linux-kernel - Re: [PATCH] overlayfs: add FS_ALLOW_IDMAP flag to enable idmapped mounts

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Message-ID: <CAOQ4uxjX_YYVvVj7NqwAaX-LMkbTgJwkXY3a=p9F+h6810e9CA@mail.gmail.com>
Date: Fri, 15 Aug 2025 17:30:09 +0200
From: Amir Goldstein <amir73il@...il.com>
To: Sohan Kunkerkar <sohank2602@...il.com>, Christian Brauner <brauner@...nel.org>
Cc: miklos@...redi.hu, linux-unionfs@...r.kernel.org, 
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] overlayfs: add FS_ALLOW_IDMAP flag to enable idmapped mounts

On Fri, Aug 15, 2025 at 5:06 PM Sohan Kunkerkar <sohank2602@...il.com> 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@...il.com>
> ---
>  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.