(r, name, perm)
 }

 // Chown changes the numeric uid and gid of the named file in the root.
 // See [Chown] for more details.
 func (r *Root) Chown(name string, uid, gid int) error {
 	return rootChown(r, name, uid, gid)
 }

 // Lchown changes the numeric uid and gid of the named file in the root.
 // See [Lchown] for more details.
 func (r *Root) Lchown(name string, uid, gid int) error {
 	return rootLchown(r, name, uid, gid)
 }

 // Chtimes changes the access and modification times of the named file in the root.
 // See [Chtimes] for more details.
 func (r *Root) Chtimes(name string, atime time.Time, mtime time.Time) error {
 	return rootChtimes(r, name, atime, mtime)
 }

 // Remove removes the named file or (empty) directory in the root.
 // See [Remove] for more details.
 func (r *Root) Remove(name string) error {
 	return rootRemove(r, name)
 }

 // RemoveAll removes the named file or directory and any children that it contains.
 // See [RemoveAll] for more details.
 func (r *Root) RemoveAll(name string) error {
 	return rootRemoveAll(r, name)
 }

 // Stat returns a [FileInfo] describing the named file in the root.
 // See [Stat] for more details.
 func (r *Root) Stat(name string) (FileInfo, error) {
 	r.logStat(name)
 	return rootStat(r, name, false)
 }

 // Lstat returns a [FileInfo] describing the named file in the root.
 // If the file is a symbolic link, the returned FileInfo
 // describes the symbolic link.
 // See [Lstat] for more details.
 func (r *Root) Lstat(name string) (FileInfo, error) {
 	r.logStat(name)
 	return rootStat(r, name, true)
 }

 // Readlink returns the destination of the named symbolic link in the root.
 // See [Readlink] for more details.
 func (r *Root) Readlink(name string) (string, error) {
 	return rootReadlink(r, name)
 }

 // Rename renames (moves) oldname to newname.
 // Both paths are relative to the root.
 // See [Rename] for more details.
 func (r *Root) Rename(oldname, newname string) error {
 	return rootRename(r, oldname, newname)
 }

 // Link creates newname as a hard link to the oldname file.
 // Both paths are relative to the root.
 // See [Link] for more details.
 //
 // If oldname is a symbolic link, Link creates new link to oldname and not its target.
 // This behavior may differ from that of [Link] on some platforms.
 //
 // When GOOS=js, Link returns an error if oldname is a symbolic link.
 func (r *Root) Link(oldname, newname string) error {
 	return rootLink(r, oldname, newname)
 }

 // Symlink creates newname as a symbolic link to oldname.
 // See [Symlink] for more details.
 //
 // Symlink does not validate oldname,
 // which may reference a location outside the root.
 //
 // On Windows, a directory link is created if oldname references
 // a directory within the root. Otherwise a file link is created.
 func (r *Root) Symlink(oldname, newname string) error {
 	return rootSymlink(r, oldname, newname)
 }

 // ReadFile reads the named file in the root and returns its contents.
 // See [ReadFile] for more details.
 func (r *Root) ReadFile(name string) ([]byte, error) {
 	f, err := r.Open(name)
 	if err != nil {
 		return nil, err
 	}
 	defer f.Close()
 	return readFileContents(statOrZero(f), f.Read)
 }

 // WriteFile writes data to the named file in the root, creating it if necessary.
 // See [WriteFile] for more details.
 func (r *Root) WriteFile(name string, data []byte, perm FileMode) error {
 	f, err := r.OpenFile(name, O_WRONLY|O_CREATE|O_TRUNC, perm)
 	if err != nil {
 		return err
 	}
 	_, err = f.Write(data)
 	if err1 := f.Close(); err == nil {
 		err = err1
 	}
 	return err
 }

 func (r *Root) logOpen(name string) {
 	if log := testlog.Logger(); log != nil {
 		// This won't be right if r's name has changed since it was opened,
 		// but it's the best we can do.
 		log.Open(joinPath(r.Name(), name))
 	}
 }

 func (r *Root) logStat(name string) {
 	if log := testlog.Logger(); log != nil {
 		// This won't be right if r's name has changed since it was opened,
 		// but it's the best we can do.
 		log.Stat(joinPath(r.Name(), name))
 	}
 }

