On Wed, Nov 19, 2025 at 01:55:21PM +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]>
> ---
> drivers/net/tun_vnet.h | 2 +-
> drivers/net/virtio_net.c | 8 ++++--
> include/linux/virtio_net.h | 58 ++++++++++++++++++++++++++++++--------
> 3 files changed, 54 insertions(+), 14 deletions(-)
>
> diff --git a/drivers/net/tun_vnet.h b/drivers/net/tun_vnet.h
> index 81662328b2c7..b06aa6f2aade 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)) {
> + false, vlan_hlen)) {
> 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 0369dda5ed60..a62acfaf631b 100644
> --- a/drivers/net/virtio_net.c
> +++ b/drivers/net/virtio_net.c
> @@ -3317,9 +3317,12 @@ static int xmit_skb(struct send_queue *sq, struct
> sk_buff *skb, bool orphan)
> const unsigned char *dest = ((struct ethhdr *)skb->data)->h_dest;
> struct virtnet_info *vi = sq->vq->vdev->priv;
> struct virtio_net_hdr_v1_hash_tunnel *hdr;
> - int num_sg;
> unsigned hdr_len = vi->hdr_len;
> + bool guest_hdrlen;
> bool can_push;
> + int num_sg;
> +
> + guest_hdrlen = virtio_has_feature(vi->vdev, VIRTIO_NET_F_GUEST_HDRLEN);
>
> pr_debug("%s: xmit %p %pM\n", vi->dev->name, skb, dest);
>
> @@ -3339,7 +3342,8 @@ static int xmit_skb(struct send_queue *sq, struct
> sk_buff *skb, bool orphan)
> hdr = &skb_vnet_common_hdr(skb)->tnl_hdr;
>
> if (virtio_net_hdr_tnl_from_skb(skb, hdr, vi->tx_tnl,
> - virtio_is_little_endian(vi->vdev), 0))
> + virtio_is_little_endian(vi->vdev),
> + guest_hdrlen, 0))
> return -EPROTO;
>
> if (vi->mergeable_rx_bufs)
> diff --git a/include/linux/virtio_net.h b/include/linux/virtio_net.h
> index b673c31569f3..ee960ec9a35e 100644
> --- a/include/linux/virtio_net.h
> +++ b/include/linux/virtio_net.h
> @@ -207,20 +207,40 @@ 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 int virtio_net_hdr_from_skb(const struct sk_buff *skb,
> - struct virtio_net_hdr *hdr,
> - bool little_endian,
> - bool has_data_valid,
> - int vlan_hlen)
> +static inline void virtio_net_set_hdrlen(const struct sk_buff *skb,
> + struct virtio_net_hdr *hdr,
> + bool little_endian,
> + bool guest_hdrlen)
> +{
> + u16 hdr_len;
> +
> + if (guest_hdrlen) {
> + 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);
> + } else {
> + /* This is a hint as to how much should be linear. */
> + hdr_len = skb_headlen(skb);
> + }
> +
> + hdr->hdr_len = __cpu_to_virtio16(little_endian, hdr_len);
> +}
So this adds code that is broken for tunnels, then the follow up patch
fixes it up. I don't feel it's worth it, just squash the two patches
together please.
> +
> +static inline int __virtio_net_hdr_from_skb(const struct sk_buff *skb,
> + struct virtio_net_hdr *hdr,
> + bool little_endian,
> + bool has_data_valid,
> + bool guest_hdrlen,
> + int vlan_hlen)
> {
> memset(hdr, 0, sizeof(*hdr)); /* no info leak */
>
> if (skb_is_gso(skb)) {
> struct skb_shared_info *sinfo = skb_shinfo(skb);
>
> - /* This is a hint as to how much should be linear. */
> - hdr->hdr_len = __cpu_to_virtio16(little_endian,
> - skb_headlen(skb));
> hdr->gso_size = __cpu_to_virtio16(little_endian,
> sinfo->gso_size);
> if (sinfo->gso_type & SKB_GSO_TCPV4)
> @@ -231,6 +251,10 @@ static inline int virtio_net_hdr_from_skb(const struct
> sk_buff *skb,
> hdr->gso_type = VIRTIO_NET_HDR_GSO_UDP_L4;
> else
> return -EINVAL;
> +
> + virtio_net_set_hdrlen(skb, hdr, little_endian,
> + guest_hdrlen);
> +
> if (sinfo->gso_type & SKB_GSO_TCP_ECN)
> hdr->gso_type |= VIRTIO_NET_HDR_GSO_ECN;
> } else
> @@ -250,6 +274,16 @@ static inline int virtio_net_hdr_from_skb(const struct
> sk_buff *skb,
> return 0;
> }
>
> +static inline int virtio_net_hdr_from_skb(const struct sk_buff *skb,
> + struct virtio_net_hdr *hdr,
> + bool little_endian,
> + bool has_data_valid,
> + int vlan_hlen)
> +{
> + return __virtio_net_hdr_from_skb(skb, hdr, little_endian,
> + has_data_valid, false, vlan_hlen);
> +}
> +
> static inline unsigned int virtio_l3min(bool is_ipv6)
> {
> return is_ipv6 ? sizeof(struct ipv6hdr) : sizeof(struct iphdr);
> @@ -384,6 +418,7 @@ virtio_net_hdr_tnl_from_skb(const struct sk_buff *skb,
> struct virtio_net_hdr_v1_hash_tunnel *vhdr,
> bool tnl_hdr_negotiated,
> bool little_endian,
> + bool guest_hdrlen,
> int vlan_hlen)
> {
> struct virtio_net_hdr *hdr = (struct virtio_net_hdr *)vhdr;
> @@ -394,8 +429,8 @@ 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, false,
> - vlan_hlen);
> + return __virtio_net_hdr_from_skb(skb, hdr, little_endian, false,
> + guest_hdrlen, vlan_hlen);
>
> /* Tunnel support not negotiated but skb ask for it. */
> if (!tnl_hdr_negotiated)
> @@ -408,7 +443,8 @@ virtio_net_hdr_tnl_from_skb(const struct sk_buff *skb,
>
> /* Let the basic parsing deal with plain GSO features. */
> skb_shinfo(skb)->gso_type &= ~tnl_gso_type;
> - ret = virtio_net_hdr_from_skb(skb, hdr, true, false, vlan_hlen);
> + ret = __virtio_net_hdr_from_skb(skb, hdr, true, false,
> + guest_hdrlen, vlan_hlen);
> skb_shinfo(skb)->gso_type |= tnl_gso_type;
> if (ret)
> return ret;
> --
> 2.32.0.3.g01195cf9f
