Re: [PATCH net-next 3/8] udp: properly complete L4 GRO over UDP tunnel packet

[Paolo Abeni <pabeni@redhat.com>]

On Mon, 2021-03-22 at 09:30 -0400, Willem de Bruijn wrote:
> On Sun, Mar 21, 2021 at 1:01 PM Paolo Abeni <pabeni@redhat.com> wrote:
> > After the previous patch the stack can do L4 UDP aggregation
> > on top of an UDP tunnel.
> > 
> > The current GRO complete code tries frag based aggregation first;
> > in the above scenario will generate corrupted frames.
> > 
> > We need to try first UDP tunnel based aggregation, if the GRO
> > packet requires that. We can use time GRO 'encap_mark' field
> > to track the need GRO complete action. If encap_mark is set,
> > skip the frag_list aggregation.
> > 
> > On tunnel encap GRO complete clear such field, so that an inner
> > frag_list GRO complete could take action.
> > 
> > Signed-off-by: Paolo Abeni <pabeni@redhat.com>
> > ---
> >  net/ipv4/udp_offload.c | 8 +++++++-
> >  net/ipv6/udp_offload.c | 3 ++-
> >  2 files changed, 9 insertions(+), 2 deletions(-)
> > 
> > diff --git a/net/ipv4/udp_offload.c b/net/ipv4/udp_offload.c
> > index 25134a3548e99..54e06b88af69a 100644
> > --- a/net/ipv4/udp_offload.c
> > +++ b/net/ipv4/udp_offload.c
> > @@ -642,6 +642,11 @@ int udp_gro_complete(struct sk_buff *skb, int nhoff,
> >                 skb_shinfo(skb)->gso_type = uh->check ? SKB_GSO_UDP_TUNNEL_CSUM
> >                                         : SKB_GSO_UDP_TUNNEL;
> > 
> > +               /* clear the encap mark, so that inner frag_list gro_complete
> > +                * can take place
> > +                */
> > +               NAPI_GRO_CB(skb)->encap_mark = 0;
> > +
> >                 /* Set encapsulation before calling into inner gro_complete()
> >                  * functions to make them set up the inner offsets.
> >                  */
> > @@ -665,7 +670,8 @@ INDIRECT_CALLABLE_SCOPE int udp4_gro_complete(struct sk_buff *skb, int nhoff)
> >         const struct iphdr *iph = ip_hdr(skb);
> >         struct udphdr *uh = (struct udphdr *)(skb->data + nhoff);
> > 
> > -       if (NAPI_GRO_CB(skb)->is_flist) {
> > +       /* do fraglist only if there is no outer UDP encap (or we already processed it) */
> > +       if (NAPI_GRO_CB(skb)->is_flist && !NAPI_GRO_CB(skb)->encap_mark) {
> 
> Sorry, I don't follow. I thought the point was to avoid fraglist if an
> outer udp tunnel header is present. But the above code clears the mark
> and allows entering the fraglist branch exactly when such a header is
> encountered?

The relevant UDP packet has gone through:

[l2/l3 GRO] -> udp_gro_receive  -> udp_sk(sk)->gro_receive -> [some
more GRO layers] -> udp_gro_receive (again)

The first udp_gro_receive set NAPI_GRO_CB(skb)->encap_mark, the
latter udp_gro_receive set NAPI_GRO_CB(skb)->is_flist.

Then, at GRO complete time:

[l2/l3 GRO] -> udp{4,6}_gro_complete -> udp_sk(sk)->gro_complete ->
[more GRO layers] -> udp{4,6}_gro_complete (again).

In the first udp{4,6}_gro_complete invocation 'encap_mark' is 1, so
with this patch we do the 'udp_sk(sk)->gro_complete' path. In the
second udp{4,6}_gro_complete invocation 'encap_mark' has been cleared
(by udp_gro_complete), so we do the SKB_GSO_FRAGLIST completion.

In case SKB_GSO_FRAGLIST with no UDP tunnel, 'encap_mark' is 0 and we
do the SKB_GSO_FRAGLIST completion.

Another alternative, possibly more readable, would be avoid clearing
'encap_mark' in udp_gro_complete() and replacing the above check with:

	if (NAPI_GRO_CB(skb)->is_flist &&
            (!NAPI_GRO_CB(skb)->encap_mark ||
 	     (NAPI_GRO_CB(skb)->encap_mark && skb->encapsulation))) {

I opted otherwise to simplify the conditional expression.

Please let me know if the above is somewhat more clear and if you have
preferecens between the two options.

Cheers,

Paolo