Re: [PATCH net v4 1/4] virtio-net: fix incorrect flags recording in big mode

["Michael S. Tsirkin" <mst@redhat.com>]

On Wed, Oct 29, 2025 at 11:09:10AM +0800, Xuan Zhuo wrote:
> The purpose of commit 703eec1b2422 ("virtio_net: fixing XDP for fully
> checksummed packets handling") is to record the flags in advance, as
> their value may be overwritten in the XDP case. However, the flags
> recorded under big mode are incorrect, because in big mode, the passed
> buf does not point to the rx buffer, but rather to the page of the
> submitted buffer. This commit fixes this issue.
> 
> For the small mode, the commit c11a49d58ad2 ("virtio_net: Fix mismatched
> buf address when unmapping for small packets") fixed it.
> 
> Fixes: 703eec1b2422 ("virtio_net: fixing XDP for fully checksummed packets handling")
> Signed-off-by: Xuan Zhuo <xuanzhuo@linux.alibaba.com>
> Acked-by: Jason Wang <jasowang@redhat.com>

Acked-by: Michael S. Tsirkin <mst@redhat.com>

> ---
>  drivers/net/virtio_net.c | 16 +++++++++++-----
>  1 file changed, 11 insertions(+), 5 deletions(-)
> 
> diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
> index 8e8a179aaa49..59f116b609f6 100644
> --- a/drivers/net/virtio_net.c
> +++ b/drivers/net/virtio_net.c
> @@ -2625,22 +2625,28 @@ static void receive_buf(struct virtnet_info *vi, struct receive_queue *rq,
>  		return;
>  	}
>  
> -	/* 1. Save the flags early, as the XDP program might overwrite them.
> +	/* About the flags below:
> +	 * 1. Save the flags early, as the XDP program might overwrite them.
>  	 * These flags ensure packets marked as VIRTIO_NET_HDR_F_DATA_VALID
>  	 * stay valid after XDP processing.
>  	 * 2. XDP doesn't work with partially checksummed packets (refer to
>  	 * virtnet_xdp_set()), so packets marked as
>  	 * VIRTIO_NET_HDR_F_NEEDS_CSUM get dropped during XDP processing.
>  	 */
> -	flags = ((struct virtio_net_common_hdr *)buf)->hdr.flags;
>  
> -	if (vi->mergeable_rx_bufs)
> +	if (vi->mergeable_rx_bufs) {
> +		flags = ((struct virtio_net_common_hdr *)buf)->hdr.flags;
>  		skb = receive_mergeable(dev, vi, rq, buf, ctx, len, xdp_xmit,
>  					stats);
> -	else if (vi->big_packets)
> +	} else if (vi->big_packets) {
> +		void *p = page_address((struct page *)buf);
> +
> +		flags = ((struct virtio_net_common_hdr *)p)->hdr.flags;
>  		skb = receive_big(dev, vi, rq, buf, len, stats);
> -	else
> +	} else {
> +		flags = ((struct virtio_net_common_hdr *)buf)->hdr.flags;
>  		skb = receive_small(dev, vi, rq, buf, ctx, len, xdp_xmit, stats);
> +	}
>  
>  	if (unlikely(!skb))
>  		return;
> -- 
> 2.32.0.3.g01195cf9f