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

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Message-ID: <CACGkMEscCsCf8RC-xQzfTNMp94Ty4wTrBgLkF50OAQ+yF8xD-A@mail.gmail.com>
Date: Fri, 19 Sep 2025 10:11:55 +0800
From: Jason Wang <jasowang@...hat.com>
To: Xuan Zhuo <xuanzhuo@...ux.alibaba.com>
Cc: netdev@...r.kernel.org, "Michael S. Tsirkin" <mst@...hat.com>, 
	Eugenio Pérez <eperezma@...hat.com>, 
	Andrew Lunn <andrew+netdev@...n.ch>, "David S. Miller" <davem@...emloft.net>, 
	Eric Dumazet <edumazet@...gle.com>, Jakub Kicinski <kuba@...nel.org>, Paolo Abeni <pabeni@...hat.com>, 
	Heng Qi <hengqi@...ux.alibaba.com>, virtualization@...ts.linux.dev
Subject: Re: [PATCH net] virtio-net: fix incorrect flags recording in big mode

On Fri, Sep 19, 2025 at 9:35 AM Xuan Zhuo <xuanzhuo@...ux.alibaba.com> 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@...ux.alibaba.com>
> ---
>  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);
> +               flags = ((struct virtio_net_common_hdr *)p)->hdr.flags;
> +

Patch looks good but a I have a nit:

It looks better to move this above?

if (vi->big_packets) {
               void *p = page_address((struct page *)buf);
               flags = ((struct virtio_net_common_hdr *)p)->hdr.flags;
} else
               flags = ((struct virtio_net_common_hdr *)buf)->hdr.flags;

To avoid twice the calculations and reuse the comment.

>                 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;
> --
> 2.32.0.3.g01195cf9f
>

Thanks