[PATCH net v5 2/2] virtio-net: correct hdr_len handling for tunnel gso

[Paolo Abeni]

On 11/11/25 12:12 PM, 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 at linux.alibaba.com>
> ---
>  include/linux/virtio_net.h | 24 ++++++++++++++++--------
>  1 file changed, 16 insertions(+), 8 deletions(-)
> 
> diff --git a/include/linux/virtio_net.h b/include/linux/virtio_net.h
> index 3cd8b2ebc197..432b17979d17 100644
> --- a/include/linux/virtio_net.h
> +++ b/include/linux/virtio_net.h
> @@ -232,12 +232,23 @@ static inline int virtio_net_hdr_from_skb(const struct sk_buff *skb,
>  			return -EINVAL;
>  
>  		if (hdrlen_negotiated) {
> -			hdr_len = skb_transport_offset(skb);
> +			if (sinfo->gso_type & (SKB_GSO_UDP_TUNNEL |
> +					       SKB_GSO_UDP_TUNNEL_CSUM)) {

I'm personally not a huge fan of adding UDP tunnel specific check to the
generic code, did you tried something along the lines suggested here:

https://lore.kernel.org/netdev/CAF6piCLkv6kFqoq7OQfJ=Su9AVHSQ9J7DzaumOSf5xuf9w-kyA@mail.gmail.com/

?

Thanks,

Paolo