Re: [PATCH bpf-next v2 3/6] net: remove duplicate reuseport_lookup functions

[Kuniyuki Iwashima]

From: Lorenz Bauer <lmb@xxxxxxxxxxxxx>
Date: Tue, 20 Jun 2023 15:26:05 +0100
> On Tue, Jun 13, 2023 at 7:57 PM Kuniyuki Iwashima <kuniyu@xxxxxxxxxx> wrote:
> >
> > The assignment to result below is buggy.  Let's say SO_REUSEPROT group
> > have TCP_CLOSE and TCP_ESTABLISHED sockets.
> >
> >   1. Find TCP_CLOSE sk and do SO_REUSEPORT lookup
> >   2. result is not NULL, but the group has TCP_ESTABLISHED sk
> >   3. result = result
> >   4. Find TCP_ESTABLISHED sk, which has a higher score
> >   5. result = result (TCP_CLOSE) <-- should be sk.
> >
> > Same for v6 function.
> 
> Thanks for your explanation, I think I get it now. I misunderstood
> that you were worried about returning TCP_ESTABLISHED instead of
> TCP_CLOSE, but it's exactly the other way around.
> 
> I have a follow up question regarding the existing code:
> 
>     result = lookup_reuseport(net, sk, skb,
>                     saddr, sport, daddr, hnum);
>     /* Fall back to scoring if group has connections */
>     if (result && !reuseport_has_conns(sk))
>         return result;
> 
>     result = result ? : sk;
>     badness = score;
> 
> Assuming that result != NULL but reuseport_has_conns() == true, we use
> the reuseport socket as the result, but assign the score of sk to
> badness. Shouldn't we use the score of the reuseport socket?

Good point.  This is based on an assumption that all SO_REUSEPORT
sockets have the same score, which is wrong for two corner cases
if reuseport_has_conns() == true :

  1) SO_INCOMING_CPU is set
     -> selected sk might have +1 score

  2) BPF prog returns ESTABLISHED and/or SO_INCOMING_CPU sk
     -> selected sk will have more than 8

Using the old score could trigger more lookups depending on the
order that sockets are created.

  sk -> sk (SO_INCOMING_CPU) -> sk (ESTABLISHED)
  |     |
  `-> select the next SO_INCOMING_CPU sk
        |
	`-> select itself (We should save this lookup)

So, yes, we should update badness like

  if (unlikely(result)) {
      badness = compute_score(result, ...);
  } else {
      result = sk;
      badness = score;
  }