On Wed, Mar 18, 2026 at 12:07 PM Jason Wang <[email protected]> wrote:
>
> On Fri, Mar 13, 2026 at 3:59 PM Xuan Zhuo <[email protected]> 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 | 6 +++++-
> > include/linux/virtio_net.h | 33 +++++++++++++++++++++++++++++----
> > 3 files changed, 35 insertions(+), 6 deletions(-)
> >
> > diff --git a/drivers/net/tun_vnet.h b/drivers/net/tun_vnet.h
> > index a5f93b6c4482..fa5cab9d3e55 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, true)) {
> > + vlan_hlen, true, false)) {
> > 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 72d6a9c6a5a2..7106333ef904 100644
> > --- a/drivers/net/virtio_net.c
> > +++ b/drivers/net/virtio_net.c
> > @@ -3267,8 +3267,12 @@ static int xmit_skb(struct send_queue *sq, struct
> > sk_buff *skb, bool orphan)
> > 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);
> >
> > /* Make sure it's safe to cast between formats */
> > @@ -3288,7 +3292,7 @@ static int xmit_skb(struct send_queue *sq, struct
> > sk_buff *skb, bool orphan)
> >
> > 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)
> > diff --git a/include/linux/virtio_net.h b/include/linux/virtio_net.h
> > index 75dabb763c65..48de4a16a96a 100644
> > --- a/include/linux/virtio_net.h
> > +++ b/include/linux/virtio_net.h
> > @@ -207,6 +207,22 @@ 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 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);
>
> Ok, I think this depends on the logic inside virtio_net_hdr_from_skb()
>
> if (sinfo->gso_type & SKB_GSO_TCPV4)
> hdr->gso_type = VIRTIO_NET_HDR_GSO_TCPV4;
> 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->gso_type = VIRTIO_NET_HDR_GSO_UDP_L4;
> else
> return -EINVAL;
>
> To be more robust, I'd suggest moving it there.
>
> But I have another question, the logic above depends on the headlen is
> correctly set:
>
> /* This is a hint as to how much should be linear. */
> hdr->hdr_len = __cpu_to_virtio16(little_endian,
> skb_headlen(skb));
>
> Is the headlen guaranteed to be correct in all cases (e.g for nested
> setups or dodgy packets?)
Speak too fast, I miss
hdr_len = skb_transport_offset(skb);
This is probably another call to
1) Move virtio_net_set_hdrlen() inside virtio_net_hdr_from_skb() or
2) call virtio_net_set_hdrlen() inside virtio_net_hdr_from_skb().
Thanks
>
> Thanks
>
> > +
> > + 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,
> > @@ -385,7 +401,8 @@ virtio_net_hdr_tnl_from_skb(const struct sk_buff *skb,
> > 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;
> > @@ -394,9 +411,17 @@ 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,
> > - 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)
> > --
> > 2.32.0.3.g01195cf9f
> >
