On Wed, Oct 29, 2025 at 11:09:12AM +0800, Xuan Zhuo 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 <[email protected]>
ca you supply some examples when this is wrong please?
> ---
> include/linux/virtio_net.h | 22 ++++++++++++++++------
> 1 file changed, 16 insertions(+), 6 deletions(-)
>
> diff --git a/include/linux/virtio_net.h b/include/linux/virtio_net.h
> index 710ae0d2d336..6ef0b737d548 100644
> --- a/include/linux/virtio_net.h
> +++ b/include/linux/virtio_net.h
> @@ -217,25 +217,35 @@ 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 = 0;
>
> /* In certain code paths (such as the af_packet.c receive path),
> * this function may be called without a transport header.
> * In this case, we do not need to set the hdr_len.
> */
you actually do initialize it, just to 0.
the comment is confusing.
> if (skb_transport_header_was_set(skb))
> - hdr->hdr_len = __cpu_to_virtio16(little_endian,
> - skb_headlen(skb));
> + hdr_len = skb_transport_offset(skb);
better:
else
hdr_len = 0;
>
> 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)
> + if (hdr_len)
> + 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)
> + if (hdr_len)
> + hdr_len += tcp_hdrlen(skb);
> + } else if (sinfo->gso_type & SKB_GSO_UDP_L4) {
> hdr->gso_type = VIRTIO_NET_HDR_GSO_UDP_L4;
> - else
> + if (hdr_len)
> + hdr_len += sizeof(struct udphdr);
> + } else {
> return -EINVAL;
> + }
> +
> + hdr->hdr_len = __cpu_to_virtio16(little_endian, hdr_len);
> +
> if (sinfo->gso_type & SKB_GSO_TCP_ECN)
> hdr->gso_type |= VIRTIO_NET_HDR_GSO_ECN;
> } else
> --
> 2.32.0.3.g01195cf9f
