[PATCH 03/10] fs: tweak fsuidgid_has_mapping()

[Christian Brauner <brauner@kernel.org>]

From: Christian Brauner <christian.brauner@ubuntu.com>

If the caller's fs{g,u}id aren't mapped in the mount's idmapping we can
return early and skip the check whether the mapped fs{g,u}id also have a
mapping in the filesystem's idmapping. If the fs{g,u}id aren't mapped in
the mount's idmapping they consequently can't be mapped in the
filesystem's idmapping. So there's no point in checking that.

Cc: Seth Forshee <sforshee@digitalocean.com>
Cc: Christoph Hellwig <hch@lst.de>
Cc: Al Viro <viro@zeniv.linux.org.uk>
CC: linux-fsdevel@vger.kernel.org
Signed-off-by: Christian Brauner <christian.brauner@ubuntu.com>
---
 include/linux/fs.h | 14 +++++++++++---
 1 file changed, 11 insertions(+), 3 deletions(-)

diff --git a/include/linux/fs.h b/include/linux/fs.h
index eb69e8b035fa..161b5936094e 100644
--- a/include/linux/fs.h
+++ b/include/linux/fs.h
@@ -1695,10 +1695,18 @@ static inline void inode_fsgid_set(struct inode *inode,
 static inline bool fsuidgid_has_mapping(struct super_block *sb,
 					struct user_namespace *mnt_userns)
 {
-	struct user_namespace *s_user_ns = sb->s_user_ns;
+	struct user_namespace *fs_userns = sb->s_user_ns;
+	kuid_t kuid;
+	kgid_t kgid;
 
-	return kuid_has_mapping(s_user_ns, mapped_fsuid(mnt_userns)) &&
-	       kgid_has_mapping(s_user_ns, mapped_fsgid(mnt_userns));
+	kuid = mapped_fsuid(mnt_userns);
+	if (!uid_valid(kuid))
+		return false;
+	kgid = mapped_fsgid(mnt_userns);
+	if (!gid_valid(kgid))
+		return false;
+	return kuid_has_mapping(fs_userns, kuid) &&
+	       kgid_has_mapping(fs_userns, kgid);
 }
 
 extern struct timespec64 current_time(struct inode *inode);
-- 
2.30.2