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

[Mike Pattrick]

On Thu, Nov 23, 2023 at 7:06 PM Willem de Bruijn
<willemdebruijn.kernel@xxxxxxxxx> wrote:
>
> Mike Pattrick wrote:
> > On Thu, Nov 23, 2023 at 4:25 PM Willem de Bruijn
> > <willemdebruijn.kernel@xxxxxxxxx> 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?

Just for maximum clarity I'll include the extracted commands below,
but roughly there is a vlan device on top of a vxlan device on top of
a vlan device on top of a veth, in a namespace.

ip netns add at_ns0
ip netns exec at_ns0 ip link add dev at_vxlan1 type vxlan remote
172.31.1.100 id 0 dstport 4789
ip netns exec at_ns0 ip addr add dev at_vxlan1 10.2.1.1/24
ip netns exec at_ns0 ip link set dev at_vxlan1 mtu 1450 up
ip link add p0 type veth peer name ovs-p0
ethtool -K p0 sg on
ethtool -K p0 tso on
ip link set p0 netns at_ns0
ip link set dev ovs-p0 up
ip netns exec at_ns0 ip addr add "172.31.2.1/24" dev p0
ip netns exec at_ns0 ip link set dev p0 up
ip netns exec at_ns0 ip link add link at_vxlan1 name at_vxlan1.100
type vlan proto 802.1q id 100
ip netns exec at_ns0 ip link set dev at_vxlan1.100 up
ip netns exec at_ns0 ip addr add dev at_vxlan1.100 "10.1.1.1/24"
ip netns exec at_ns0 ip link add link p0 name p0.42 type vlan proto 802.1q id 42
ip netns exec at_ns0 ip link set dev p0.42 up
ip netns exec at_ns0 ip addr add dev p0.42 "172.31.1.1/24"
ip addr add "172.31.1.100/24" dev p0
ip link set dev p0 up
ip netns exec at_ns0 ping 10.1.1.100

An AF_PACKET socket on ovs-p0 receives the incorrect csum_start.
Setting up the same with a geneve tunnel and udpcsum enabled produces
the same result. Removing vlan 100 also yields an incorrect
csum_start. Removing only vlan 42 yields a correct csum_start.

>
> 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.
>