os: add Root.Chmod

For #67002

Change-Id: Id6c3a2096bd10f5f5f6921a0441dc6d9e6cdeb3b
Reviewed-on: https://go-review.googlesource.com/c/go/+/645718
Commit-Queue: Damien Neil <dneil@google.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Auto-Submit: Damien Neil <dneil@google.com>

api/next/67002.txt

@@ -0,0 +1 @@
+pkg os, method (*Root) Chmod(string, fs.FileMode) error #67002

doc/next/6-stdlib/99-minor/os/67002.md

@@ -0,0 +1,3 @@
+The [os.Root] type supports the following additional methods:
+
+  * [os.Root.Chmod]

src/internal/syscall/unix/asm_darwin.s

@@ -25,3 +25,4 @@ TEXT ·libc_sysconf_trampoline(SB),NOSPLIT,$0-0; JMP libc_sysconf(SB)
 TEXT ·libc_faccessat_trampoline(SB),NOSPLIT,$0-0; JMP libc_faccessat(SB)
 TEXT ·libc_readlinkat_trampoline(SB),NOSPLIT,$0-0; JMP libc_readlinkat(SB)
 TEXT ·libc_mkdirat_trampoline(SB),NOSPLIT,$0-0; JMP libc_mkdirat(SB)
+TEXT ·libc_fchmodat_trampoline(SB),NOSPLIT,$0-0; JMP libc_fchmodat(SB)

src/internal/syscall/unix/asm_openbsd.s

@@ -14,3 +14,5 @@ TEXT ·libc_readlinkat_trampoline(SB),NOSPLIT,$0-0
         JMP	libc_readlinkat(SB)
 TEXT ·libc_mkdirat_trampoline(SB),NOSPLIT,$0-0
         JMP	libc_mkdirat(SB)
+TEXT ·libc_fchmodat_trampoline(SB),NOSPLIT,$0-0
+        JMP	libc_fchmodat(SB)

src/internal/syscall/unix/at.go

@@ -79,3 +79,20 @@ func Mkdirat(dirfd int, path string, mode uint32) error {
 	}
 	return nil
 }
+
+func Fchmodat(dirfd int, path string, mode uint32, flags int) error {
+	p, err := syscall.BytePtrFromString(path)
+	if err != nil {
+		return err
+	}
+	_, _, errno := syscall.Syscall6(fchmodatTrap,
+		uintptr(dirfd),
+		uintptr(unsafe.Pointer(p)),
+		uintptr(mode),
+		uintptr(flags),
+		0, 0)
+	if errno != 0 {
+		return errno
+	}
+	return nil
+}

src/internal/syscall/unix/at_darwin.go

@@ -58,3 +58,25 @@ func Mkdirat(dirfd int, path string, mode uint32) error {
 	}
 	return nil
 }
+
+func libc_fchmodat_trampoline()
+
+//go:cgo_import_dynamic libc_fchmodat fchmodat "/usr/lib/libSystem.B.dylib"
+
+func Fchmodat(dirfd int, path string, mode uint32, flags int) error {
+	p, err := syscall.BytePtrFromString(path)
+	if err != nil {
+		return err
+	}
+	_, _, errno := syscall_syscall6(abi.FuncPCABI0(libc_fchmodat_trampoline),
+		uintptr(dirfd),
+		uintptr(unsafe.Pointer(p)),
+		uintptr(mode),
+		uintptr(flags),
+		0,
+		0)
+	if errno != 0 {
+		return errno
+	}
+	return nil
+}

src/internal/syscall/unix/at_libc.go

@@ -16,13 +16,15 @@ import (
 //go:linkname procUnlinkat libc_unlinkat
 //go:linkname procReadlinkat libc_readlinkat
 //go:linkname procMkdirat libc_mkdirat
+//go:linkname procFchmodat libc_fchmodat
 
 var (
 	procFstatat,
 	procOpenat,
 	procUnlinkat,
 	procReadlinkat,
-	procMkdirat uintptr
+	procMkdirat,
+	procFchmodat uintptr
 )
 
