Re: [PATCH net] virtio-net: fix incorrect flags recording in big mode

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

On Fri, Sep 19, 2025 at 09:34:50AM +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>


Thanks for the patch! Yet something to improve:

> ---
>  drivers/net/virtio_net.c | 12 +++++++++---
>  1 file changed, 9 insertions(+), 3 deletions(-)
> 
> diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
> index 975bdc5dab84..6e6e74390955 100644
> --- a/drivers/net/virtio_net.c
> +++ b/drivers/net/virtio_net.c
> @@ -2630,13 +2630,19 @@ static void receive_buf(struct virtnet_info *vi, struct receive_queue *rq,
>  	 */
>  	flags = ((struct virtio_net_common_hdr *)buf)->hdr.flags;
>  
> -	if (vi->mergeable_rx_bufs)
> +	if (vi->mergeable_rx_bufs) {
>  		skb = receive_mergeable(dev, vi, rq, buf, ctx, len, xdp_xmit,
>  					stats);
> -	else if (vi->big_packets)
> +	} else if (vi->big_packets) {
> +		void *p;
> +
> +		p = page_address((struct page *)buf);

I'd move this assignment to where p is declared:
		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 {
>  		skb = receive_small(dev, vi, rq, buf, ctx, len, xdp_xmit, stats);
> +	}
>  
>  	if (unlikely(!skb))
>  		return;


Indeed but let's please not do the initial 
       flags = ((struct virtio_net_common_hdr *)buf)->hdr.flags;
for the big mode at all, it's confusing, and only works because struct
page is bigger than struct virtio_net_common_hdr.

pls move it into the clauses for mergeable and small.


> -- 
> 2.32.0.3.g01195cf9f