Re: [PATCH net v10 1/2] virtio-net: correct hdr_len handling for VIRTIO_NET_F_GUEST_HDRLEN

[Paolo Abeni <pabeni@redhat.com>]

On 3/13/26 8:59 AM, 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 <xuanzhuo@linux.alibaba.com>
> ---
>  drivers/net/tun_vnet.h     |  2 +-
>  drivers/net/virtio_net.c   |  6 +++++-
>  include/linux/virtio_net.h | 33 +++++++++++++++++++++++++++++----
>  3 files changed, 35 insertions(+), 6 deletions(-)
> 
> diff --git a/drivers/net/tun_vnet.h b/drivers/net/tun_vnet.h
> index a5f93b6c4482..fa5cab9d3e55 100644
> --- a/drivers/net/tun_vnet.h
> +++ b/drivers/net/tun_vnet.h
> @@ -244,7 +244,7 @@ tun_vnet_hdr_tnl_from_skb(unsigned int flags,
>  
>  	if (virtio_net_hdr_tnl_from_skb(skb, tnl_hdr, has_tnl_offload,
>  					tun_vnet_is_little_endian(flags),
> -					vlan_hlen, true)) {
> +					vlan_hlen, true, false)) {
>  		struct virtio_net_hdr_v1 *hdr = &tnl_hdr->hash_hdr.hdr;
>  		struct skb_shared_info *sinfo = skb_shinfo(skb);
>  
> diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
> index 72d6a9c6a5a2..7106333ef904 100644
> --- a/drivers/net/virtio_net.c
> +++ b/drivers/net/virtio_net.c
> @@ -3267,8 +3267,12 @@ static int xmit_skb(struct send_queue *sq, struct sk_buff *skb, bool orphan)
>  	struct virtio_net_hdr_v1_hash_tunnel *hdr;
>  	int num_sg;
>  	unsigned hdr_len = vi->hdr_len;
> +	bool feature_hdrlen;
>  	bool can_push;
>  
> +	feature_hdrlen = virtio_has_feature(vi->vdev,
> +					    VIRTIO_NET_F_GUEST_HDRLEN);
> +
>  	pr_debug("%s: xmit %p %pM\n", vi->dev->name, skb, dest);
>  
>  	/* Make sure it's safe to cast between formats */
> @@ -3288,7 +3292,7 @@ static int xmit_skb(struct send_queue *sq, struct sk_buff *skb, bool orphan)
>  
>  	if (virtio_net_hdr_tnl_from_skb(skb, hdr, vi->tx_tnl,
>  					virtio_is_little_endian(vi->vdev), 0,
> -					false))
> +					false, feature_hdrlen))
>  		return -EPROTO;
>  
>  	if (vi->mergeable_rx_bufs)
> diff --git a/include/linux/virtio_net.h b/include/linux/virtio_net.h
> index 75dabb763c65..48de4a16a96a 100644
> --- a/include/linux/virtio_net.h
> +++ b/include/linux/virtio_net.h
> @@ -207,6 +207,22 @@ static inline int virtio_net_hdr_to_skb(struct sk_buff *skb,
>  	return __virtio_net_hdr_to_skb(skb, hdr, little_endian, hdr->gso_type);
>  }
>  
> +static inline void virtio_net_set_hdrlen(const struct sk_buff *skb,
> +					 struct virtio_net_hdr *hdr,
> +					 bool little_endian)
> +{
> +	u16 hdr_len;
> +
> +	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);
> +
> +	hdr->hdr_len = __cpu_to_virtio16(little_endian, hdr_len);
> +}

The series LGTM, let's wait for Jason and or Michael.

If another revision is needed, please consider factoring out a
__virtio_net_set_hdrlen() helper taking an additional explicit
transport_offset argument, to deduplicate a bit the code between patch
1/2 and 2.2. Not a blocked anyway for me.

/P