 func Unlinkat(dirfd int, path string, flags int) error {
@@ -107,3 +109,20 @@ func Mkdirat(dirfd int, path string, mode uint32) error {
 	}
 	return nil
 }
+
+func Fchmodat(dirfd int, path string, mode uint32, flags int) error {
+	p, err := syscall.BytePtrFromString(path)
+	if err != nil {
+		return err
+	}
+	_, _, errno := syscall6(uintptr(unsafe.Pointer(&procFchmodat)), 4,
+		uintptr(dirfd),
+		uintptr(unsafe.Pointer(p)),
+		uintptr(mode),
+		uintptr(flags),
+		0, 0)
+	if errno != 0 {
+		return errno
+	}
+	return nil
+}

src/internal/syscall/unix/at_openbsd.go

@@ -49,3 +49,25 @@ func Mkdirat(dirfd int, path string, mode uint32) error {
 	}
 	return nil
 }
+
+//go:cgo_import_dynamic libc_fchmodat fchmodat "libc.so"
+
+func libc_fchmodat_trampoline()
+
+func Fchmodat(dirfd int, path string, mode uint32, flags int) error {
+	p, err := syscall.BytePtrFromString(path)
+	if err != nil {
+		return err
+	}
+	_, _, errno := syscall_syscall6(abi.FuncPCABI0(libc_fchmodat_trampoline),
+		uintptr(dirfd),
+		uintptr(unsafe.Pointer(p)),
+		uintptr(mode),
+		uintptr(flags),
+		0,
+		0)
+	if errno != 0 {
+		return errno
+	}
+	return nil
+}

src/internal/syscall/unix/at_sysnum_dragonfly.go

@@ -12,6 +12,7 @@ const (
 	fstatatTrap    uintptr = syscall.SYS_FSTATAT
 	readlinkatTrap uintptr = syscall.SYS_READLINKAT
 	mkdiratTrap    uintptr = syscall.SYS_MKDIRAT
+	fchmodatTrap   uintptr = syscall.SYS_FCHMODAT
 
 	AT_EACCESS          = 0x4
 	AT_FDCWD            = 0xfffafdcd

src/internal/syscall/unix/at_sysnum_freebsd.go

@@ -19,4 +19,5 @@ const (
 	posixFallocateTrap uintptr = syscall.SYS_POSIX_FALLOCATE
 	readlinkatTrap     uintptr = syscall.SYS_READLINKAT
 	mkdiratTrap        uintptr = syscall.SYS_MKDIRAT
+	fchmodatTrap       uintptr = syscall.SYS_FCHMODAT
 )

src/internal/syscall/unix/at_sysnum_linux.go

@@ -11,6 +11,7 @@ const (
 	openatTrap     uintptr = syscall.SYS_OPENAT
 	readlinkatTrap uintptr = syscall.SYS_READLINKAT
 	mkdiratTrap    uintptr = syscall.SYS_MKDIRAT
+	fchmodatTrap   uintptr = syscall.SYS_FCHMODAT
 )
 
 const (

src/internal/syscall/unix/at_sysnum_netbsd.go

@@ -12,6 +12,7 @@ const (
 	fstatatTrap    uintptr = syscall.SYS_FSTATAT
 	readlinkatTrap uintptr = syscall.SYS_READLINKAT
 	mkdiratTrap    uintptr = syscall.SYS_MKDIRAT
+	fchmodatTrap   uintptr = syscall.SYS_FCHMODAT
 )
 
 const (

src/internal/syscall/unix/at_wasip1.go

@@ -101,6 +101,11 @@ func Mkdirat(dirfd int, path string, mode uint32) error {
 	))
 }
 
+func Fchmodat(dirfd int, path string, mode uint32, flags int) error {
+	// WASI preview 1 doesn't support changing file modes.
+	return syscall.ENOSYS
+}
+
 //go:wasmimport wasi_snapshot_preview1 path_create_directory
 //go:noescape
 func path_create_directory(fd int32, path *byte, pathLen size) syscall.Errno

src/internal/syscall/windows/at_windows.go

@@ -20,9 +20,10 @@ const (
 	O_DIRECTORY    = 0x100000   // target must be a directory
 	O_NOFOLLOW_ANY = 0x20000000 // disallow symlinks anywhere in the path
 	O_OPEN_REPARSE = 0x40000000 // FILE_OPEN_REPARSE_POINT, used by Lstat
+	O_WRITE_ATTRS  = 0x80000000 // FILE_WRITE_ATTRIBUTES, used by Chmod
 )
 
