Re: [PATCH v2] io_uring/zcrx: return early from io_zcrx_recv_skb if readlen is 0

[Jens Axboe <axboe@kernel.dk>]

On 4/1/25 1:53 PM, David Wei wrote:
> When readlen is set for a recvzc request, tcp_read_sock() will call
> io_zcrx_recv_skb() one final time with len == desc->count == 0. This is
> caused by the !desc->count check happening too late. The offset + 1 !=
> skb->len happens earlier and causes the while loop to continue.
> 
> Fix this in io_zcrx_recv_skb() instead of tcp_read_sock(). Return early
> if len is 0 i.e. the read is done.
> 
> Changes in v2:
> --------------
> * Add Fixes tag
> * Return 0 directly
> * Add comment explaining why the !len check is needed
> 
> Fixes: 6699ec9a23f8 ("io_uring/zcrx: add a read limit to recvzc requests")
> Signed-off-by: David Wei <dw@davidwei.uk>
> ---
>  io_uring/zcrx.c | 7 +++++++
>  1 file changed, 7 insertions(+)
> 
> diff --git a/io_uring/zcrx.c b/io_uring/zcrx.c
> index 9c95b5b6ec4e..2c8b29c745c5 100644
> --- a/io_uring/zcrx.c
> +++ b/io_uring/zcrx.c
> @@ -818,6 +818,13 @@ io_zcrx_recv_skb(read_descriptor_t *desc, struct sk_buff *skb,
>  	int ret = 0;
>  
>  	len = min_t(size_t, len, desc->count);
> +	/* __tcp_read_sock() always calls io_zcrx_recv_skb one last time, even
> +	 * if desc->count is already 0. This is caused by the if (offset + 1 !=
> +	 * skb->len) check. Return early in this case to break out of
> +	 * __tcp_read_sock().
> +	 */
> +	if (!len)
> +		return 0;
>  	if (unlikely(args->nr_skbs++ > IO_SKBS_PER_CALL_LIMIT))
>  		return -EAGAIN;

Bad comment format, but I can fix it up while applying. Thanks!

-- 
Jens Axboe