Re: [PATCH net v2 1/2] virtio-net: xsk: rx: fix the frame's length check

[Xuan Zhuo <xuanzhuo@linux.alibaba.com>]

On Sat, 21 Jun 2025 21:49:51 +0700, Bui Quang Minh <minhquangbui99@gmail.com> wrote:
> When calling buf_to_xdp, the len argument is the frame data's length
> without virtio header's length (vi->hdr_len). We check that len with
>
> 	xsk_pool_get_rx_frame_size() + vi->hdr_len
>
> to ensure the provided len does not larger than the allocated chunk
> size. The additional vi->hdr_len is because in virtnet_add_recvbuf_xsk,
> we use part of XDP_PACKET_HEADROOM for virtio header and ask the vhost
> to start placing data from
>
> 	hard_start + XDP_PACKET_HEADROOM - vi->hdr_len
> not
> 	hard_start + XDP_PACKET_HEADROOM
>
> But the first buffer has virtio_header, so the maximum frame's length in
> the first buffer can only be
>
> 	xsk_pool_get_rx_frame_size()
> not
> 	xsk_pool_get_rx_frame_size() + vi->hdr_len
>
> like in the current check.
>
> This commit adds an additional argument to buf_to_xdp differentiate
> between the first buffer and other ones to correctly calculate the maximum
> frame's length.
>
> Fixes: a4e7ba702701 ("virtio_net: xsk: rx: support recv small mode")
> Signed-off-by: Bui Quang Minh <minhquangbui99@gmail.com>

Reviewed-by: Xuan Zhuo <xuanzhuo@linux.alibaba.com>

> ---
>  drivers/net/virtio_net.c | 22 ++++++++++++++++++----
>  1 file changed, 18 insertions(+), 4 deletions(-)
>
> diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
> index e53ba600605a..1eb237cd5d0b 100644
> --- a/drivers/net/virtio_net.c
> +++ b/drivers/net/virtio_net.c
> @@ -1127,15 +1127,29 @@ static void check_sq_full_and_disable(struct virtnet_info *vi,
>  	}
>  }
>
> +/* Note that @len is the length of received data without virtio header */
>  static struct xdp_buff *buf_to_xdp(struct virtnet_info *vi,
> -				   struct receive_queue *rq, void *buf, u32 len)
> +				   struct receive_queue *rq, void *buf,
> +				   u32 len, bool first_buf)
>  {
>  	struct xdp_buff *xdp;
>  	u32 bufsize;
>
>  	xdp = (struct xdp_buff *)buf;
>
> -	bufsize = xsk_pool_get_rx_frame_size(rq->xsk_pool) + vi->hdr_len;
> +	/* In virtnet_add_recvbuf_xsk, we use part of XDP_PACKET_HEADROOM for
> +	 * virtio header and ask the vhost to fill data from
> +	 *         hard_start + XDP_PACKET_HEADROOM - vi->hdr_len
> +	 * The first buffer has virtio header so the remaining region for frame
> +	 * data is
> +	 *         xsk_pool_get_rx_frame_size()
> +	 * While other buffers than the first one do not have virtio header, so
> +	 * the maximum frame data's length can be
> +	 *         xsk_pool_get_rx_frame_size() + vi->hdr_len
> +	 */
> +	bufsize = xsk_pool_get_rx_frame_size(rq->xsk_pool);
> +	if (!first_buf)
> +		bufsize += vi->hdr_len;
>
>  	if (unlikely(len > bufsize)) {
>  		pr_debug("%s: rx error: len %u exceeds truesize %u\n",
> @@ -1260,7 +1274,7 @@ static int xsk_append_merge_buffer(struct virtnet_info *vi,
>
>  		u64_stats_add(&stats->bytes, len);
>
> -		xdp = buf_to_xdp(vi, rq, buf, len);
> +		xdp = buf_to_xdp(vi, rq, buf, len, false);
>  		if (!xdp)
>  			goto err;
>
> @@ -1358,7 +1372,7 @@ static void virtnet_receive_xsk_buf(struct virtnet_info *vi, struct receive_queu
>
>  	u64_stats_add(&stats->bytes, len);
>
> -	xdp = buf_to_xdp(vi, rq, buf, len);
> +	xdp = buf_to_xdp(vi, rq, buf, len, true);
>  	if (!xdp)
>  		return;
>
> --
> 2.43.0
>