-func Openat(dirfd syscall.Handle, name string, flag int, perm uint32) (_ syscall.Handle, e1 error) {
+func Openat(dirfd syscall.Handle, name string, flag uint64, perm uint32) (_ syscall.Handle, e1 error) {
 	if len(name) == 0 {
 		return syscall.InvalidHandle, syscall.ERROR_FILE_NOT_FOUND
 	}
@@ -61,6 +62,9 @@ func Openat(dirfd syscall.Handle, name string, flag int, perm uint32) (_ syscall
 	if flag&syscall.O_SYNC != 0 {
 		options |= FILE_WRITE_THROUGH
 	}
+	if flag&O_WRITE_ATTRS != 0 {
+		access |= FILE_WRITE_ATTRIBUTES
+	}
 	// Allow File.Stat.
 	access |= STANDARD_RIGHTS_READ | FILE_READ_ATTRIBUTES | FILE_READ_EA
 
@@ -129,7 +133,7 @@ func Openat(dirfd syscall.Handle, name string, flag int, perm uint32) (_ syscall
 }
 
 // ntCreateFileError maps error returns from NTCreateFile to user-visible errors.
-func ntCreateFileError(err error, flag int) error {
+func ntCreateFileError(err error, flag uint64) error {
 	s, ok := err.(NTStatus)
 	if !ok {
 		// Shouldn't really be possible, NtCreateFile always returns NTStatus.

src/internal/syscall/windows/at_windows_test.go

@@ -46,7 +46,7 @@ func TestOpen(t *testing.T) {
 			continue
 		}
 		base := filepath.Base(tt.path)
-		h, err := windows.Openat(dirfd, base, tt.flag, 0o660)
+		h, err := windows.Openat(dirfd, base, uint64(tt.flag), 0o660)
 		syscall.CloseHandle(dirfd)
 		if err == nil {
 			syscall.CloseHandle(h)

src/os/root.go

@@ -54,11 +54,16 @@ func OpenInRoot(dir, name string) (*File, error) {
 //
 //   - When GOOS=windows, file names may not reference Windows reserved device names
 //     such as NUL and COM1.
+//   - On Unix, [Root.Chmod] and [Root.Chown] 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 peformed 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
 }
@@ -127,6 +132,12 @@ func (r *Root) OpenRoot(name string) (*Root, error) {
 	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.

src/os/root_noopenat.go

@@ -95,6 +95,16 @@ func rootStat(r *Root, name string, lstat bool) (FileInfo, error) {
 	return fi, nil
 }
 
+func rootChmod(r *Root, name string, mode FileMode) error {
+	if err := checkPathEscapes(r, name); err != nil {
+		return &PathError{Op: "chmodat", Path: name, Err: err}
+	}
+	if err := Chmod(joinPath(r.root.name, name), mode); err != nil {
+		return &PathError{Op: "chmodat", Path: name, Err: underlyingError(err)}
+	}
+	return nil
+}
+
 func rootMkdir(r *Root, name string, perm FileMode) error {
 	if err := checkPathEscapes(r, name); err != nil {
 		return &PathError{Op: "mkdirat", Path: name, Err: err}

src/os/root_openat.go

@@ -64,6 +64,16 @@ func (r *root) Name() string {
 	return r.name
 }
 
+func rootChmod(r *Root, name string, mode FileMode) error {
+	_, err := doInRoot(r, name, func(parent sysfdType, name string) (struct{}, error) {
+		return struct{}{}, chmodat(parent, name, mode)
+	})
+	if err != nil {
+		return &PathError{Op: "chmodat", Path: name, Err: err}
+	}
+	return err
+}
+
 func rootMkdir(r *Root, name string, perm FileMode) error {
 	_, err := doInRoot(r, name, func(parent sysfdType, name string) (struct{}, error) {
 		return struct{}{}, mkdirat(parent, name, perm)