 // splitPathInRoot splits a path into components
 // and joins it with the given prefix and suffix.
 //
 // The path is relative to a Root, and must not be
 // absolute, volume-relative, or "".
 //
 // "." components are removed, except in the last component.
 //
 // Path separators following the last component are returned in suffixSep.
 func splitPathInRoot(s string, prefix, suffix []string) (_ []string, suffixSep string, err error) {
 	if len(s) == 0 {
 		return nil, "", errors.New("empty path")
 	}
 	if IsPathSeparator(s[0]) {
 		return nil, "", errPathEscapes
 	}

 	if runtime.GOOS == "windows" {
 		// Windows cleans paths before opening them.
 		s, err = rootCleanPath(s, prefix, suffix)
 		if err != nil {
 			return nil, "", err
 		}
 		prefix = nil
 		suffix = nil
 	}

 	parts := slices.Clone(prefix)
 	i, j := 0, 1
 	for {
 		if j < len(s) && !IsPathSeparator(s[j]) {
 			// Keep looking for the end of this component.
 			j++
 			continue
 		}
 		parts = append(parts, s[i:j])
 		// Advance to the next component, or end of the path.
 		partEnd := j
 		for j < len(s) && IsPathSeparator(s[j]) {
 			j++
 		}
 		if j == len(s) {
 			// If this is the last path component,
 			// preserve any trailing path separators.
 			suffixSep = s[partEnd:]
 			break
 		}
 		if parts[len(parts)-1] == "." {
 			// Remove "." components, except at the end.
 			parts = parts[:len(parts)-1]
 		}
 		i = j
 	}
 	if len(suffix) > 0 && len(parts) > 0 && parts[len(parts)-1] == "." {
 		// Remove a trailing "." component if we're joining to a suffix.
 		parts = parts[:len(parts)-1]
 	}
 	parts = append(parts, suffix...)
 	return parts, suffixSep, nil
 }

 // FS returns a file system (an fs.FS) for the tree of files in the root.
 //
 // The result implements [io/fs.StatFS], [io/fs.ReadFileFS],
 // [io/fs.ReadDirFS], and [io/fs.ReadLinkFS].
 func (r *Root) FS() fs.FS {
 	return (*rootFS)(r)
 }

 type rootFS Root

 func (rfs *rootFS) Open(name string) (fs.File, error) {
 	r := (*Root)(rfs)
 	if !isValidRootFSPath(name) {
 		return nil, &PathError{Op: "open", Path: name, Err: ErrInvalid}
 	}
 	f, err := r.Open(name)
 	if err != nil {
 		return nil, err
 	}
 	return f, nil
 }

 func (rfs *rootFS) ReadDir(name string) ([]DirEntry, error) {
 	r := (*Root)(rfs)
 	if !isValidRootFSPath(name) {
 		return nil, &PathError{Op: "readdir", Path: name, Err: ErrInvalid}
 	}

 	// This isn't efficient: We just open a regular file and ReadDir it.
 	// Ideally, we would skip creating a *File entirely and operate directly
 	// on the file descriptor, but that will require some extensive reworking
 	// of directory reading in general.
 	//
 	// This suffices for the moment.
 	f, err := r.Open(name)
 	if err != nil {
 		return nil, err
 	}
 	defer f.Close()
 	dirs, err := f.ReadDir(-1)
 	slices.SortFunc(dirs, func(a, b DirEntry) int {
 		return bytealg.CompareString(a.Name(), b.Name())
 	})
 	return dirs, err
 }

 func (rfs *rootFS) ReadFile(name string) ([]byte, error) {
 	r := (*Root)(rfs)
 	if !isValidRootFSPath(name) {
 		return nil, &PathError{Op: "readfile", Path: name, Err: ErrInvalid}
 	}
 	f, err := r.Open(name)
 	if err != nil {
 		return nil, err
 	}
 	defer f.Close()
 	return readFileContents(statOrZero(f), f.Read)
 }

