Subject: Re: [PATCH net v6 2/2] virtio-net: correct hdr_len handling for tunnel gso

From: "Michael S. Tsirkin" <[email-masked]>
Date: Wed, 19 Nov 2025 10:35:47 -0500

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-masked]>
> ---
>  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