Re: [PATCH net v8 2/2] virtio-net: correct hdr_len handling for tunnel gso

[Xuan Zhuo <xuanzhuo@linux.alibaba.com>]

On Tue, 10 Mar 2026 11:05:33 +0100, Paolo Abeni <pabeni@redhat.com> wrote:
> On 3/5/26 4:19 AM, Xuan Zhuo wrote:
> > The commit a2fb4bc4e2a6a03 ("net: implement virtio helpers to handle UDP
> > GSO tunneling.") introduces support for the UDP GSO tunnel feature in
> > virtio-net.
> >
> > The virtio spec says:
> >
> >     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.
> >
> > The commit did not update the hdr_len to include the inner transport.
> >
> > I observed that the "hdr_len" is 116 for this packet:
> >
> >     17:36:18.241105 52:55:00:d1:27:0a > 2e:2c:df:46:a9:e1, ethertype IPv4 (0x0800), length 2912: (tos 0x0, ttl 64, id 45197, offset 0, flags [none], proto UDP (17), length 2898)
> >         192.168.122.100.50613 > 192.168.122.1.4789: [bad udp cksum 0x8106 -> 0x26a0!] VXLAN, flags [I] (0x08), vni 1
> >     fa:c3:ba:82:05:ee > ce:85:0c:31:77:e5, ethertype IPv4 (0x0800), length 2862: (tos 0x0, ttl 64, id 14678, offset 0, flags [DF], proto TCP (6), length 2848)
> >         192.168.3.1.49880 > 192.168.3.2.9898: Flags [P.], cksum 0x9266 (incorrect -> 0xaa20), seq 515667:518463, ack 1, win 64, options [nop,nop,TS val 2990048824 ecr 2798801412], length 2796
> >
> > 116 = 14(mac) + 20(ip) + 8(udp) + 8(vxlan) + 14(inner mac) + 20(inner ip) + 32(innner tcp)
> >
> > Fixes: a2fb4bc4e2a6a03 ("net: implement virtio helpers to handle UDP GSO tunneling.")
> > Signed-off-by: Xuan Zhuo <xuanzhuo@linux.alibaba.com>
> > ---
> >  include/linux/virtio_net.h | 31 ++++++++++++++++++++-----------
> >  1 file changed, 20 insertions(+), 11 deletions(-)
> >
> > diff --git a/include/linux/virtio_net.h b/include/linux/virtio_net.h
> > index 23f42bb8ece0..906157779955 100644
> > --- a/include/linux/virtio_net.h
> > +++ b/include/linux/virtio_net.h
> > @@ -209,18 +209,29 @@ static inline int virtio_net_hdr_to_skb(struct sk_buff *skb,
> >
> >  static inline void virtio_net_set_hdrlen(const struct sk_buff *skb,
> >  					 struct virtio_net_hdr *hdr,
> > +					 struct skb_shared_info *sinfo,
> >  					 bool little_endian,
> >  					 bool feature_hdrlen)
> >  {
> >  	u16 hdr_len;
> >
> >  	if (feature_hdrlen) {
> > -		hdr_len = skb_transport_offset(skb);
> > -
> > -		if (hdr->gso_type == VIRTIO_NET_HDR_GSO_UDP_L4)
> > -			hdr_len += sizeof(struct udphdr);
> > -		else
> > -			hdr_len += tcp_hdrlen(skb);
> > +		if (sinfo->gso_type & (SKB_GSO_UDP_TUNNEL |
> > +				       SKB_GSO_UDP_TUNNEL_CSUM)) {
> > +			hdr_len = skb_inner_transport_offset(skb);
> > +
> > +			if (hdr->gso_type == VIRTIO_NET_HDR_GSO_UDP_L4)
> > +				hdr_len += sizeof(struct udphdr);
> > +			else
> > +				hdr_len += inner_tcp_hdrlen(skb);
> > +		} else {
> > +			hdr_len = skb_transport_offset(skb);
> > +
> > +			if (hdr->gso_type == VIRTIO_NET_HDR_GSO_UDP_L4)
> > +				hdr_len += sizeof(struct udphdr);
> > +			else
> > +				hdr_len += tcp_hdrlen(skb);
> > +		}
>
> I already mentioned the following a couple of times, with no replies...
> perhaps the 3rd attempt will be more successful.
>
> I think it's a bad adding tunnel specific check here. It sounds like
> layering violation. Instead you could have tunnel-specific variant, say
> virtio_net_set_tnl_hdrlen(), and use it in virtio_net_hdr_tnl_from_skb().
>
> Or you could try what i suggested in:
>
> https://lore.kernel.org/netdev/CAF6piCLkv6kFqoq7OQfJ=Su9AVHSQ9J7DzaumOSf5xuf9w-kyA@mail.gmail.com/#t

I saw your suggestions, though I didn't reply directly. I proceeded to adjust
the code based on my understanding of your feedback, but it appears my
understanding was incorrect. After re-reading your suggestions, I think
virtio_net_hdr_from_skb does not need to be changed. Instead, within
virtio_net_hdr_tnl_from_skb, we should directly call the device header length
function based on whether the feature is negotiated.

Thanks.

>
> Thanks,
>
> Paolo
>