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

On Fri, Sep 19, 2025 at 10:11:55AM +0800, Jason Wang wrote:

On Fri, Sep 19, 2025 at 9:35 AM Xuan Zhuo [off-list ref] wrote:

quoted

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>
---
 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.

I think duplicating a bit of code is better than branching twice.
So I would move the flags assignment down instead.

quoted

                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