Re: [PATCH net 2/2] virtio-net: correct hdr_len handling for VIRTIO_NET_F_GUEST_HDRLEN

[Jason Wang <jasowang@redhat.com>]

On Thu, Oct 9, 2025 at 7:10 PM Xuan Zhuo <xuanzhuo@linux.alibaba.com> wrote:
>
> On Fri, 26 Sep 2025 12:52:53 +0800, Jason Wang <jasowang@redhat.com> wrote:
> > On Thu, Sep 25, 2025 at 10:25 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.
> > >
> > > 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 | 19 ++++++++++++-------
> > >  1 file changed, 12 insertions(+), 7 deletions(-)
> > >
> > > diff --git a/include/linux/virtio_net.h b/include/linux/virtio_net.h
> > > index 20e0584db1dd..4273420a9ff9 100644
> > > --- a/include/linux/virtio_net.h
> > > +++ b/include/linux/virtio_net.h
> > > @@ -217,20 +217,25 @@ 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;
> > >
> > > -               /* This is a hint as to how much should be linear. */
> > > -               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)
> > > +                       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)
> > > +                       hdr_len += tcp_hdrlen(skb);
> > > +               } else if (sinfo->gso_type & SKB_GSO_UDP_L4) {
> > >                         hdr->gso_type = VIRTIO_NET_HDR_GSO_UDP_L4;
> > > -               else
> >
> > I think we need to deal with the GSO tunnel as well?
> >
> > """
> >     If the \field{gso_type} has the VIRTIO_NET_HDR_GSO_UDP_TUNNEL_IPV4 bit or
> >     VIRTIO_NET_HDR_GSO_UDP_TUNNEL_IPV6 bit set, \field{hdr_len} accounts for
> >     all the headers up to and including the inner transport.
> > """
>
> I checked the new code, VIRTIO_NET_HDR_GSO_UDP_TUNNEL_IPV6
> and VIRTIO_NET_HDR_GSO_UDP_TUNNEL_IPV4 are not supported, so the next version
> will not include these feature.
>
> Thanks.

I may miss something but we had:

        if (skb->protocol == htons(ETH_P_IPV6))
                hdr->gso_type |= VIRTIO_NET_HDR_GSO_UDP_TUNNEL_IPV6;
        else
                hdr->gso_type |= VIRTIO_NET_HDR_GSO_UDP_TUNNEL_IPV4;

in virtio_net_hdr_tnl_from_skb() now.

Thanks

>
>
> >
> > > +                       hdr_len += sizeof(struct udphdr);
> > > +               } else {
> > >                         return -EINVAL;
> > > +               }
> > > +
> > > +               hdr->hdr_len = __cpu_to_virtio16(little_endian, hdr_len);
> >
> > Should we at least check against the feature of VIRTIO_NET_F_GUEST_HDRLEN?
> >
> > >                 if (sinfo->gso_type & SKB_GSO_TCP_ECN)
> > >                         hdr->gso_type |= VIRTIO_NET_HDR_GSO_ECN;
> > >         } else
> > > --
> > > 2.32.0.3.g01195cf9f
> > >
> >
>