Lchmod(): fix for Linux/Go 1.11

kolyshkin · kolyshkin · commit 94af8008a7b6 · 2018-07-11T17:54:58.000-07:00
Linux does not support file mode for symlinks. It does not support flags argument for fchmodat() syscall either (the only flag described by POSIX is AT_SYMLINK_NOFOLLOW to allow changing mode on symlink). This is probably the reason why the standard os package from Go does not provide Lchmod(). Go < 1.11 errorneously assumed the flags argument is supported on Linux, and implemented syscall.Fchmodat() with flags being passed on to the kernel. The kernel ignored the flags (as the kernel syscall doesn't even have flags argument). The result was changing the mode on the file that symlink points to (i.e. ignoring AT_SYMLINK_NOFOLLOW). The above bug was fixed in Go-1.11beta1 (see [1]), and since the fix, if any flags are set, an error is returned. This rendered Lchmod() useless on Linux + Go 1.11, as now it always returns an error. The best possible fix I can think of, given the constraints of keeping the API intact, is implemented in this commit. In short, chown() is not called for symlinks on Linux. [1] https://go-review.googlesource.com/c/go/+/118658 Signed-off-by: Kir Kolyshkin <kolyshkin@gmail.com>
diff --git a/driver/driver_unix.go b/driver/driver_unix.go
@@ -10,7 +10,6 @@ import (
 
 	"github.com/containerd/continuity/devices"
 	"github.com/containerd/continuity/sysx"
-	"golang.org/x/sys/unix"
 )
 
 func (d *driver) Mknod(path string, mode os.FileMode, major, minor int) error {
@@ -26,11 +25,6 @@ func (d *driver) Mkfifo(path string, mode os.FileMode) error {
 	return devices.Mknod(path, mode, 0, 0)
 }
 
-// Lchmod changes the mode of a file not following symlinks.
-func (d *driver) Lchmod(path string, mode os.FileMode) error {
-	return unix.Fchmodat(unix.AT_FDCWD, path, uint32(mode), unix.AT_SYMLINK_NOFOLLOW)
-}
-
 // Getxattr returns all of the extended attributes for the file at path p.
 func (d *driver) Getxattr(p string) (map[string][]byte, error) {
 	xattrs, err := sysx.Listxattr(p)
diff --git a/driver/lchmod_linux.go b/driver/lchmod_linux.go
@@ -0,0 +1,19 @@
+package driver
+
+import (
+	"os"
+
+	"golang.org/x/sys/unix"
+)
+
+// Lchmod changes the mode of a file not following symlinks.
+func (d *driver) Lchmod(path string, mode os.FileMode) error {
+	// On Linux, file mode is not supported for symlinks,
+	// and fchmodat() does not support AT_SYMLINK_NOFOLLOW,
+	// so symlinks need to be skipped entirely.
+	if st, err := os.Stat(path); err == nil && st.Mode()&os.ModeSymlink != 0 {
+		return nil
+	}
+
+	return unix.Fchmodat(unix.AT_FDCWD, path, uint32(mode), 0)
+}
diff --git a/driver/lchmod_unix.go b/driver/lchmod_unix.go
@@ -0,0 +1,14 @@
+// +build darwin freebsd solaris
+
+package driver
+
+import (
+	"os"
+
+	"golang.org/x/sys/unix"
+)
+
+// Lchmod changes the mode of a file not following symlinks.
+func (d *driver) Lchmod(path string, mode os.FileMode) error {
+	return unix.Fchmodat(unix.AT_FDCWD, path, uint32(mode), unix.AT_SYMLINK_NOFOLLOW)
+}
