Re: [PATCH net v4 3/4] virtio-net: correct hdr_len handling for VIRTIO_NET_F_GUEST_HDRLEN

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

On Thu, Oct 30, 2025 at 10:53:01AM +0800, Jason Wang wrote:
> On Wed, Oct 29, 2025 at 11:09 AM Xuan Zhuo <xuanzhuo@linux.alibaba.com> wrote:
> >
> > The commit be50da3e9d4a ("net: virtio_net: implement exact header length
> > guest feature") introduces support for the VIRTIO_NET_F_GUEST_HDRLEN
> > feature in virtio-net.
> >
> > This feature requires virtio-net to set hdr_len to the actual header
> > length of the packet when transmitting, the number of
> > bytes from the start of the packet to the beginning of the
> > transport-layer payload.
> >
> > However, in practice, hdr_len was being set using skb_headlen(skb),
> > which is clearly incorrect. This commit fixes that issue.
> 
> I still think it would be more safe to check the feature

which feature VIRTIO_NET_F_GUEST_HDRLEN ?


> and switch to
> the new behaviour if it is set. This seems to be more safe.
> 
> But I'm fine if it's agreed that this could be the way to go.
> 
> >
> > Fixes: be50da3e9d4a ("net: virtio_net: implement exact header length guest feature")
> > Signed-off-by: Xuan Zhuo <xuanzhuo@linux.alibaba.com>
> > ---
> >  include/linux/virtio_net.h | 22 ++++++++++++++++------
> >  1 file changed, 16 insertions(+), 6 deletions(-)
> >
> > diff --git a/include/linux/virtio_net.h b/include/linux/virtio_net.h
> > index 710ae0d2d336..6ef0b737d548 100644
> > --- a/include/linux/virtio_net.h
> > +++ b/include/linux/virtio_net.h
> > @@ -217,25 +217,35 @@ static inline int virtio_net_hdr_from_skb(const struct sk_buff *skb,
> >
> >         if (skb_is_gso(skb)) {
> >                 struct skb_shared_info *sinfo = skb_shinfo(skb);
> > +               u16 hdr_len = 0;
> >
> >                 /* In certain code paths (such as the af_packet.c receive path),
> >                  * this function may be called without a transport header.
> >                  * In this case, we do not need to set the hdr_len.
> >                  */
> >                 if (skb_transport_header_was_set(skb))
> > -                       hdr->hdr_len = __cpu_to_virtio16(little_endian,
> > -                                                        skb_headlen(skb));
> > +                       hdr_len = skb_transport_offset(skb);
> >
> >                 hdr->gso_size = __cpu_to_virtio16(little_endian,
> >                                                   sinfo->gso_size);
> > -               if (sinfo->gso_type & SKB_GSO_TCPV4)
> > +               if (sinfo->gso_type & SKB_GSO_TCPV4) {
> >                         hdr->gso_type = VIRTIO_NET_HDR_GSO_TCPV4;
> > -               else if (sinfo->gso_type & SKB_GSO_TCPV6)
> > +                       if (hdr_len)
> > +                               hdr_len += tcp_hdrlen(skb);
> > +               } else if (sinfo->gso_type & SKB_GSO_TCPV6) {
> >                         hdr->gso_type = VIRTIO_NET_HDR_GSO_TCPV6;
> > -               else if (sinfo->gso_type & SKB_GSO_UDP_L4)
> > +                       if (hdr_len)
> > +                               hdr_len += tcp_hdrlen(skb);
> > +               } else if (sinfo->gso_type & SKB_GSO_UDP_L4) {
> >                         hdr->gso_type = VIRTIO_NET_HDR_GSO_UDP_L4;
> > -               else
> > +                       if (hdr_len)
> > +                               hdr_len += sizeof(struct udphdr);
> > +               } else {
> >                         return -EINVAL;
> > +               }
> > +
> > +               hdr->hdr_len = __cpu_to_virtio16(little_endian, hdr_len);
> > +
> >                 if (sinfo->gso_type & SKB_GSO_TCP_ECN)
> >                         hdr->gso_type |= VIRTIO_NET_HDR_GSO_ECN;
> >         } else
> > --
> > 2.32.0.3.g01195cf9f
> >
> 
> Thanks