loopback: preserve owner when running as root

If we are running as root have caller information from
the context, try to set the owner of newly created files
and directories.

Lays the foundation for fixing xfstests generic/087
(xfstests always run as root).

Change-Id: Ib8d768153f3ec82ce572021a433a6398560efd44

fs/loopback.go

@@ -89,12 +89,26 @@ func (n *loopbackNode) Lookup(ctx context.Context, name string, out *fuse.EntryO
 	return ch, 0
 }
 
+// preserveOwner sets uid and gid of `path` according to the caller information
+// in `ctx`.
+func (n *loopbackNode) preserveOwner(ctx context.Context, path string) error {
+	if os.Getuid() != 0 {
+		return nil
+	}
+	caller, ok := fuse.FromContext(ctx)
+	if !ok {
+		return nil
+	}
+	return syscall.Lchown(path, int(caller.Uid), int(caller.Gid))
+}
+
 func (n *loopbackNode) Mknod(ctx context.Context, name string, mode, rdev uint32, out *fuse.EntryOut) (*Inode, syscall.Errno) {
 	p := filepath.Join(n.path(), name)
 	err := syscall.Mknod(p, mode, int(rdev))
 	if err != nil {
 		return nil, ToErrno(err)
 	}
+	n.preserveOwner(ctx, p)
 	st := syscall.Stat_t{}
 	if err := syscall.Lstat(p, &st); err != nil {
 		syscall.Rmdir(p)
@@ -115,6 +129,7 @@ func (n *loopbackNode) Mkdir(ctx context.Context, name string, mode uint32, out
 	if err != nil {
 		return nil, ToErrno(err)
 	}
+	n.preserveOwner(ctx, p)
 	st := syscall.Stat_t{}
 	if err := syscall.Lstat(p, &st); err != nil {
 		syscall.Rmdir(p)
@@ -189,7 +204,7 @@ func (n *loopbackNode) Create(ctx context.Context, name string, flags uint32, mo
 	if err != nil {
 		return nil, nil, 0, ToErrno(err)
 	}
-
+	n.preserveOwner(ctx, p)
 	st := syscall.Stat_t{}
 	if err := syscall.Fstat(fd, &st); err != nil {
 		syscall.Close(fd)
@@ -210,6 +225,7 @@ func (n *loopbackNode) Symlink(ctx context.Context, target, name string, out *fu
 	if err != nil {
 		return nil, ToErrno(err)
 	}
+	n.preserveOwner(ctx, p)
 	st := syscall.Stat_t{}
 	if err := syscall.Lstat(p, &st); err != nil {
 		syscall.Unlink(p)