[PATCH net v5 0/2] virtio-net: fix for VIRTIO_NET_F_GUEST_HDRLEN

[PATCH net v5 0/2] virtio-net: fix for VIRTIO_NET_F_GUEST_HDRLEN

[Xuan Zhuo]

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 path set fixes that issue.

As discussed in [0], this version checks the VIRTIO_NET_F_GUEST_HDRLEN is
negotiated.

[0]: http://lore.kernel.org/all/20251029030913.20423-1-xuanzhuo@linux.alibaba.com

Xuan Zhuo (2):
  virtio-net: correct hdr_len handling for VIRTIO_NET_F_GUEST_HDRLEN
  virtio-net: correct hdr_len handling for tunnel gso

 arch/um/drivers/vector_transports.c |  1 +
 drivers/net/tun_vnet.h              |  4 +--
 drivers/net/virtio_net.c            |  9 +++++--
 include/linux/virtio_net.h          | 40 +++++++++++++++++++++++------
 net/packet/af_packet.c              |  5 ++--
 5 files changed, 45 insertions(+), 14 deletions(-)

--
2.32.0.3.g01195cf9f

[PATCH net v5 1/2] virtio-net: correct hdr_len handling for VIRTIO_NET_F_GUEST_HDRLEN

[Xuan Zhuo]

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@linux.alibaba.com>
---
 arch/um/drivers/vector_transports.c |  1 +
 drivers/net/tun_vnet.h              |  4 ++--
 drivers/net/virtio_net.c            |  9 +++++++--
 include/linux/virtio_net.h          | 26 +++++++++++++++++++++-----
 net/packet/af_packet.c              |  5 +++--
 5 files changed, 34 insertions(+), 11 deletions(-)

diff --git a/arch/um/drivers/vector_transports.c b/arch/um/drivers/vector_transports.c
index 0794d23f07cb..03c5baa1d0c1 100644
--- a/arch/um/drivers/vector_transports.c
+++ b/arch/um/drivers/vector_transports.c
@@ -121,6 +121,7 @@ static int raw_form_header(uint8_t *header,
 		vheader,
 		virtio_legacy_is_little_endian(),
 		false,
+		false,
 		0
 	);
 
diff --git a/drivers/net/tun_vnet.h b/drivers/net/tun_vnet.h
index 81662328b2c7..0d376bc70dd7 100644
--- a/drivers/net/tun_vnet.h
+++ b/drivers/net/tun_vnet.h
@@ -214,7 +214,7 @@ static inline int tun_vnet_hdr_from_skb(unsigned int flags,
 
 	if (virtio_net_hdr_from_skb(skb, hdr,
 				    tun_vnet_is_little_endian(flags), true,
-				    vlan_hlen)) {
+				    false, vlan_hlen)) {
 		struct skb_shared_info *sinfo = skb_shinfo(skb);
 
 		if (net_ratelimit()) {
@@ -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..b335c88a8cd6 100644
--- a/drivers/net/virtio_net.c
+++ b/drivers/net/virtio_net.c
@@ -3317,9 +3317,13 @@ 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 hdrlen_negotiated;
 	bool can_push;
+	int num_sg;
+
+	hdrlen_negotiated = virtio_has_feature(vi->vdev,
+					       VIRTIO_NET_F_GUEST_HDRLEN);
 
 	pr_debug("%s: xmit %p %pM\n", vi->dev->name, skb, dest);
 
@@ -3339,7 +3343,8 @@ 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),
+					hdrlen_negotiated, 0))
 		return -EPROTO;
 
 	if (vi->mergeable_rx_bufs)
diff --git a/include/linux/virtio_net.h b/include/linux/virtio_net.h
index b673c31569f3..3cd8b2ebc197 100644
--- a/include/linux/virtio_net.h
+++ b/include/linux/virtio_net.h
@@ -211,16 +211,15 @@ 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 hdrlen_negotiated,
 					  int vlan_hlen)
 {
 	memset(hdr, 0, sizeof(*hdr));   /* no info leak */
 
 	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->gso_size = __cpu_to_virtio16(little_endian,
 						  sinfo->gso_size);
 		if (sinfo->gso_type & SKB_GSO_TCPV4)
@@ -231,6 +230,21 @@ 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;
+
+		if (hdrlen_negotiated) {
+			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);
+
 		if (sinfo->gso_type & SKB_GSO_TCP_ECN)
 			hdr->gso_type |= VIRTIO_NET_HDR_GSO_ECN;
 	} else
@@ -384,6 +398,7 @@ 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 hdrlen_negotiated,
 			    int vlan_hlen)
 {
 	struct virtio_net_hdr *hdr = (struct virtio_net_hdr *)vhdr;
@@ -395,7 +410,7 @@ virtio_net_hdr_tnl_from_skb(const struct sk_buff *skb,
 						    SKB_GSO_UDP_TUNNEL_CSUM);
 	if (!tnl_gso_type)
 		return virtio_net_hdr_from_skb(skb, hdr, little_endian, false,
-					       vlan_hlen);
+					       hdrlen_negotiated, vlan_hlen);
 
