loopback: use Lgetxattr and friends

Stop following symlinks when working with extended
attributes, and add a test for it.

Problem found by xfstests generic/062.

Change-Id: I67f94451322cdfebdcbcc3af21679ccd4e2800d7

fs/loopback_linux.go

@@ -14,22 +14,22 @@ import (
 )
 
 func (n *loopbackNode) Getxattr(ctx context.Context, attr string, dest []byte) (uint32, syscall.Errno) {
-	sz, err := syscall.Getxattr(n.path(), attr, dest)
+	sz, err := unix.Lgetxattr(n.path(), attr, dest)
 	return uint32(sz), ToErrno(err)
 }
 
 func (n *loopbackNode) Setxattr(ctx context.Context, attr string, data []byte, flags uint32) syscall.Errno {
-	err := syscall.Setxattr(n.path(), attr, data, int(flags))
+	err := unix.Lsetxattr(n.path(), attr, data, int(flags))
 	return ToErrno(err)
 }
 
 func (n *loopbackNode) Removexattr(ctx context.Context, attr string) syscall.Errno {
-	err := syscall.Removexattr(n.path(), attr)
+	err := unix.Lremovexattr(n.path(), attr)
 	return ToErrno(err)
 }
 
 func (n *loopbackNode) Listxattr(ctx context.Context, dest []byte) (uint32, syscall.Errno) {
-	sz, err := syscall.Listxattr(n.path(), dest)
+	sz, err := unix.Llistxattr(n.path(), dest)
 	return uint32(sz), ToErrno(err)
 }
 

fs/loopback_linux_test.go

@@ -163,6 +163,28 @@ func TestXAttr(t *testing.T) {
 	}
 }
 
+// TestXAttrSymlink verifies that we did not forget to use Lgetxattr instead
+// of Getxattr. This test is Linux-specific because it depends on the behavoir
+// of the `security` namespace.
+//
+// On Linux, symlinks can not have xattrs in the `user` namespace, so we
+// try to read something from `security`. Writing would need root rights,
+// so don't even bother. See `man 7 xattr` for more info.
+func TestXAttrSymlink(t *testing.T) {
+	tc := newTestCase(t, nil)
+	defer tc.Clean()
+
+	path := tc.mntDir + "/symlink"
+	if err := syscall.Symlink("target/does/not/exist", path); err != nil {
+		t.Fatal(err)
+	}
+	buf := make([]byte, 10)
+	_, err := unix.Lgetxattr(path, "security.foo", buf)
+	if err != unix.ENODATA {
+		t.Errorf("want %d=ENODATA, got error %d=%q instead", unix.ENODATA, err, err)
+	}
+}
+
 func TestCopyFileRange(t *testing.T) {
 	tc := newTestCase(t, &testOptions{attrCache: true, entryCache: true})
 	defer tc.Clean()

fs/simple_test.go

@@ -32,6 +32,7 @@ type testCase struct {
 	server   *fuse.Server
 }
 
+// writeOrig writes a file into the backing directory of the loopback mount
 func (tc *testCase) writeOrig(path, content string, mode os.FileMode) {
 	if err := ioutil.WriteFile(filepath.Join(tc.origDir, path), []byte(content), mode); err != nil {
 		tc.Fatal(err)
@@ -54,6 +55,8 @@ type testOptions struct {
 	testDir       string
 }
 
+// newTestCase creates the directories `orig` and `mnt` inside a temporary
+// directory and mounts a loopback filesystem, backed by `orig`, on `mnt`.
 func newTestCase(t *testing.T, opts *testOptions) *testCase {
 	if opts == nil {
 		opts = &testOptions{}