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
>