 	/* Tunnel support not negotiated but skb ask for it. */
 	if (!tnl_hdr_negotiated)
@@ -408,7 +423,8 @@ 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, hdrlen_negotiated,
+				      vlan_hlen);
 	skb_shinfo(skb)->gso_type |= tnl_gso_type;
 	if (ret)
 		return ret;
diff --git a/net/packet/af_packet.c b/net/packet/af_packet.c
index 173e6edda08f..6982f4ab1c73 100644
--- a/net/packet/af_packet.c
+++ b/net/packet/af_packet.c
@@ -2093,7 +2093,8 @@ static int packet_rcv_vnet(struct msghdr *msg, const struct sk_buff *skb,
 		return -EINVAL;
 	*len -= vnet_hdr_sz;
 
-	if (virtio_net_hdr_from_skb(skb, (struct virtio_net_hdr *)&vnet_hdr, vio_le(), true, 0))
+	if (virtio_net_hdr_from_skb(skb, (struct virtio_net_hdr *)&vnet_hdr,
+				    vio_le(), true, false, 0))
 		return -EINVAL;
 
 	return memcpy_to_msg(msg, (void *)&vnet_hdr, vnet_hdr_sz);
@@ -2361,7 +2362,7 @@ static int tpacket_rcv(struct sk_buff *skb, struct net_device *dev,
 	if (vnet_hdr_sz &&
 	    virtio_net_hdr_from_skb(skb, h.raw + macoff -
 				    sizeof(struct virtio_net_hdr),
-				    vio_le(), true, 0)) {
+				    vio_le(), true, false, 0)) {
 		if (po->tp_version == TPACKET_V3)
 			prb_clear_blk_fill_status(&po->rx_ring);
 		goto drop_n_account;
-- 
2.32.0.3.g01195cf9f

Re: [PATCH net v5 1/2] virtio-net: correct hdr_len handling for VIRTIO_NET_F_GUEST_HDRLEN

[Michael S. Tsirkin]

On Tue, Nov 11, 2025 at 07:12:11PM +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 <xuanzhuo@linux.alibaba.com>
> ---
>  arch/um/drivers/vector_transports.c |  1 +
>  drivers/net/tun_vnet.h              |  4 ++--
>  drivers/net/virtio_net.c            |  9 +++++++--
>  include/linux/virtio_net.h          | 26 +++++++++++++++++++++-----
>  net/packet/af_packet.c              |  5 +++--
>  5 files changed, 34 insertions(+), 11 deletions(-)
> 
> diff --git a/arch/um/drivers/vector_transports.c b/arch/um/drivers/vector_transports.c
> index 0794d23f07cb..03c5baa1d0c1 100644
> --- a/arch/um/drivers/vector_transports.c
> +++ b/arch/um/drivers/vector_transports.c
> @@ -121,6 +121,7 @@ static int raw_form_header(uint8_t *header,
>  		vheader,
>  		virtio_legacy_is_little_endian(),
>  		false,
> +		false,
>  		0
>  	);
>  
> diff --git a/drivers/net/tun_vnet.h b/drivers/net/tun_vnet.h
> index 81662328b2c7..0d376bc70dd7 100644
> --- a/drivers/net/tun_vnet.h
> +++ b/drivers/net/tun_vnet.h
> @@ -214,7 +214,7 @@ static inline int tun_vnet_hdr_from_skb(unsigned int flags,
>  
>  	if (virtio_net_hdr_from_skb(skb, hdr,
>  				    tun_vnet_is_little_endian(flags), true,
> -				    vlan_hlen)) {
> +				    false, vlan_hlen)) {
>  		struct skb_shared_info *sinfo = skb_shinfo(skb);
>  
>  		if (net_ratelimit()) {
> @@ -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..b335c88a8cd6 100644
> --- a/drivers/net/virtio_net.c
> +++ b/drivers/net/virtio_net.c
> @@ -3317,9 +3317,13 @@ 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 hdrlen_negotiated;
>  	bool can_push;
> +	int num_sg;
> +
> +	hdrlen_negotiated = virtio_has_feature(vi->vdev,
> +					       VIRTIO_NET_F_GUEST_HDRLEN);
>  
>  	pr_debug("%s: xmit %p %pM\n", vi->dev->name, skb, dest);
>  
> @@ -3339,7 +3343,8 @@ 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),
> +					hdrlen_negotiated, 0))
>  		return -EPROTO;
>  
>  	if (vi->mergeable_rx_bufs)
> diff --git a/include/linux/virtio_net.h b/include/linux/virtio_net.h
> index b673c31569f3..3cd8b2ebc197 100644
> --- a/include/linux/virtio_net.h
> +++ b/include/linux/virtio_net.h
> @@ -211,16 +211,15 @@ 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 hdrlen_negotiated,
>  					  int vlan_hlen)

