syscall: support Faccessat flags argument

The Linux kernel faccessat system call does not take a flags parameter.
The flag parameter to the C library faccessat function is implemented in C.
The syscall.Faccessat function takes a flags parameter. In older releases
we have passed the flags parameter to the kernel, which ignored it.
In CL 120015 we started returning an error if any flags were set.
That seems clearly better than ignoring them, but it turns out that some
code was using the flags. The code was previously subtly broken.
Now it is obviously broken. That is better, but we can do better still:
we can implement the flags as the C library does. That is what this CL does.

Change-Id: I259bd6f240c3951e939b81c3032dead3d9c567b4
Reviewed-on: https://go-review.googlesource.com/126415
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/syscall/syscall_linux.go

@@ -40,10 +40,70 @@ func Creat(path string, mode uint32) (fd int, err error) {
 func Faccessat(dirfd int, path string, mode uint32, flags int) (err error) {
 	if flags & ^(_AT_SYMLINK_NOFOLLOW|_AT_EACCESS) != 0 {
 		return EINVAL
-	} else if flags&(_AT_SYMLINK_NOFOLLOW|_AT_EACCESS) != 0 {
-		return EOPNOTSUPP
 	}
-	return faccessat(dirfd, path, mode)
+
+	// The Linux kernel faccessat system call does not take any flags.
+	// The glibc faccessat implements the flags itself; see
+	// https://sourceware.org/git/?p=glibc.git;a=blob;f=sysdeps/unix/sysv/linux/faccessat.c;hb=HEAD
+	// Because people naturally expect syscall.Faccessat to act
+	// like C faccessat, we do the same.
+
+	if flags == 0 {
+		return faccessat(dirfd, path, mode)
+	}
+
+	var st Stat_t
+	if err := fstatat(dirfd, path, &st, flags&_AT_SYMLINK_NOFOLLOW); err != nil {
+		return err
+	}
+
+	mode &= 7
+	if mode == 0 {
+		return nil
+	}
+
+	var uid int
+	if flags&_AT_EACCESS != 0 {
+		uid = Geteuid()
+	} else {
+		uid = Getuid()
+	}
+
+	if uid == 0 {
+		if mode&1 == 0 {
+			// Root can read and write any file.
+			return nil
+		}
+		if st.Mode&0111 != 0 {
+			// Root can execute any file that anybody can execute.
+			return nil
+		}
+		return EACCES
+	}
+
+	var fmode uint32
+	if uint32(uid) == st.Uid {
+		fmode = (st.Mode >> 6) & 7
+	} else {
+		var gid int
+		if flags&_AT_EACCESS != 0 {
+			gid = Getegid()
+		} else {
+			gid = Getgid()
+		}
+
+		if uint32(gid) == st.Gid {
+			fmode = (st.Mode >> 3) & 7
+		} else {
+			fmode = st.Mode & 7
+		}
+	}
+
+	if fmode&mode == mode {
+		return nil
+	}
+
+	return EACCES
 }
 
 //sys	fchmodat(dirfd int, path string, mode uint32) (err error)

src/syscall/syscall_linux_386.go

@@ -53,6 +53,7 @@ func Pipe2(p []int, flags int) (err error) {
 //sys	Dup2(oldfd int, newfd int) (err error)
 //sys	Fchown(fd int, uid int, gid int) (err error) = SYS_FCHOWN32
 //sys	Fstat(fd int, stat *Stat_t) (err error) = SYS_FSTAT64
+//sys	fstatat(dirfd int, path string, stat *Stat_t, flags int) (err error) = SYS_FSTATAT64
 //sys	Ftruncate(fd int, length int64) (err error) = SYS_FTRUNCATE64
 //sysnb	Getegid() (egid int) = SYS_GETEGID32
 //sysnb	Geteuid() (euid int) = SYS_GETEUID32

src/syscall/syscall_linux_arm.go

@@ -77,6 +77,7 @@ func Seek(fd int, offset int64, whence int) (newoffset int64, err error) {
 //sys	Dup2(oldfd int, newfd int) (err error)
 //sys	Fchown(fd int, uid int, gid int) (err error) = SYS_FCHOWN32
 //sys	Fstat(fd int, stat *Stat_t) (err error) = SYS_FSTAT64
+//sys	fstatat(dirfd int, path string, stat *Stat_t, flags int) (err error) = SYS_FSTATAT64
 //sysnb	Getegid() (egid int) = SYS_GETEGID32
 //sysnb	Geteuid() (euid int) = SYS_GETEUID32
 //sysnb	Getgid() (gid int) = SYS_GETGID32

