userns: Convert stat to return values mapped from kuids and kgids

- Store uids and gids with kuid_t and kgid_t in struct kstat
- Convert uid and gids to userspace usable values with
  from_kuid and from_kgid

Acked-by: Serge Hallyn <serge.hallyn@canonical.com>
Signed-off-by: Eric W. Biederman <ebiederm@xmission.com>
diff --git a/fs/stat.c b/fs/stat.c
index c733dc5..31acca5 100644
--- a/fs/stat.c
+++ b/fs/stat.c
@@ -137,8 +137,8 @@
 	tmp.st_nlink = stat->nlink;
 	if (tmp.st_nlink != stat->nlink)
 		return -EOVERFLOW;
-	SET_UID(tmp.st_uid, stat->uid);
-	SET_GID(tmp.st_gid, stat->gid);
+	SET_UID(tmp.st_uid, from_kuid_munged(current_user_ns(), stat->uid));
+	SET_GID(tmp.st_gid, from_kgid_munged(current_user_ns(), stat->gid));
 	tmp.st_rdev = old_encode_dev(stat->rdev);
 #if BITS_PER_LONG == 32
 	if (stat->size > MAX_NON_LFS)
@@ -215,8 +215,8 @@
 	tmp.st_nlink = stat->nlink;
 	if (tmp.st_nlink != stat->nlink)
 		return -EOVERFLOW;
-	SET_UID(tmp.st_uid, stat->uid);
-	SET_GID(tmp.st_gid, stat->gid);
+	SET_UID(tmp.st_uid, from_kuid_munged(current_user_ns(), stat->uid));
+	SET_GID(tmp.st_gid, from_kgid_munged(current_user_ns(), stat->gid));
 #if BITS_PER_LONG == 32
 	tmp.st_rdev = old_encode_dev(stat->rdev);
 #else
@@ -350,8 +350,8 @@
 #endif
 	tmp.st_mode = stat->mode;
 	tmp.st_nlink = stat->nlink;
-	tmp.st_uid = stat->uid;
-	tmp.st_gid = stat->gid;
+	tmp.st_uid = from_kuid_munged(current_user_ns(), stat->uid);
+	tmp.st_gid = from_kgid_munged(current_user_ns(), stat->gid);
 	tmp.st_atime = stat->atime.tv_sec;
 	tmp.st_atime_nsec = stat->atime.tv_nsec;
 	tmp.st_mtime = stat->mtime.tv_sec;