Re: [PATCH net-next] packet: Account for VLAN_HLEN in csum_start when virtio_net_hdr is enabled

[Willem de Bruijn <willemdebruijn.kernel@gmail.com>]

Mike Pattrick wrote:
> On Thu, Nov 23, 2023 at 4:25 PM Willem de Bruijn
> <willemdebruijn.kernel@gmail.com> wrote:
> >
> > Mike Pattrick wrote:
> > > Af_packet provides checksum offload offsets to usermode applications
> > > through struct virtio_net_hdr when PACKET_VNET_HDR is enabled on the
> > > socket. For skbuffs with a vlan being sent to a SOCK_RAW socket,
> > > af_packet will include the link level header and so csum_start needs
> > > to be adjusted accordingly.
> >
> > Is this patch based on observing an incorrect offset in a workload,
> > or on code inspection?
> 
> Based on an incorrect offset in a workload. The setup involved sending
> vxlan traffic though a veth interface configured with a vlan. The
> vnet_hdr's csum_start value was off by 4, and this problem went away
> when the vlan was removed.
> 
> I'll take another look at this patch.

This is a vlan device on top of a veth device? On which device and at
which point (ingress or egress) are you receiving the packet over the
packet socket?

From a quick glance, in all cases that I see the VLAN tag is kept in
skb->vlan_tci, so is never part of the packet payload.

But checksum offload with VXLAN can be non-trivial on its own. If
type & SKB_GSO_UDP_TUNNEL_CSUM | SKB_GSO_TUNNEL_REMCSUM, say. Then
csum_start will point to the checksum in vxlanhdr.