src/syscall/syscall_linux_arm64.go

@@ -13,6 +13,7 @@ const (
 //sys	Fchown(fd int, uid int, gid int) (err error)
 //sys	Fstat(fd int, stat *Stat_t) (err error)
 //sys	Fstatat(fd int, path string, stat *Stat_t, flags int) (err error)
+//sys	fstatat(dirfd int, path string, stat *Stat_t, flags int) (err error)
 //sys	Fstatfs(fd int, buf *Statfs_t) (err error)
 //sys	Ftruncate(fd int, length int64) (err error)
 //sysnb	Getegid() (egid int)

src/syscall/syscall_linux_mips64x.go

@@ -15,6 +15,7 @@ const (
 //sys	Dup2(oldfd int, newfd int) (err error)
 //sys	Fchown(fd int, uid int, gid int) (err error)
 //sys	Fstatfs(fd int, buf *Statfs_t) (err error)
+//sys	fstatat(dirfd int, path string, stat *Stat_t, flags int) (err error) = SYS_NEWFSTATAT
 //sys	Ftruncate(fd int, length int64) (err error)
 //sysnb	Getegid() (egid int)
 //sysnb	Geteuid() (euid int)

src/syscall/syscall_linux_mipsx.go

@@ -18,6 +18,7 @@ func Syscall9(trap, a1, a2, a3, a4, a5, a6, a7, a8, a9 uintptr) (r1, r2 uintptr,
 
 //sys	Dup2(oldfd int, newfd int) (err error)
 //sys	Fchown(fd int, uid int, gid int) (err error)
+//sys	fstatat(dirfd int, path string, stat *Stat_t, flags int) (err error) = SYS_FSTATAT64
 //sys	Ftruncate(fd int, length int64) (err error) = SYS_FTRUNCATE64
 //sysnb	Getegid() (egid int)
 //sysnb	Geteuid() (euid int)

src/syscall/syscall_linux_ppc64x.go

@@ -15,6 +15,7 @@ const (
 //sys	Dup2(oldfd int, newfd int) (err error)
 //sys	Fchown(fd int, uid int, gid int) (err error)
 //sys	Fstat(fd int, stat *Stat_t) (err error)
+//sys	fstatat(dirfd int, path string, stat *Stat_t, flags int) (err error) = SYS_NEWFSTATAT
 //sys	Fstatfs(fd int, buf *Statfs_t) (err error)
 //sys	Ftruncate(fd int, length int64) (err error)
 //sysnb	Getegid() (egid int)

src/syscall/syscall_linux_s390x.go

@@ -14,6 +14,7 @@ const (
 //sys	Dup2(oldfd int, newfd int) (err error)
 //sys	Fchown(fd int, uid int, gid int) (err error)
 //sys	Fstat(fd int, stat *Stat_t) (err error)
+//sys	fstatat(dirfd int, path string, stat *Stat_t, flags int) (err error) = SYS_NEWFSTATAT
 //sys	Fstatfs(fd int, buf *Statfs_t) (err error)
 //sys	Ftruncate(fd int, length int64) (err error)
 //sysnb	Getegid() (egid int)

src/syscall/syscall_linux_test.go

@@ -57,25 +57,27 @@ const (
 	_AT_SYMLINK_NOFOLLOW = 0x100
 	_AT_FDCWD            = -0x64
 	_AT_EACCESS          = 0x200
+	_F_OK                = 0
+	_R_OK                = 4
 )
 
 func TestFaccessat(t *testing.T) {
 	defer chtmpdir(t)()
 	touch(t, "file1")
 
-	err := syscall.Faccessat(_AT_FDCWD, "file1", syscall.O_RDONLY, 0)
+	err := syscall.Faccessat(_AT_FDCWD, "file1", _R_OK, 0)
 	if err != nil {
 		t.Errorf("Faccessat: unexpected error: %v", err)
 	}
 
-	err = syscall.Faccessat(_AT_FDCWD, "file1", syscall.O_RDONLY, 2)
+	err = syscall.Faccessat(_AT_FDCWD, "file1", _R_OK, 2)
 	if err != syscall.EINVAL {
 		t.Errorf("Faccessat: unexpected error: %v, want EINVAL", err)
 	}
 
