fs/mnt_idmapping.c: Return -EINVAL when no map is written

Currently, it is possible to create an idmapped mount using a user
namespace without any mappings. However, this yields an idmapped
mount that doesn't actually map the ids. With the following change,
it will no longer be possible to create an idmapped mount when using
a user namespace with no mappings, and will instead return EINVAL,
an “invalid argument” error code.

Reviewed-by: Christian Brauner <brauner@kernel.org>
Signed-off-by: Taylor Jackson <taylor.a.jackson@me.com>
Link: https://lore.kernel.org/r/20240208-mnt-idmap-inval-v2-1-58ef26d194e0@me.com
Signed-off-by: Christian Brauner <brauner@kernel.org>

diff --git a/fs/mnt_idmapping.c b/fs/mnt_idmapping.c
index 64c5205e2b5e7dac35c3ee8470d0f65d3254d4fb..3c60f1eaca615a24588a3a7e1645dbaf71234774 100644 (file)
--- a/fs/mnt_idmapping.c
+++ b/fs/mnt_idmapping.c
@@ -214,7 +214,7 @@ static int copy_mnt_idmap(struct uid_gid_map *map_from,
         * anything at all.
         */
        if (nr_extents == 0)
-               return 0;
+               return -EINVAL;
 
        /*
         * Here we know that nr_extents is greater than zero which means