virtio-net: correct hdr_len handling for VIRTIO_NET_F_GUEST_HDRLEN

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 <[e-mail]>
Link: https://patch.msgid.link/[e-mail]
Acked-by: Michael S. Tsirkin <[e-mail]>
Signed-off-by: Paolo Abeni <[e-mail]>

drivers/net/tun_vnet.h

···
 
 	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);
 

drivers/net/virtio_net.c

···
 	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);
 
···
 
 	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)

include/linux/virtio_net.h

···
 	return __virtio_net_hdr_to_skb(skb, hdr, little_endian, hdr->gso_type);
 }
 
+/* This function must be called after virtio_net_hdr_from_skb(). */
+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);
+}
+
 static inline int virtio_net_hdr_from_skb(const struct sk_buff *skb,
 					  struct virtio_net_hdr *hdr,
 					  bool little_endian,
···
 			    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;
···
 
 	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)