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

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Message-ID: <CACGkMEvhABOtHTCVW9sX7p0wo1QCMXMvOAD+u4pzBueoU=MCpg@mail.gmail.com>
Date: Fri, 26 Sep 2025 12:52:53 +0800
From: Jason Wang <jasowang@...hat.com>
To: Xuan Zhuo <xuanzhuo@...ux.alibaba.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 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.
"""

> +                       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?

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