Took me a while to figure out why does tun pass false here.

The reason is that this flag is really only dealing with guest
hdrlen.  so how about guest_hdrlen to mirror spec
or if you like xmit_hdrlen?





>  {
>  	memset(hdr, 0, sizeof(*hdr));   /* no info leak */
>  
>  	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->gso_size = __cpu_to_virtio16(little_endian,
>  						  sinfo->gso_size);
>  		if (sinfo->gso_type & SKB_GSO_TCPV4)
> @@ -231,6 +230,21 @@ 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;
> +
> +		if (hdrlen_negotiated) {
> +			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);
> +
>  		if (sinfo->gso_type & SKB_GSO_TCP_ECN)
>  			hdr->gso_type |= VIRTIO_NET_HDR_GSO_ECN;
>  	} else
> @@ -384,6 +398,7 @@ 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 hdrlen_negotiated,
>  			    int vlan_hlen)
>  {
>  	struct virtio_net_hdr *hdr = (struct virtio_net_hdr *)vhdr;
> @@ -395,7 +410,7 @@ virtio_net_hdr_tnl_from_skb(const struct sk_buff *skb,
>  						    SKB_GSO_UDP_TUNNEL_CSUM);
>  	if (!tnl_gso_type)
>  		return virtio_net_hdr_from_skb(skb, hdr, little_endian, false,
> -					       vlan_hlen);
> +					       hdrlen_negotiated, vlan_hlen);
>  
>  	/* Tunnel support not negotiated but skb ask for it. */
>  	if (!tnl_hdr_negotiated)
> @@ -408,7 +423,8 @@ 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, hdrlen_negotiated,
> +				      vlan_hlen);
>  	skb_shinfo(skb)->gso_type |= tnl_gso_type;
>  	if (ret)
>  		return ret;
> diff --git a/net/packet/af_packet.c b/net/packet/af_packet.c
> index 173e6edda08f..6982f4ab1c73 100644
> --- a/net/packet/af_packet.c
> +++ b/net/packet/af_packet.c
> @@ -2093,7 +2093,8 @@ static int packet_rcv_vnet(struct msghdr *msg, const struct sk_buff *skb,
>  		return -EINVAL;
>  	*len -= vnet_hdr_sz;
>  
> -	if (virtio_net_hdr_from_skb(skb, (struct virtio_net_hdr *)&vnet_hdr, vio_le(), true, 0))
> +	if (virtio_net_hdr_from_skb(skb, (struct virtio_net_hdr *)&vnet_hdr,
> +				    vio_le(), true, false, 0))
>  		return -EINVAL;
>  
>  	return memcpy_to_msg(msg, (void *)&vnet_hdr, vnet_hdr_sz);
> @@ -2361,7 +2362,7 @@ static int tpacket_rcv(struct sk_buff *skb, struct net_device *dev,
>  	if (vnet_hdr_sz &&
>  	    virtio_net_hdr_from_skb(skb, h.raw + macoff -
>  				    sizeof(struct virtio_net_hdr),
> -				    vio_le(), true, 0)) {
> +				    vio_le(), true, false, 0)) {
>  		if (po->tp_version == TPACKET_V3)
>  			prb_clear_blk_fill_status(&po->rx_ring);
>  		goto drop_n_account;
> -- 
> 2.32.0.3.g01195cf9f

Re: [PATCH net v5 1/2] virtio-net: correct hdr_len handling for VIRTIO_NET_F_GUEST_HDRLEN

[Paolo Abeni]

On 11/11/25 12:12 PM, 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 <xuanzhuo@linux.alibaba.com>

IMHO this looks like more a new feature - namely,
VIRTIO_NET_F_GUEST_HDRLEN support - than a fix.

