On Fri, Mar 13, 2026 at 3:59 PM Xuan Zhuo <[email protected]> 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]>
> ---
> 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);
Ok, I think this depends on the logic inside virtio_net_hdr_from_skb()
if (sinfo->gso_type & SKB_GSO_TCPV4)
hdr->gso_type = VIRTIO_NET_HDR_GSO_TCPV4;
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->gso_type = VIRTIO_NET_HDR_GSO_UDP_L4;
else
return -EINVAL;
To be more robust, I'd suggest moving it there.
But I have another question, the logic above depends on the headlen is
correctly set:
/* This is a hint as to how much should be linear. */
hdr->hdr_len = __cpu_to_virtio16(little_endian,
skb_headlen(skb));
Is the headlen guaranteed to be correct in all cases (e.g for nested
setups or dodgy packets?)
Thanks
> +
> + hdr->hdr_len = __cpu_to_virtio16(little_endian, hdr_len);
> +}
> +
> static inline int virtio_net_hdr_from_skb(const struct sk_buff *skb,
> struct virtio_net_hdr *hdr,
> bool little_endian,
> @@ -385,7 +401,8 @@ virtio_net_hdr_tnl_from_skb(const struct sk_buff *skb,
> bool tnl_hdr_negotiated,
> bool little_endian,
> int vlan_hlen,
> - bool has_data_valid)
> + bool has_data_valid,
> + bool feature_hdrlen)
> {
> struct virtio_net_hdr *hdr = (struct virtio_net_hdr *)vhdr;
> unsigned int inner_nh, outer_th;
> @@ -394,9 +411,17 @@ virtio_net_hdr_tnl_from_skb(const struct sk_buff *skb,
>
> tnl_gso_type = skb_shinfo(skb)->gso_type & (SKB_GSO_UDP_TUNNEL |
> SKB_GSO_UDP_TUNNEL_CSUM);
> - if (!tnl_gso_type)
> - return virtio_net_hdr_from_skb(skb, hdr, little_endian,
> - has_data_valid, vlan_hlen);
> + if (!tnl_gso_type) {
> + ret = virtio_net_hdr_from_skb(skb, hdr, little_endian,
> + has_data_valid, vlan_hlen);
> + if (ret)
> + return ret;
> +
> + if (feature_hdrlen && hdr->hdr_len)
> + virtio_net_set_hdrlen(skb, hdr, little_endian);
> +
> + return ret;
> + }
>
> /* Tunnel support not negotiated but skb ask for it. */
> if (!tnl_hdr_negotiated)
> --
> 2.32.0.3.g01195cf9f
>
