On Fri, Aug 15, 2025 at 5:06 PM Sohan Kunkerkar <sohank2602@xxxxxxxxx> wrote:
>
> OverlayFS already has comprehensive support for idmapped mounts through
> its ovl_copyattr() function and proper mnt_idmap() handling throughout
> the codebase. The infrastructure correctly maps UIDs/GIDs from idmapped
> upper and lower layers.
>
> However, the filesystem was missing the FS_ALLOW_IDMAP flag, which
> caused mount_setattr() calls with MOUNT_ATTR_IDMAP to fail with -EINVAL.
>
> This change enables idmapped mount support by adding the FS_ALLOW_IDMAP
> flag to the overlayfs file_system_type, allowing containers and other
> applications to use idmapped mounts with overlay filesystems.
>
> Signed-off-by: Sohan Kunkerkar <sohank2602@xxxxxxxxx>
> ---
> fs/overlayfs/super.c | 2 +-
> 1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/fs/overlayfs/super.c b/fs/overlayfs/super.c
> index e19940d64..c628f9179 100644
> --- a/fs/overlayfs/super.c
> +++ b/fs/overlayfs/super.c
> @@ -1516,7 +1516,7 @@ struct file_system_type ovl_fs_type = {
> .name = "overlay",
> .init_fs_context = ovl_init_fs_context,
> .parameters = ovl_parameter_spec,
> - .fs_flags = FS_USERNS_MOUNT,
> + .fs_flags = FS_USERNS_MOUNT | FS_ALLOW_IDMAP,
> .kill_sb = kill_anon_super,
> };
> MODULE_ALIAS_FS("overlay");
> --
> 2.50.1
So Christian just forgot to do that?
Somehow I find that hard to believe.
I am guessing there were either some known issues or
more code audits that needed to be done.
Christian? WDYT?
Thanks,
Amir.