 func (rfs *rootFS) ReadLink(name string) (string, error) {
 	r := (*Root)(rfs)
 	if !isValidRootFSPath(name) {
 		return "", &PathError{Op: "readlink", Path: name, Err: ErrInvalid}
 	}
 	return r.Readlink(name)
 }

 func (rfs *rootFS) Stat(name string) (FileInfo, error) {
 	r := (*Root)(rfs)
 	if !isValidRootFSPath(name) {
 		return nil, &PathError{Op: "stat", Path: name, Err: ErrInvalid}
 	}
 	return r.Stat(name)
 }

 func (rfs *rootFS) Lstat(name string) (FileInfo, error) {
 	r := (*Root)(rfs)
 	if !isValidRootFSPath(name) {
 		return nil, &PathError{Op: "lstat", Path: name, Err: ErrInvalid}
 	}
 	return r.Lstat(name)
 }

 // isValidRootFSPath reports whether name is a valid filename to pass a Root.FS method.
 func isValidRootFSPath(name string) bool {
 	if !fs.ValidPath(name) {
 		return false
 	}
 	if runtime.GOOS == "windows" {
 		// fs.FS paths are /-separated.
 		// On Windows, reject the path if it contains any \ separators.
 		// Other forms of invalid path (for example, "NUL") are handled by
 		// Root's usual file lookup mechanisms.
 		if stringslite.IndexByte(name, '\\') >= 0 {
 			return false
 		}
 	}
 	return true
 }
	// Copyright 2024 The Go Authors. All rights reserved.
	// Use of this source code is governed by a BSD-style
	// license that can be found in the LICENSE file.

	package os

	import (
	"errors"
	"internal/bytealg"
	"internal/stringslite"
	"internal/testlog"
	"io/fs"
	"runtime"
	"slices"
	"time"
	)

	// OpenInRoot opens the file name in the directory dir.
	// It is equivalent to OpenRoot(dir) followed by opening the file in the root.
	//
	// OpenInRoot returns an error if any component of the name
	// references a location outside of dir.
	//
	// See [Root] for details and limitations.
	func OpenInRoot(dir, name string) (*File, error) {
	r, err := OpenRoot(dir)
	if err != nil {
	return nil, err
	}
	defer r.Close()
	return r.Open(name)
	}

	// Root may be used to only access files within a single directory tree.
	//
	// Methods on Root can only access files and directories beneath a root directory.
	// If any component of a file name passed to a method of Root references a location
	// outside the root, the method returns an error.
	// File names may reference the directory itself (.).
	//
	// Methods on Root will follow symbolic links, but symbolic links may not
	// reference a location outside the root.
	// Symbolic links must not be absolute.
	//
	// Methods on Root do not prohibit traversal of filesystem boundaries,
	// Linux bind mounts, /proc special files, or access to Unix device files.
	//
	// Methods on Root are safe to be used from multiple goroutines simultaneously.
	//
	// On most platforms, creating a Root opens a file descriptor or handle referencing
	// the directory. If the directory is moved, methods on Root reference the original
	// directory in its new location.
	//
	// Root's behavior differs on some platforms:
	//
	// - When GOOS=windows, file names may not reference Windows reserved device names
	// such as NUL and COM1.
	// - On Unix, [Root.Chmod], [Root.Chown], and [Root.Chtimes] are vulnerable to a race condition.
	// If the target of the operation is changed from a regular file to a symlink
	// while the operation is in progress, the operation may be performed on the link
	// rather than the link target.
	// - When GOOS=js, Root is vulnerable to TOCTOU (time-of-check-time-of-use)
	// attacks in symlink validation, and cannot ensure that operations will not
	// escape the root.
	// - When GOOS=plan9 or GOOS=js, Root does not track directories across renames.
	// On these platforms, a Root references a directory name, not a file descriptor.
	// - WASI preview 1 (GOOS=wasip1) does not support [Root.Chmod].
	type Root struct {
	root *root
	}

	const (
	// Maximum number of symbolic links we will follow when resolving a file in a root.
	// 8 is __POSIX_SYMLOOP_MAX (the minimum allowed value for SYMLOOP_MAX),
	// and a common limit.
	rootMaxSymlinks = 8
	)

