Re: [PATCH] io_uring: revert consumed iov_iter bytes on error

[Stefano Garzarella <sgarzare@redhat.com>]

On Mon, Aug 24, 2020 at 11:48:44AM -0600, Jens Axboe wrote:
> Some consumers of the iov_iter will return an error, but still have
> bytes consumed in the iterator. This is an issue for -EAGAIN, since we
> rely on a sane iov_iter state across retries.
> 
> Fix this by ensuring that we revert consumed bytes, if any, if the file
> operations have consumed any bytes from iterator. This is similar to what
> generic_file_read_iter() does, and is always safe as we have the previous
> bytes count handy already.
> 
> Fixes: ff6165b2d7f6 ("io_uring: retain iov_iter state over io_read/io_write calls")
> Reported-by: Dmitry Shulyak <yashulyak@gmail.com>
> Signed-off-by: Jens Axboe <axboe@kernel.dk>
> 
> ---
> 
> diff --git a/fs/io_uring.c b/fs/io_uring.c
> index c9d526ff55e0..e030b33fa53e 100644
> --- a/fs/io_uring.c
> +++ b/fs/io_uring.c
> @@ -3153,6 +3153,8 @@ static int io_read(struct io_kiocb *req, bool force_nonblock,
>  	} else if (ret == -EAGAIN) {
>  		if (!force_nonblock)
>  			goto done;
> +		/* some cases will consume bytes even on error returns */
> +		iov_iter_revert(iter, iov_count - iov_iter_count(iter));
>  		ret = io_setup_async_rw(req, iovec, inline_vecs, iter, false);
>  		if (ret)
>  			goto out_free;
> @@ -3294,6 +3296,8 @@ static int io_write(struct io_kiocb *req, bool force_nonblock,
>  	if (!force_nonblock || ret2 != -EAGAIN) {
>  		kiocb_done(kiocb, ret2, cs);
>  	} else {
> +		/* some cases will consume bytes even on error returns */
> +		iov_iter_revert(iter, iov_count - iov_iter_count(iter));
>  copy_iov:
>  		ret = io_setup_async_rw(req, iovec, inline_vecs, iter, false);
>  		if (!ret)
> 

What about moving iov_iter_revert() in io_setup_async_rw(), passing
iov_initial_count as parameter?

Maybe it's out of purpose since we use it even when we're not trying again.


Anyway the patch LGTM:

Reviewed-by: Stefano Garzarella <sgarzare@redhat.com>

Thanks,
Stefano