[...]
> @@ -2361,7 +2362,7 @@ static int tpacket_rcv(struct sk_buff *skb, struct net_device *dev,
>  	if (vnet_hdr_sz &&
>  	    virtio_net_hdr_from_skb(skb, h.raw + macoff -
>  				    sizeof(struct virtio_net_hdr),
> -				    vio_le(), true, 0)) {
> +				    vio_le(), true, false, 0)) {
>  		if (po->tp_version == TPACKET_V3)
>  			prb_clear_blk_fill_status(&po->rx_ring);
>  		goto drop_n_account;
To reduce the diffstat, what about creating a __virtio_net_hdr_from_skb()
variant (please find a better name) allowing the extra `hdrlen_negotiated`
argument, define virtio_net_hdr_from_skb() as a wrapper of such helper
withthe extra arg == false, and use the helper in the few places that
really could use hdrlen?

Re: [PATCH net v5 1/2] virtio-net: correct hdr_len handling for VIRTIO_NET_F_GUEST_HDRLEN

[Michael S. Tsirkin]

On Thu, Nov 13, 2025 at 03:39:35PM +0100, Paolo Abeni wrote:
> On 11/11/25 12:12 PM, 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 <xuanzhuo@linux.alibaba.com>
> 
> IMHO this looks like more a new feature - namely,
> VIRTIO_NET_F_GUEST_HDRLEN support - than a fix.


I mean if guest negotiates VIRTIO_NET_F_GUEST_HDRLEN but the header
length is wrong then yes it is broken and this is a fix.


> [...]
> > @@ -2361,7 +2362,7 @@ static int tpacket_rcv(struct sk_buff *skb, struct net_device *dev,
> >  	if (vnet_hdr_sz &&
> >  	    virtio_net_hdr_from_skb(skb, h.raw + macoff -
> >  				    sizeof(struct virtio_net_hdr),
> > -				    vio_le(), true, 0)) {
> > +				    vio_le(), true, false, 0)) {
> >  		if (po->tp_version == TPACKET_V3)
> >  			prb_clear_blk_fill_status(&po->rx_ring);
> >  		goto drop_n_account;
> To reduce the diffstat, what about creating a __virtio_net_hdr_from_skb()
> variant (please find a better name) allowing the extra `hdrlen_negotiated`
> argument, define virtio_net_hdr_from_skb() as a wrapper of such helper
> withthe extra arg == false, and use the helper in the few places that
> really could use hdrlen?

[PATCH net v5 2/2] virtio-net: correct hdr_len handling for tunnel gso

[Xuan Zhuo]

The commit a2fb4bc4e2a6a03 ("net: implement virtio helpers to handle UDP
GSO tunneling.") introduces support for the UDP GSO tunnel feature in
virtio-net.

The virtio spec says:

    If the \field{gso_type} has the VIRTIO_NET_HDR_GSO_UDP_TUNNEL_IPV4 bit or
    VIRTIO_NET_HDR_GSO_UDP_TUNNEL_IPV6 bit set, \field{hdr_len} accounts for
    all the headers up to and including the inner transport.

The commit did not update the hdr_len to include the inner transport.

I observed that the "hdr_len" is 116 for this packet:

    17:36:18.241105 52:55:00:d1:27:0a > 2e:2c:df:46:a9:e1, ethertype IPv4 (0x0800), length 2912: (tos 0x0, ttl 64, id 45197, offset 0, flags [none], proto UDP (17), length 2898)
        192.168.122.100.50613 > 192.168.122.1.4789: [bad udp cksum 0x8106 -> 0x26a0!] VXLAN, flags [I] (0x08), vni 1
    fa:c3:ba:82:05:ee > ce:85:0c:31:77:e5, ethertype IPv4 (0x0800), length 2862: (tos 0x0, ttl 64, id 14678, offset 0, flags [DF], proto TCP (6), length 2848)
        192.168.3.1.49880 > 192.168.3.2.9898: Flags [P.], cksum 0x9266 (incorrect -> 0xaa20), seq 515667:518463, ack 1, win 64, options [nop,nop,TS val 2990048824 ecr 2798801412], length 2796

116 = 14(mac) + 20(ip) + 8(udp) + 8(vxlan) + 14(inner mac) + 20(inner ip) + 32(innner tcp)

