netdev - [PATCH net v4 2/4] virtio-net: Ensure hdr_len is not set unless the header is forwarded to the device.

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Message-Id: <20251029030913.20423-3-xuanzhuo@linux.alibaba.com>
Date: Wed, 29 Oct 2025 11:09:11 +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>,
	Heng Qi <hengqi@...ux.alibaba.com>,
	Willem de Bruijn <willemb@...gle.com>,
	Jiri Pirko <jiri@...nulli.us>,
	Alvaro Karsz <alvaro.karsz@...id-run.com>,
	virtualization@...ts.linux.dev
Subject: [PATCH net v4 2/4] virtio-net: Ensure hdr_len is not set unless the header is forwarded to the device.

Although `virtio_net_hdr_from_skb` is used in several places outside of
`virtio-net.c`, the `hdr_len` field is only utilized by the device
according to the specification. Therefore, we do not need to set
`hdr_len` unless the header is actually passed to the device.

Signed-off-by: Xuan Zhuo <xuanzhuo@...ux.alibaba.com>
---
 include/linux/virtio_net.h | 11 ++++++++---
 1 file changed, 8 insertions(+), 3 deletions(-)

diff --git a/include/linux/virtio_net.h b/include/linux/virtio_net.h
index 4d1780848d0e..710ae0d2d336 100644
--- a/include/linux/virtio_net.h
+++ b/include/linux/virtio_net.h
@@ -218,9 +218,14 @@ 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);
 
-		/* This is a hint as to how much should be linear. */
-		hdr->hdr_len = __cpu_to_virtio16(little_endian,
-						 skb_headlen(skb));
+		/* In certain code paths (such as the af_packet.c receive path),
+		 * this function may be called without a transport header.
+		 * In this case, we do not need to set the hdr_len.
+		 */
+		if (skb_transport_header_was_set(skb))
+			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)
-- 
2.32.0.3.g01195cf9f