Re: [PATCH net-next v2 4/8] udp: never accept GSO_FRAGLIST packets

[Paolo Abeni <pabeni@redhat.com>]

On Mon, 2021-03-29 at 08:31 -0400, Willem de Bruijn wrote:
> On Mon, Mar 29, 2021 at 4:14 AM Paolo Abeni <pabeni@redhat.com> wrote:
> > On Fri, 2021-03-26 at 14:15 -0400, Willem de Bruijn wrote:
> > > On Thu, Mar 25, 2021 at 1:24 PM Paolo Abeni <pabeni@redhat.com> wrote:
> > > > Currently the UDP protocol delivers GSO_FRAGLIST packets to
> > > > the sockets without the expected segmentation.
> > > > 
> > > > This change addresses the issue introducing and maintaining
> > > > a couple of new fields to explicitly accept SKB_GSO_UDP_L4
> > > > or GSO_FRAGLIST packets. Additionally updates  udp_unexpected_gso()
> > > > accordingly.
> > > > 
> > > > UDP sockets enabling UDP_GRO stil keep accept_udp_fraglist
> > > > zeroed.
> > > > 
> > > > v1 -> v2:
> > > >  - use 2 bits instead of a whole GSO bitmask (Willem)
> > > > 
> > > > Fixes: 9fd1ff5d2ac7 ("udp: Support UDP fraglist GRO/GSO.")
> > > > Signed-off-by: Paolo Abeni <pabeni@redhat.com>
> > > 
> > > This looks good to me in principle, thanks for the revision.
> > > 
> > > I hadn't fully appreciated that gro_enabled implies accept_udp_l4, but
> > > not necessarily vice versa.
> > > 
> > > It is equivalent to (accept_udp_l4 && !up->gro_receive), right?
> > 
> > In this series, yes.
> > 
> > > Could the extra bit be avoided with
> > > 
> > > "
> > > +      /* Prefer fraglist GRO unless target is a socket with UDP_GRO,
> > > +       * which requires all but last segments to be of same gso_size,
> > > passed in cmsg */
> > >         if (skb->dev->features & NETIF_F_GRO_FRAGLIST)
> > > -                NAPI_GRO_CB(skb)->is_flist = sk ? !udp_sk(sk)->gro_enabled: 1;
> > > +               NAPI_GRO_CB(skb)->is_flist = sk ?
> > > (!udp_sk(sk)->gro_enabled || udp_sk(sk)->accept_udp_fraglist) : 1;
> > 
> > This is not ovious at all to me.
> > 
> > > +     /* Apply transport layer GRO if forwarding is enabled or the
> > > flow lands at a local socket */
> > >        if ((!sk && (skb->dev->features & NETIF_F_GRO_UDP_FWD)) ||
> > >             (sk && udp_sk(sk)->gro_enabled && !up->encap_rcv) ||
> > > NAPI_GRO_CB(skb)->is_flist) {
> > >                 pp = call_gro_receive(udp_gro_receive_segment, head, skb);
> > >                 return pp;
> > >         }
> > > 
> > > +      /* Continue with tunnel GRO */
> > > "
> > > 
> > > .. not that the extra bit matters a lot. And these two conditions with
> > > gro_enabled are not very obvious.
> > > 
> > > Just a thought.
> > 
> > Overall looks more complex to me. I would keep the extra bit, unless
> > you have strong opinion.
> 
> Sounds good.
> 
> > Side note: I was wondering about a follow-up to simplify the condition:
> > 
> >         if ((!sk && (skb->dev->features & NETIF_F_GRO_UDP_FWD)) ||
> >              (sk && udp_sk(sk)->gro_enabled && !up->encap_rcv) || NAPI_GRO_CB(skb)->is_flist) {
> > 
> > Since UDP sockets could process (segmenting as needed) unexpected GSO
> > packets, we could always do 'NETIF_F_GRO_UDP_FWD', when enabled on the
> > device level. The above becomes:
> > 
> >         if (skb->dev->features & NETIF_F_GRO_UDP_FWD) ||
> >             (sk && udp_sk(sk)->gro_enabled && !up->encap_rcv) ||
> >             NAPI_GRO_CB(skb)->is_flist) {
> > 
> > which is hopefully more clear (and simpler). As said, non for this
> > series anyhow.
> 
> UDP sockets can segment, but it is expensive. In this case I think the
> simplification is not worth the possible regression.

No strong opinion here, I will not do the thing mentioned above.

Thanks!

Paolo