src/os/root_test.go

@@ -389,6 +389,43 @@ func TestRootCreate(t *testing.T) {
 	}
 }
 
+func TestRootChmod(t *testing.T) {
+	if runtime.GOOS == "wasip1" {
+		t.Skip("Chmod not supported on " + runtime.GOOS)
+	}
+	for _, test := range rootTestCases {
+		test.run(t, func(t *testing.T, target string, root *os.Root) {
+			if target != "" {
+				// Create a file with no read/write permissions,
+				// to ensure we can use Chmod on an inaccessible file.
+				if err := os.WriteFile(target, nil, 0o000); err != nil {
+					t.Fatal(err)
+				}
+			}
+			if runtime.GOOS == "windows" {
+				// On Windows, Chmod("symlink") affects the link, not its target.
+				// See issue 71492.
+				fi, err := root.Lstat(test.open)
+				if err == nil && !fi.Mode().IsRegular() {
+					t.Skip("https://go.dev/issue/71492")
+				}
+			}
+			want := os.FileMode(0o666)
+			err := root.Chmod(test.open, want)
+			if errEndsTest(t, err, test.wantError, "root.Chmod(%q)", test.open) {
+				return
+			}
+			st, err := os.Stat(target)
+			if err != nil {
+				t.Fatalf("os.Stat(%q) = %v", target, err)
+			}
+			if got := st.Mode(); got != want {
+				t.Errorf("after root.Chmod(%q, %v): file mode = %v, want %v", test.open, want, got, want)
+			}
+		})
+	}
+}
+
 func TestRootMkdir(t *testing.T) {
 	for _, test := range rootTestCases {
 		test.run(t, func(t *testing.T, target string, root *os.Root) {
@@ -877,6 +914,35 @@ func TestRootConsistencyCreate(t *testing.T) {
 	}
 }
 
+func TestRootConsistencyChmod(t *testing.T) {
+	if runtime.GOOS == "wasip1" {
+		t.Skip("Chmod not supported on " + runtime.GOOS)
+	}
+	for _, test := range rootConsistencyTestCases {
+		test.run(t, func(t *testing.T, path string, r *os.Root) (string, error) {
+			chmod := os.Chmod
+			lstat := os.Lstat
+			if r != nil {
+				chmod = r.Chmod
+				lstat = r.Lstat
+			}
+
+			var m1, m2 os.FileMode
+			err := chmod(path, 0o555)
+			fi, err := lstat(path)
+			if err == nil {
+				m1 = fi.Mode()
+			}
+			err = chmod(path, 0o777)
+			fi, err = lstat(path)
+			if err == nil {
+				m2 = fi.Mode()
+			}
+			return fmt.Sprintf("%v %v", m1, m2), err
+		})
+	}
+}
+
 func TestRootConsistencyMkdir(t *testing.T) {
 	for _, test := range rootConsistencyTestCases {
 		test.run(t, func(t *testing.T, path string, r *os.Root) (string, error) {

src/os/root_unix.go

@@ -131,6 +131,32 @@ func rootStat(r *Root, name string, lstat bool) (FileInfo, error) {
 	return fi, nil
 }
 
+// On systems which use fchmodat, fchownat, etc., we have a race condition:
+// When "name" is a symlink, Root.Chmod("name") should act on the target of that link.
+// However, fchmodat doesn't allow us to chmod a file only if it is not a symlink;
+// the AT_SYMLINK_NOFOLLOW parameter causes the operation to act on the symlink itself.
+//
+// We do the best we can by first checking to see if the target of the operation is a symlink,
+// and only attempting the fchmodat if it is not. If the target is replaced between the check
+// and the fchmodat, we will chmod the symlink rather than following it.
+//
+// This race condition is unfortunate, but does not permit escaping a root:
+// We may act on the wrong file, but that file will be contained within the root.
+func afterResolvingSymlink(parent int, name string, f func() error) error {
+	if err := checkSymlink(parent, name, nil); err != nil {
+		return err
+	}
+	return f()
+}
+
+func chmodat(parent int, name string, mode FileMode) error {
+	return afterResolvingSymlink(parent, name, func() error {
+		return ignoringEINTR(func() error {
+			return unix.Fchmodat(parent, name, syscallMode(mode), unix.AT_SYMLINK_NOFOLLOW)
+		})
+	})
+}
+
 func mkdirat(fd int, name string, perm FileMode) error {
 	return ignoringEINTR(func() error {
 		return unix.Mkdirat(fd, name, syscallMode(perm))