Re: [PATCH] loop: Add sanity check for read/write_iter

[Ming Lei]

On Fri, Apr 25, 2025 at 11:41 AM Lizhi Xu <lizhi.xu@xxxxxxxxxxxxx> wrote:
>
> Some file systems do not support read_iter or write_iter, such as selinuxfs
> in this issue.
> So before calling them, first confirm that the interface is supported and
> then call it.
>
> Reported-by: syzbot+6af973a3b8dfd2faefdc@xxxxxxxxxxxxxxxxxxxxxxxxx
> Closes: https://syzkaller.appspot.com/bug?extid=6af973a3b8dfd2faefdc
> Signed-off-by: Lizhi Xu <lizhi.xu@xxxxxxxxxxxxx>
> ---
>  drivers/block/loop.c | 13 +++++++++----
>  1 file changed, 9 insertions(+), 4 deletions(-)
>
> diff --git a/drivers/block/loop.c b/drivers/block/loop.c
> index 674527d770dc..4f968e3071ed 100644
> --- a/drivers/block/loop.c
> +++ b/drivers/block/loop.c
> @@ -449,10 +449,15 @@ static int lo_rw_aio(struct loop_device *lo, struct loop_cmd *cmd,
>         cmd->iocb.ki_flags = IOCB_DIRECT;
>         cmd->iocb.ki_ioprio = IOPRIO_PRIO_VALUE(IOPRIO_CLASS_NONE, 0);
>
> -       if (rw == ITER_SOURCE)
> -               ret = file->f_op->write_iter(&cmd->iocb, &iter);
> -       else
> -               ret = file->f_op->read_iter(&cmd->iocb, &iter);
> +       ret = 0;
> +       if (rw == ITER_SOURCE) {
> +               if (likely(file->f_op->write_iter))
> +                       ret = file->f_op->write_iter(&cmd->iocb, &iter);
> +       }
> +       else {
> +               if (likely(file->f_op->read_iter))
> +                       ret = file->f_op->read_iter(&cmd->iocb, &iter);
> +       }

The check can be added in loop_configure()/loop_change_fd()
instead of fast IO path.

Thanks,