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

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Message-ID: <1760007608.9282165-1-xuanzhuo@linux.alibaba.com>
Date: Thu, 9 Oct 2025 19:00:08 +0800
From: Xuan Zhuo <xuanzhuo@...ux.alibaba.com>
To: Jason Wang <jasowang@...hat.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>,
 Willem de Bruijn <willemb@...gle.com>,
 Jiri Pirko <jiri@...nulli.us>,
 Alvaro Karsz <alvaro.karsz@...id-run.com>,
 Heng Qi <hengqi@...ux.alibaba.com>,
 virtualization@...ts.linux.dev
Subject: Re: [PATCH net 2/2] virtio-net: correct hdr_len handling for VIRTIO_NET_F_GUEST_HDRLEN

On Fri, 26 Sep 2025 12:52:53 +0800, Jason Wang <jasowang@...hat.com> wrote:
> On Thu, Sep 25, 2025 at 10:25 AM Xuan Zhuo <xuanzhuo@...ux.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@...ux.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.
> """

Sorry, I referred to the old spec. According to the latest spec, this case should indeed be considered.


>
> > +                       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?

I think we don't need to check for VIRTIO_NET_F_GUEST_HDRLEN, because regardless
of whether this feature has been negotiated or not, we always pass this value
hdr_len.

Previously, the value might have been incorrect but still acceptable; however,
once VIRTIO_NET_F_GUEST_HDRLEN is negotiated, incorrect values must no
longer be accepted. Therefore, we don't need to check VIRTIO_NET_F_GUEST_HDRLEN.
On the other hand, VIRTIO_NET_HDR_GSO_UDP_TUNNEL_IPV4 should be checked.

Thanks.

>
> >                 if (sinfo->gso_type & SKB_GSO_TCP_ECN)
> >                         hdr->gso_type |= VIRTIO_NET_HDR_GSO_ECN;
> >         } else
> > --
> > 2.32.0.3.g01195cf9f
> >
>