	// OpenRoot opens the named directory.
	// It follows symbolic links in the directory name.
	// If there is an error, it will be of type [*PathError].
	func OpenRoot(name string) (*Root, error) {
	testlog.Open(name)
	return openRootNolog(name)
	}

	// Name returns the name of the directory presented to OpenRoot.
	//
	// It is safe to call Name after [Close].
	func (r *Root) Name() string {
	return r.root.Name()
	}

	// Close closes the Root.
	// After Close is called, methods on Root return errors.
	func (r *Root) Close() error {
	return r.root.Close()
	}

	// Open opens the named file in the root for reading.
	// See [Open] for more details.
	func (r Root) Open(name string) (File, error) {
	return r.OpenFile(name, O_RDONLY, 0)
	}

	// Create creates or truncates the named file in the root.
	// See [Create] for more details.
	func (r Root) Create(name string) (File, error) {
	return r.OpenFile(name, O_RDWR\|O_CREATE\|O_TRUNC, 0666)
	}

	// OpenFile opens the named file in the root.
	// See [OpenFile] for more details.
	//
	// If perm contains bits other than the nine least-significant bits (0o777),
	// OpenFile returns an error.
	func (r Root) OpenFile(name string, flag int, perm FileMode) (File, error) {
	if perm&0o777 != perm {
	return nil, &PathError{Op: "openat", Path: name, Err: errors.New("unsupported file mode")}
	}
	r.logOpen(name)
	rf, err := rootOpenFileNolog(r, name, flag, perm)
	if err != nil {
	return nil, err
	}
	rf.appendMode = flag&O_APPEND != 0
	return rf, nil
	}

	// OpenRoot opens the named directory in the root.
	// If there is an error, it will be of type [*PathError].
	func (r Root) OpenRoot(name string) (Root, error) {
	r.logOpen(name)
	return openRootInRoot(r, name)
	}

	// Chmod changes the mode of the named file in the root to mode.
	// See [Chmod] for more details.
	func (r *Root) Chmod(name string, mode FileMode) error {
	return rootChmod(r, name, mode)
	}

	// Mkdir creates a new directory in the root
	// with the specified name and permission bits (before umask).
	// See [Mkdir] for more details.
	//
	// If perm contains bits other than the nine least-significant bits (0o777),
	// Mkdir returns an error.
	func (r *Root) Mkdir(name string, perm FileMode) error {
	if perm&0o777 != perm {
	return &PathError{Op: "mkdirat", Path: name, Err: errors.New("unsupported file mode")}
	}
	return rootMkdir(r, name, perm)
	}

	// MkdirAll creates a new directory in the root, along with any necessary parents.
	// See [MkdirAll] for more details.
	//
	// If perm contains bits other than the nine least-significant bits (0o777),
	// MkdirAll returns an error.
	func (r *Root) MkdirAll(name string, perm FileMode) error {
	if perm&0o777 != perm {
	return &PathError{Op: "mkdirat", Path: name, Err: errors.New("unsupported file mode")}
	}
	return rootMkdirAll(r, name, perm)
	}

	// Chown changes the numeric uid and gid of the named file in the root.
	// See [Chown] for more details.
	func (r *Root) Chown(name string, uid, gid int) error {
	return rootChown(r, name, uid, gid)
	}

	// Lchown changes the numeric uid and gid of the named file in the root.
	// See [Lchown] for more details.
	func (r *Root) Lchown(name string, uid, gid int) error {
	return rootLchown(r, name, uid, gid)
	}

	// Chtimes changes the access and modification times of the named file in the root.
	// See [Chtimes] for more details.
	func (r *Root) Chtimes(name string, atime time.Time, mtime time.Time) error {
	return rootChtimes(r, name, atime, mtime)
	}

	// Remove removes the named file or (empty) directory in the root.
	// See [Remove] for more details.
	func (r *Root) Remove(name string) error {
	return rootRemove(r, name)
	}

	// RemoveAll removes the named file or directory and any children that it contains.
	// See [RemoveAll] for more details.
	func (r *Root) RemoveAll(name string) error {
	return rootRemoveAll(r, name)
	}