Fixes: a2fb4bc4e2a6a03 ("net: implement virtio helpers to handle UDP GSO tunneling.")
Signed-off-by: Xuan Zhuo <xuanzhuo@linux.alibaba.com>
---
 include/linux/virtio_net.h | 24 ++++++++++++++++--------
 1 file changed, 16 insertions(+), 8 deletions(-)

diff --git a/include/linux/virtio_net.h b/include/linux/virtio_net.h
index 3cd8b2ebc197..432b17979d17 100644
--- a/include/linux/virtio_net.h
+++ b/include/linux/virtio_net.h
@@ -232,12 +232,23 @@ static inline int virtio_net_hdr_from_skb(const struct sk_buff *skb,
 			return -EINVAL;
 
 		if (hdrlen_negotiated) {
-			hdr_len = skb_transport_offset(skb);
+			if (sinfo->gso_type & (SKB_GSO_UDP_TUNNEL |
+					       SKB_GSO_UDP_TUNNEL_CSUM)) {
+				hdr_len = skb_inner_transport_offset(skb);
+
+				if (hdr->gso_type == VIRTIO_NET_HDR_GSO_UDP_L4)
+					hdr_len += sizeof(struct udphdr);
+				else
+					hdr_len += inner_tcp_hdrlen(skb);
+			} else {
+				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);
+			}
 
-			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);
@@ -421,11 +432,8 @@ virtio_net_hdr_tnl_from_skb(const struct sk_buff *skb,
         vhdr->hash_hdr.hash_report = 0;
         vhdr->hash_hdr.padding = 0;
 
-	/* 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, hdrlen_negotiated,
 				      vlan_hlen);
-	skb_shinfo(skb)->gso_type |= tnl_gso_type;
 	if (ret)
 		return ret;
 
-- 
2.32.0.3.g01195cf9f

Re: [PATCH net v5 2/2] virtio-net: correct hdr_len handling for tunnel gso

[Paolo Abeni]

On 11/11/25 12:12 PM, Xuan Zhuo wrote:
> The commit a2fb4bc4e2a6a03 ("net: implement virtio helpers to handle UDP
> GSO tunneling.") introduces support for the UDP GSO tunnel feature in
> virtio-net.
> 
> The virtio spec says:
> 
>     If the \field{gso_type} has the VIRTIO_NET_HDR_GSO_UDP_TUNNEL_IPV4 bit or
>     VIRTIO_NET_HDR_GSO_UDP_TUNNEL_IPV6 bit set, \field{hdr_len} accounts for
>     all the headers up to and including the inner transport.
> 
> The commit did not update the hdr_len to include the inner transport.
> 
> I observed that the "hdr_len" is 116 for this packet:
> 
>     17:36:18.241105 52:55:00:d1:27:0a > 2e:2c:df:46:a9:e1, ethertype IPv4 (0x0800), length 2912: (tos 0x0, ttl 64, id 45197, offset 0, flags [none], proto UDP (17), length 2898)
>         192.168.122.100.50613 > 192.168.122.1.4789: [bad udp cksum 0x8106 -> 0x26a0!] VXLAN, flags [I] (0x08), vni 1
>     fa:c3:ba:82:05:ee > ce:85:0c:31:77:e5, ethertype IPv4 (0x0800), length 2862: (tos 0x0, ttl 64, id 14678, offset 0, flags [DF], proto TCP (6), length 2848)
>         192.168.3.1.49880 > 192.168.3.2.9898: Flags [P.], cksum 0x9266 (incorrect -> 0xaa20), seq 515667:518463, ack 1, win 64, options [nop,nop,TS val 2990048824 ecr 2798801412], length 2796
> 
> 116 = 14(mac) + 20(ip) + 8(udp) + 8(vxlan) + 14(inner mac) + 20(inner ip) + 32(innner tcp)
> 
> Fixes: a2fb4bc4e2a6a03 ("net: implement virtio helpers to handle UDP GSO tunneling.")
> Signed-off-by: Xuan Zhuo <xuanzhuo@linux.alibaba.com>
> ---
>  include/linux/virtio_net.h | 24 ++++++++++++++++--------
>  1 file changed, 16 insertions(+), 8 deletions(-)
> 
> diff --git a/include/linux/virtio_net.h b/include/linux/virtio_net.h
> index 3cd8b2ebc197..432b17979d17 100644
> --- a/include/linux/virtio_net.h
> +++ b/include/linux/virtio_net.h
> @@ -232,12 +232,23 @@ static inline int virtio_net_hdr_from_skb(const struct sk_buff *skb,
>  			return -EINVAL;
>  
>  		if (hdrlen_negotiated) {
> -			hdr_len = skb_transport_offset(skb);
> +			if (sinfo->gso_type & (SKB_GSO_UDP_TUNNEL |
> +					       SKB_GSO_UDP_TUNNEL_CSUM)) {

I'm personally not a huge fan of adding UDP tunnel specific check to the
generic code, did you tried something along the lines suggested here:

https://lore.kernel.org/netdev/CAF6piCLkv6kFqoq7OQfJ=Su9AVHSQ9J7DzaumOSf5xuf9w-kyA@mail.gmail.com/

?

Thanks,

Paolo