Subject: Re: [PATCH net v4 3/4] virtio-net: correct hdr_len handling for VIRTIO_NET_F_GUEST_HDRLEN

From: "Michael S. Tsirkin" <[email-masked]>
Date: Sun, 9 Nov 2025 16:42:47 -0500

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-masked]>

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