	// Stat returns a [FileInfo] describing the named file in the root.
	// See [Stat] for more details.
	func (r *Root) Stat(name string) (FileInfo, error) {
	r.logStat(name)
	return rootStat(r, name, false)
	}

	// Lstat returns a [FileInfo] describing the named file in the root.
	// If the file is a symbolic link, the returned FileInfo
	// describes the symbolic link.
	// See [Lstat] for more details.
	func (r *Root) Lstat(name string) (FileInfo, error) {
	r.logStat(name)
	return rootStat(r, name, true)
	}

	// Readlink returns the destination of the named symbolic link in the root.
	// See [Readlink] for more details.
	func (r *Root) Readlink(name string) (string, error) {
	return rootReadlink(r, name)
	}

	// Rename renames (moves) oldname to newname.
	// Both paths are relative to the root.
	// See [Rename] for more details.
	func (r *Root) Rename(oldname, newname string) error {
	return rootRename(r, oldname, newname)
	}

	// Link creates newname as a hard link to the oldname file.
	// Both paths are relative to the root.
	// See [Link] for more details.
	//
	// If oldname is a symbolic link, Link creates new link to oldname and not its target.
	// This behavior may differ from that of [Link] on some platforms.
	//
	// When GOOS=js, Link returns an error if oldname is a symbolic link.
	func (r *Root) Link(oldname, newname string) error {
	return rootLink(r, oldname, newname)
	}

	// Symlink creates newname as a symbolic link to oldname.
	// See [Symlink] for more details.
	//
	// Symlink does not validate oldname,
	// which may reference a location outside the root.
	//
	// On Windows, a directory link is created if oldname references
	// a directory within the root. Otherwise a file link is created.
	func (r *Root) Symlink(oldname, newname string) error {
	return rootSymlink(r, oldname, newname)
	}

	// ReadFile reads the named file in the root and returns its contents.
	// See [ReadFile] for more details.
	func (r *Root) ReadFile(name string) ([]byte, error) {
	f, err := r.Open(name)
	if err != nil {
	return nil, err
	}
	defer f.Close()
	return readFileContents(statOrZero(f), f.Read)
	}

	// WriteFile writes data to the named file in the root, creating it if necessary.
	// See [WriteFile] for more details.
	func (r *Root) WriteFile(name string, data []byte, perm FileMode) error {
	f, err := r.OpenFile(name, O_WRONLY\|O_CREATE\|O_TRUNC, perm)
	if err != nil {
	return err
	}
	_, err = f.Write(data)
	if err1 := f.Close(); err == nil {
	err = err1
	}
	return err
	}

	func (r *Root) logOpen(name string) {
	if log := testlog.Logger(); log != nil {
	// This won't be right if r's name has changed since it was opened,
	// but it's the best we can do.
	log.Open(joinPath(r.Name(), name))
	}
	}

	func (r *Root) logStat(name string) {
	if log := testlog.Logger(); log != nil {
	// This won't be right if r's name has changed since it was opened,
	// but it's the best we can do.
	log.Stat(joinPath(r.Name(), name))
	}
	}

	// splitPathInRoot splits a path into components
	// and joins it with the given prefix and suffix.
	//
	// The path is relative to a Root, and must not be
	// absolute, volume-relative, or "".
	//
	// "." components are removed, except in the last component.
	//
	// Path separators following the last component are returned in suffixSep.
	func splitPathInRoot(s string, prefix, suffix []string) (_ []string, suffixSep string, err error) {
	if len(s) == 0 {
	return nil, "", errors.New("empty path")
	}
	if IsPathSeparator(s[0]) {
	return nil, "", errPathEscapes
	}

	if runtime.GOOS == "windows" {
	// Windows cleans paths before opening them.
	s, err = rootCleanPath(s, prefix, suffix)
	if err != nil {
	return nil, "", err
	}
	prefix = nil
	suffix = nil
	}