-	err = syscall.Faccessat(_AT_FDCWD, "file1", syscall.O_RDONLY, _AT_EACCESS)
-	if err != syscall.EOPNOTSUPP {
-		t.Errorf("Faccessat: unexpected error: %v, want EOPNOTSUPP", err)
+	err = syscall.Faccessat(_AT_FDCWD, "file1", _R_OK, _AT_EACCESS)
+	if err != nil {
+		t.Errorf("Faccessat: unexpected error: %v", err)
 	}
 
 	err = os.Symlink("file1", "symlink1")
@@ -83,9 +85,31 @@ func TestFaccessat(t *testing.T) {
 		t.Fatal(err)
 	}
 
-	err = syscall.Faccessat(_AT_FDCWD, "symlink1", syscall.O_RDONLY, _AT_SYMLINK_NOFOLLOW)
-	if err != syscall.EOPNOTSUPP {
-		t.Errorf("Faccessat: unexpected error: %v, want EOPNOTSUPP", err)
+	err = syscall.Faccessat(_AT_FDCWD, "symlink1", _R_OK, _AT_SYMLINK_NOFOLLOW)
+	if err != nil {
+		t.Errorf("Faccessat SYMLINK_NOFOLLOW: unexpected error %v", err)
+	}
+
+	// We can't really test _AT_SYMLINK_NOFOLLOW, because there
+	// doesn't seem to be any way to change the mode of a symlink.
+	// We don't test _AT_EACCESS because such tests are only
+	// meaningful if run as root.
+
+	err = syscall.Fchmodat(_AT_FDCWD, "file1", 0, 0)
+	if err != nil {
+		t.Errorf("Fchmodat: unexpected error %v", err)
+	}
+
+	err = syscall.Faccessat(_AT_FDCWD, "file1", _F_OK, _AT_SYMLINK_NOFOLLOW)
+	if err != nil {
+		t.Errorf("Faccessat: unexpected error: %v", err)
+	}
+
+	err = syscall.Faccessat(_AT_FDCWD, "file1", _R_OK, _AT_SYMLINK_NOFOLLOW)
+	if err != syscall.EACCES {
+		if syscall.Getuid() != 0 {
+			t.Errorf("Faccessat: unexpected error: %v, want EACCES", err)
+		}
 	}
 }
 

src/syscall/zsyscall_linux_386.go

@@ -1188,6 +1188,21 @@ func Fstat(fd int, stat *Stat_t) (err error) {
 
 // THIS FILE IS GENERATED BY THE COMMAND AT THE TOP; DO NOT EDIT
 
+func fstatat(dirfd int, path string, stat *Stat_t, flags int) (err error) {
+	var _p0 *byte
+	_p0, err = BytePtrFromString(path)
+	if err != nil {
+		return
+	}
+	_, _, e1 := Syscall6(SYS_FSTATAT64, uintptr(dirfd), uintptr(unsafe.Pointer(_p0)), uintptr(unsafe.Pointer(stat)), uintptr(flags), 0, 0)
+	if e1 != 0 {
+		err = errnoErr(e1)
+	}
+	return
+}
+
+// THIS FILE IS GENERATED BY THE COMMAND AT THE TOP; DO NOT EDIT
+
 func Ftruncate(fd int, length int64) (err error) {
 	_, _, e1 := Syscall(SYS_FTRUNCATE64, uintptr(fd), uintptr(length), uintptr(length>>32))
 	if e1 != 0 {

src/syscall/zsyscall_linux_arm.go

@@ -1357,6 +1357,21 @@ func Fstat(fd int, stat *Stat_t) (err error) {
 
 // THIS FILE IS GENERATED BY THE COMMAND AT THE TOP; DO NOT EDIT
 
+func fstatat(dirfd int, path string, stat *Stat_t, flags int) (err error) {
+	var _p0 *byte
+	_p0, err = BytePtrFromString(path)
+	if err != nil {
+		return
+	}
+	_, _, e1 := Syscall6(SYS_FSTATAT64, uintptr(dirfd), uintptr(unsafe.Pointer(_p0)), uintptr(unsafe.Pointer(stat)), uintptr(flags), 0, 0)
+	if e1 != 0 {
+		err = errnoErr(e1)
+	}
+	return
+}
+
+// THIS FILE IS GENERATED BY THE COMMAND AT THE TOP; DO NOT EDIT
+
 func Getegid() (egid int) {
 	r0, _ := rawSyscallNoError(SYS_GETEGID32, 0, 0, 0)
 	egid = int(r0)

src/syscall/zsyscall_linux_arm64.go

@@ -1190,6 +1190,21 @@ func Fstatat(fd int, path string, stat *Stat_t, flags int) (err error) {
 
