netdev - [PATCH net 2/2] virtio-net: correct hdr_len handling for VIRTIO_NET_F_GUEST

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Message-Id: <20250925022537.91774-3-xuanzhuo@linux.alibaba.com>
Date: Thu, 25 Sep 2025 10:25:37 +0800
From: Xuan Zhuo <xuanzhuo@...ux.alibaba.com>
To: netdev@...r.kernel.org
Cc: "Michael S. Tsirkin" <mst@...hat.com>,
	Jason Wang <jasowang@...hat.com>,
	Xuan Zhuo <xuanzhuo@...ux.alibaba.com>,
	Eugenio Pérez <eperezma@...hat.com>,
	Andrew Lunn <andrew+netdev@...n.ch>,
	"David S. Miller" <davem@...emloft.net>,
	Eric Dumazet <edumazet@...gle.com>,
	Jakub Kicinski <kuba@...nel.org>,
	Paolo Abeni <pabeni@...hat.com>,
	Willem de Bruijn <willemb@...gle.com>,
	Jiri Pirko <jiri@...nulli.us>,
	Alvaro Karsz <alvaro.karsz@...id-run.com>,
	Heng Qi <hengqi@...ux.alibaba.com>,
	virtualization@...ts.linux.dev
Subject: [PATCH net 2/2] 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 <xuanzhuo@...ux.alibaba.com>
---
 include/linux/virtio_net.h | 19 ++++++++++++-------
 1 file changed, 12 insertions(+), 7 deletions(-)

diff --git a/include/linux/virtio_net.h b/include/linux/virtio_net.h
index 20e0584db1dd..4273420a9ff9 100644
--- a/include/linux/virtio_net.h
+++ b/include/linux/virtio_net.h
@@ -217,20 +217,25 @@ 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;
 
-		/* This is a hint as to how much should be linear. */
-		hdr->hdr_len = __cpu_to_virtio16(little_endian,
-						 skb_headlen(skb));
+		hdr_len = skb_transport_offset(skb);
 		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)
+			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)
+			hdr_len += tcp_hdrlen(skb);
+		} else if (sinfo->gso_type & SKB_GSO_UDP_L4) {
 			hdr->gso_type = VIRTIO_NET_HDR_GSO_UDP_L4;
-		else
+			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