fs: tweak fsuidgid_has_mapping()

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.

Link: https://lore.kernel.org/r/[email protected] (v1)
Link: https://lore.kernel.org/r/[email protected] (v2)
Link: https://lore.kernel.org/r/[email protected]
Cc: Seth Forshee <[email protected]>
Cc: Christoph Hellwig <[email protected]>
Cc: Al Viro <[email protected]>
CC: [email protected]
Reviewed-by: Amir Goldstein <[email protected]>
Reviewed-by: Seth Forshee <[email protected]>
Signed-off-by: Christian Brauner <[email protected]>

include/linux/fs.h

@@ -1697,10 +1697,18 @@ 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);