 // THIS FILE IS GENERATED BY THE COMMAND AT THE TOP; DO NOT EDIT
 
+func fstatat(dirfd int, path string, stat *Stat_t, flags int) (err error) {
+	var _p0 *byte
+	_p0, err = BytePtrFromString(path)
+	if err != nil {
+		return
+	}
+	_, _, e1 := Syscall6(SYS_FSTATAT, uintptr(dirfd), uintptr(unsafe.Pointer(_p0)), uintptr(unsafe.Pointer(stat)), uintptr(flags), 0, 0)
+	if e1 != 0 {
+		err = errnoErr(e1)
+	}
+	return
+}
+
+// THIS FILE IS GENERATED BY THE COMMAND AT THE TOP; DO NOT EDIT
+
 func Fstatfs(fd int, buf *Statfs_t) (err error) {
 	_, _, e1 := Syscall(SYS_FSTATFS, uintptr(fd), uintptr(unsafe.Pointer(buf)), 0)
 	if e1 != 0 {

src/syscall/zsyscall_linux_mips.go

@@ -1158,6 +1158,21 @@ func Fchown(fd int, uid int, gid int) (err error) {
 
 // THIS FILE IS GENERATED BY THE COMMAND AT THE TOP; DO NOT EDIT
 
+func fstatat(dirfd int, path string, stat *Stat_t, flags int) (err error) {
+	var _p0 *byte
+	_p0, err = BytePtrFromString(path)
+	if err != nil {
+		return
+	}
+	_, _, e1 := Syscall6(SYS_FSTATAT64, uintptr(dirfd), uintptr(unsafe.Pointer(_p0)), uintptr(unsafe.Pointer(stat)), uintptr(flags), 0, 0)
+	if e1 != 0 {
+		err = errnoErr(e1)
+	}
+	return
+}
+
+// THIS FILE IS GENERATED BY THE COMMAND AT THE TOP; DO NOT EDIT
+
 func Ftruncate(fd int, length int64) (err error) {
 	_, _, e1 := Syscall6(SYS_FTRUNCATE64, uintptr(fd), 0, uintptr(length>>32), uintptr(length), 0, 0)
 	if e1 != 0 {

src/syscall/zsyscall_linux_mips64.go

@@ -1168,6 +1168,21 @@ func Fstatfs(fd int, buf *Statfs_t) (err error) {
 
 // THIS FILE IS GENERATED BY THE COMMAND AT THE TOP; DO NOT EDIT
 
+func fstatat(dirfd int, path string, stat *Stat_t, flags int) (err error) {
+	var _p0 *byte
+	_p0, err = BytePtrFromString(path)
+	if err != nil {
+		return
+	}
+	_, _, e1 := Syscall6(SYS_NEWFSTATAT, uintptr(dirfd), uintptr(unsafe.Pointer(_p0)), uintptr(unsafe.Pointer(stat)), uintptr(flags), 0, 0)
+	if e1 != 0 {
+		err = errnoErr(e1)
+	}
+	return
+}
+
+// THIS FILE IS GENERATED BY THE COMMAND AT THE TOP; DO NOT EDIT
+
 func Ftruncate(fd int, length int64) (err error) {
 	_, _, e1 := Syscall(SYS_FTRUNCATE, uintptr(fd), uintptr(length), 0)
 	if e1 != 0 {

src/syscall/zsyscall_linux_mips64le.go

@@ -1168,6 +1168,21 @@ func Fstatfs(fd int, buf *Statfs_t) (err error) {
 
 // THIS FILE IS GENERATED BY THE COMMAND AT THE TOP; DO NOT EDIT
 
+func fstatat(dirfd int, path string, stat *Stat_t, flags int) (err error) {
+	var _p0 *byte
+	_p0, err = BytePtrFromString(path)
+	if err != nil {
+		return
+	}
+	_, _, e1 := Syscall6(SYS_NEWFSTATAT, uintptr(dirfd), uintptr(unsafe.Pointer(_p0)), uintptr(unsafe.Pointer(stat)), uintptr(flags), 0, 0)
+	if e1 != 0 {
+		err = errnoErr(e1)
+	}
+	return
+}
+
+// THIS FILE IS GENERATED BY THE COMMAND AT THE TOP; DO NOT EDIT
+
 func Ftruncate(fd int, length int64) (err error) {
 	_, _, e1 := Syscall(SYS_FTRUNCATE, uintptr(fd), uintptr(length), 0)
 	if e1 != 0 {