	parts := slices.Clone(prefix)
	i, j := 0, 1
	for {
	if j < len(s) && !IsPathSeparator(s[j]) {
	// Keep looking for the end of this component.
	j++
	continue
	}
	parts = append(parts, s[i:j])
	// Advance to the next component, or end of the path.
	partEnd := j
	for j < len(s) && IsPathSeparator(s[j]) {
	j++
	}
	if j == len(s) {
	// If this is the last path component,
	// preserve any trailing path separators.
	suffixSep = s[partEnd:]
	break
	}
	if parts[len(parts)-1] == "." {
	// Remove "." components, except at the end.
	parts = parts[:len(parts)-1]
	}
	i = j
	}
	if len(suffix) > 0 && len(parts) > 0 && parts[len(parts)-1] == "." {
	// Remove a trailing "." component if we're joining to a suffix.
	parts = parts[:len(parts)-1]
	}
	parts = append(parts, suffix...)
	return parts, suffixSep, nil
	}

	// FS returns a file system (an fs.FS) for the tree of files in the root.
	//
	// The result implements [io/fs.StatFS], [io/fs.ReadFileFS],
	// [io/fs.ReadDirFS], and [io/fs.ReadLinkFS].
	func (r *Root) FS() fs.FS {
	return (*rootFS)(r)
	}

	type rootFS Root

	func (rfs *rootFS) Open(name string) (fs.File, error) {
	r := (*Root)(rfs)
	if !isValidRootFSPath(name) {
	return nil, &PathError{Op: "open", Path: name, Err: ErrInvalid}
	}
	f, err := r.Open(name)
	if err != nil {
	return nil, err
	}
	return f, nil
	}

	func (rfs *rootFS) ReadDir(name string) ([]DirEntry, error) {
	r := (*Root)(rfs)
	if !isValidRootFSPath(name) {
	return nil, &PathError{Op: "readdir", Path: name, Err: ErrInvalid}
	}

	// This isn't efficient: We just open a regular file and ReadDir it.
	// Ideally, we would skip creating a *File entirely and operate directly
	// on the file descriptor, but that will require some extensive reworking
	// of directory reading in general.
	//
	// This suffices for the moment.
	f, err := r.Open(name)
	if err != nil {
	return nil, err
	}
	defer f.Close()
	dirs, err := f.ReadDir(-1)
	slices.SortFunc(dirs, func(a, b DirEntry) int {
	return bytealg.CompareString(a.Name(), b.Name())
	})
	return dirs, err
	}

	func (rfs *rootFS) ReadFile(name string) ([]byte, error) {
	r := (*Root)(rfs)
	if !isValidRootFSPath(name) {
	return nil, &PathError{Op: "readfile", Path: name, Err: ErrInvalid}
	}
	f, err := r.Open(name)
	if err != nil {
	return nil, err
	}
	defer f.Close()
	return readFileContents(statOrZero(f), f.Read)
	}

	func (rfs *rootFS) ReadLink(name string) (string, error) {
	r := (*Root)(rfs)
	if !isValidRootFSPath(name) {
	return "", &PathError{Op: "readlink", Path: name, Err: ErrInvalid}
	}
	return r.Readlink(name)
	}

	func (rfs *rootFS) Stat(name string) (FileInfo, error) {
	r := (*Root)(rfs)
	if !isValidRootFSPath(name) {
	return nil, &PathError{Op: "stat", Path: name, Err: ErrInvalid}
	}
	return r.Stat(name)
	}

	func (rfs *rootFS) Lstat(name string) (FileInfo, error) {
	r := (*Root)(rfs)
	if !isValidRootFSPath(name) {
	return nil, &PathError{Op: "lstat", Path: name, Err: ErrInvalid}
	}
	return r.Lstat(name)
	}

	// isValidRootFSPath reports whether name is a valid filename to pass a Root.FS method.
	func isValidRootFSPath(name string) bool {
	if !fs.ValidPath(name) {
	return false
	}
	if runtime.GOOS == "windows" {
	// fs.FS paths are /-separated.
	// On Windows, reject the path if it contains any \ separators.
	// Other forms of invalid path (for example, "NUL") are handled by
	// Root's usual file lookup mechanisms.
	if stringslite.IndexByte(name, '\\') >= 0 {
	return false
	}
	}
	return true
	}