unix: check Fchmodat flags parameter on Linux

Currently Linux' fchmodat(2) syscall implementation doesn't support the
flags parameter (though it might in future versions [1]). Fchmodat in
x/sys/unix takes the parameter and (wrongly) passes it on to the syscall
which will ignore it.

According to the POSIX.1-2008 manual page [2], AT_SYMLINK_NOFOLLOW is
the only valid value for the flags parameter and EOPNOTSUPP should be
returned in case changing the mode of a symbolic link is not supported
by the underlying system.  EINVAL should be returned for any other value
of the flags parameter.

  [1] https://patchwork.kernel.org/patch/9596301/
  [2] http://pubs.opengroup.org/onlinepubs/9699919799/functions/chmod.html

Change the Fchmodat implementation accordingly and also add the
corresponding test.

Fixes golang/go#20130

Change-Id: I62e677e6674d3702eaf388af0ac3d7e623a35c24
Reviewed-on: https://go-review.googlesource.com/46474
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

Author: tklauser

unix/syscall_linux.go

@@ -36,6 +36,20 @@ func Creat(path string, mode uint32) (fd int, err error) {
 	return Open(path, O_CREAT|O_WRONLY|O_TRUNC, mode)
 }
 
+//sys	fchmodat(dirfd int, path string, mode uint32) (err error)
+
+func Fchmodat(dirfd int, path string, mode uint32, flags int) (err error) {
+	// Linux fchmodat doesn't support the flags parameter. Mimick glibc's behavior
+	// and check the flags. Otherwise the mode would be applied to the symlink
+	// destination which is not what the user expects.
+	if flags&^AT_SYMLINK_NOFOLLOW != 0 {
+		return EINVAL
+	} else if flags&AT_SYMLINK_NOFOLLOW != 0 {
+		return EOPNOTSUPP
+	}
+	return fchmodat(dirfd, path, mode)
+}
+
 //sys	ioctl(fd int, req uint, arg uintptr) (err error)
 
 // ioctl itself should not be exposed directly, but additional get/set
@@ -1177,7 +1191,6 @@ func Mount(source string, target string, fstype string, flags uintptr, data stri
 //sys	Fallocate(fd int, mode uint32, off int64, len int64) (err error)
 //sys	Fchdir(fd int) (err error)
 //sys	Fchmod(fd int, mode uint32) (err error)
-//sys	Fchmodat(dirfd int, path string, mode uint32, flags int) (err error)
 //sys	Fchownat(dirfd int, path string, uid int, gid int, flags int) (err error)
 //sys	fcntl(fd int, cmd int, arg int) (val int, err error)
 //sys	Fdatasync(fd int) (err error)

unix/syscall_linux_test.go

@@ -15,6 +15,32 @@ import (
 	"golang.org/x/sys/unix"
 )
 
+func TestFchmodat(t *testing.T) {
+	defer chtmpdir(t)()
+
+	touch(t, "file1")
+	os.Symlink("file1", "symlink1")
+
+	err := unix.Fchmodat(unix.AT_FDCWD, "symlink1", 0444, 0)
+	if err != nil {
+		t.Fatalf("Fchmodat: unexpected error: %v", err)
+	}
+
+	fi, err := os.Stat("file1")
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	if fi.Mode() != 0444 {
+		t.Errorf("Fchmodat: failed to change mode: expected %v, got %v", 0444, fi.Mode())
+	}
+
+	err = unix.Fchmodat(unix.AT_FDCWD, "symlink1", 0444, unix.AT_SYMLINK_NOFOLLOW)
+	if err != unix.EOPNOTSUPP {
+		t.Fatalf("Fchmodat: unexpected error: %v, expected EOPNOTSUPP", err)
+	}
+}
+
 func TestIoctlGetInt(t *testing.T) {
 	f, err := os.Open("/dev/random")
 	if err != nil {