On Wed, Nov 19, 2025 at 01:55:22PM +0800, 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 <[email protected]>
> ---
> include/linux/virtio_net.h | 25 ++++++++++++++++---------
> 1 file changed, 16 insertions(+), 9 deletions(-)
>
> diff --git a/include/linux/virtio_net.h b/include/linux/virtio_net.h
> index ee960ec9a35e..ee8231eb759b 100644
> --- a/include/linux/virtio_net.h
> +++ b/include/linux/virtio_net.h
> @@ -215,12 +215,22 @@ static inline void virtio_net_set_hdrlen(const struct
> sk_buff *skb,
> u16 hdr_len;
>
> if (guest_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);
> + }
> } else {
> /* This is a hint as to how much should be linear. */
> hdr_len = skb_headlen(skb);
BTW I noticed that include/linux/virtio_net.h really should include
linux/tcp.h for tcp_hdrlen and uapi/linux/udp.h for struct udphdr
Not a new issue so you do not have to resolve it in this patchset,
though.
> @@ -441,11 +451,8 @@ virtio_net_hdr_tnl_from_skb(const struct sk_buff *skb,
> vhdr->hash_hdr.hash_report = 0;
> vhdr->hash_hdr.padding = 0;
>
> - /* Let the basic parsing deal with plain GSO features. */
> - skb_shinfo(skb)->gso_type &= ~tnl_gso_type;
> ret = __virtio_net_hdr_from_skb(skb, hdr, true, false,
> guest_hdrlen, vlan_hlen);
> - skb_shinfo(skb)->gso_type |= tnl_gso_type;
> if (ret)
> return ret;
>
> --
> 2.32.0.3.g01195cf9f