src/syscall/zsyscall_linux_mipsle.go

@@ -1158,6 +1158,21 @@ func Fchown(fd int, uid int, gid int) (err error) {
 
 // THIS FILE IS GENERATED BY THE COMMAND AT THE TOP; DO NOT EDIT
 
+func fstatat(dirfd int, path string, stat *Stat_t, flags int) (err error) {
+	var _p0 *byte
+	_p0, err = BytePtrFromString(path)
+	if err != nil {
+		return
+	}
+	_, _, e1 := Syscall6(SYS_FSTATAT64, uintptr(dirfd), uintptr(unsafe.Pointer(_p0)), uintptr(unsafe.Pointer(stat)), uintptr(flags), 0, 0)
+	if e1 != 0 {
+		err = errnoErr(e1)
+	}
+	return
+}
+
+// THIS FILE IS GENERATED BY THE COMMAND AT THE TOP; DO NOT EDIT
+
 func Ftruncate(fd int, length int64) (err error) {
 	_, _, e1 := Syscall6(SYS_FTRUNCATE64, uintptr(fd), 0, uintptr(length), uintptr(length>>32), 0, 0)
 	if e1 != 0 {

src/syscall/zsyscall_linux_ppc64.go

@@ -1168,6 +1168,21 @@ func Fstat(fd int, stat *Stat_t) (err error) {
 
 // THIS FILE IS GENERATED BY THE COMMAND AT THE TOP; DO NOT EDIT
 
+func fstatat(dirfd int, path string, stat *Stat_t, flags int) (err error) {
+	var _p0 *byte
+	_p0, err = BytePtrFromString(path)
+	if err != nil {
+		return
+	}
+	_, _, e1 := Syscall6(SYS_NEWFSTATAT, uintptr(dirfd), uintptr(unsafe.Pointer(_p0)), uintptr(unsafe.Pointer(stat)), uintptr(flags), 0, 0)
+	if e1 != 0 {
+		err = errnoErr(e1)
+	}
+	return
+}
+
+// THIS FILE IS GENERATED BY THE COMMAND AT THE TOP; DO NOT EDIT
+
 func Fstatfs(fd int, buf *Statfs_t) (err error) {
 	_, _, e1 := Syscall(SYS_FSTATFS, uintptr(fd), uintptr(unsafe.Pointer(buf)), 0)
 	if e1 != 0 {

src/syscall/zsyscall_linux_ppc64le.go

@@ -1168,6 +1168,21 @@ func Fstat(fd int, stat *Stat_t) (err error) {
 
 // THIS FILE IS GENERATED BY THE COMMAND AT THE TOP; DO NOT EDIT
 
+func fstatat(dirfd int, path string, stat *Stat_t, flags int) (err error) {
+	var _p0 *byte
+	_p0, err = BytePtrFromString(path)
+	if err != nil {
+		return
+	}
+	_, _, e1 := Syscall6(SYS_NEWFSTATAT, uintptr(dirfd), uintptr(unsafe.Pointer(_p0)), uintptr(unsafe.Pointer(stat)), uintptr(flags), 0, 0)
+	if e1 != 0 {
+		err = errnoErr(e1)
+	}
+	return
+}
+
+// THIS FILE IS GENERATED BY THE COMMAND AT THE TOP; DO NOT EDIT
+
 func Fstatfs(fd int, buf *Statfs_t) (err error) {
 	_, _, e1 := Syscall(SYS_FSTATFS, uintptr(fd), uintptr(unsafe.Pointer(buf)), 0)
 	if e1 != 0 {

src/syscall/zsyscall_linux_s390x.go

@@ -1168,6 +1168,21 @@ func Fstat(fd int, stat *Stat_t) (err error) {
 
 // THIS FILE IS GENERATED BY THE COMMAND AT THE TOP; DO NOT EDIT
 
+func fstatat(dirfd int, path string, stat *Stat_t, flags int) (err error) {
+	var _p0 *byte
+	_p0, err = BytePtrFromString(path)
+	if err != nil {
+		return
+	}
+	_, _, e1 := Syscall6(SYS_NEWFSTATAT, uintptr(dirfd), uintptr(unsafe.Pointer(_p0)), uintptr(unsafe.Pointer(stat)), uintptr(flags), 0, 0)
+	if e1 != 0 {
+		err = errnoErr(e1)
+	}
+	return
+}
+
+// THIS FILE IS GENERATED BY THE COMMAND AT THE TOP; DO NOT EDIT
+
 func Fstatfs(fd int, buf *Statfs_t) (err error) {
 	_, _, e1 := Syscall(SYS_FSTATFS, uintptr(fd), uintptr(unsafe.Pointer(buf)), 0)
 	if e1 != 0 {