Re: [PATCH] ovl: reserve ability to reconfigure mount options with new mount api

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On Tue, Jun 20, 2023 at 2:42 PM Christian Brauner <brauner@xxxxxxxxxx> wrote:
>
> Using the old mount api to remount an overlayfs superblock via
> mount(MS_REMOUNT) all mount options will be silently ignored. For
> example, if you create an overlayfs mount:
>
>         mount -t overlay overlay -o lowerdir=/mnt/a:/mnt/b,upperdir=/mnt/upper,workdir=/mnt/work /mnt/merged
>
> and then issue a remount via:
>
>         # force mount(8) to use mount(2)
>         export LIBMOUNT_FORCE_MOUNT2=always
>         mount -t overlay overlay -o remount,WOOTWOOT,lowerdir=/DOESNT-EXIST /mnt/merged
>
> with completely nonsensical mount options whatsoever it will succeed
> nonetheless. This prevents us from every changing any mount options we
> might introduce in the future that could reasonably be changed during a
> remount.
>
> We don't need to carry this issue into the new mount api port. Similar
> to FUSE we can use the fs_context::oldapi member to figure out that this
> is a request coming through the legacy mount api. If we detect it we
> continue silently ignoring all mount options.
>
> But for the new mount api we simply report that mount options cannot
> currently be changed. This will allow us to potentially alter mount
> properties for new or even old properties. It any case, silently
> ignoring everything is not something new apis should do.
>
> Signed-off-by: Christian Brauner <brauner@xxxxxxxxxx>
> ---
>

Looks good and tests passed.
I Applied this to overlayfs-next on my github.

Miklos,

Please update your branch.

Thanks,
Amir.

> ---
>  fs/overlayfs/super.c | 25 ++++++++++++++++++-------
>  1 file changed, 18 insertions(+), 7 deletions(-)
>
> diff --git a/fs/overlayfs/super.c b/fs/overlayfs/super.c
> index ed4b35c9d647..c14c52560fd6 100644
> --- a/fs/overlayfs/super.c
> +++ b/fs/overlayfs/super.c
> @@ -499,13 +499,24 @@ static int ovl_parse_param(struct fs_context *fc, struct fs_parameter *param)
>         struct ovl_fs_context *ctx = fc->fs_private;
>         int opt;
>
> -       /*
> -        * On remount overlayfs has always ignored all mount options no
> -        * matter if malformed or not so for backwards compatibility we
> -        * do the same here.
> -        */
> -       if (fc->purpose == FS_CONTEXT_FOR_RECONFIGURE)
> -               return 0;
> +       if (fc->purpose == FS_CONTEXT_FOR_RECONFIGURE) {
> +               /*
> +                * On remount overlayfs has always ignored all mount
> +                * options no matter if malformed or not so for
> +                * backwards compatibility we do the same here.
> +                */
> +               if (fc->oldapi)
> +                       return 0;
> +
> +               /*
> +                * Give us the freedom to allow changing mount options
> +                * with the new mount api in the future. So instead of
> +                * silently ignoring everything we report a proper
> +                * error. This is only visible for users of the new
> +                * mount api.
> +                */
> +               return invalfc(fc, "No changes allowed in reconfigure");
> +       }
>
>         opt = fs_parse(fc, ovl_parameter_spec, param, &result);
>         if (opt < 0)
>
> ---
> base-commit: cc7e4d7ce5ea183b9ca735f7466b4491a1ee440e
> change-id: 20230620-fs-overlayfs-mount-api-remount-93b210e6ccd8
>




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