netdev - Re: Is bug 200755 in anyone's queue??

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Message-ID: <8f4bda24-5bd4-3f12-4c98-5e1097dde84a@gmail.com>
Date:   Fri, 30 Aug 2019 10:54:38 +0200
From:   Eric Dumazet <eric.dumazet@...il.com>
To:     Willem de Bruijn <willemdebruijn.kernel@...il.com>,
        Steve Zabele <zabele@...cast.net>
Cc:     Network Development <netdev@...r.kernel.org>, shum@...ndrew.org,
        vladimir116@...il.com, saifi.khan@...asynergy.org,
        saifi.khan@...ikr.in, Daniel Borkmann <daniel@...earbox.net>,
        on2k16nm@...il.com, Stephen Hemminger <stephen@...workplumber.org>
Subject: Re: Is bug 200755 in anyone's queue??



On 8/29/19 9:26 PM, Willem de Bruijn wrote:

> SO_REUSEPORT was not intended to be used in this way. Opening
> multiple connected sockets with the same local port.
> 
> But since the interface allowed connect after joining a group, and
> that is being used, I guess that point is moot. Still, I'm a bit
> surprised that it ever worked as described.
> 
> Also note that the default distribution algorithm is not round robin
> assignment, but hash based. So multiple consecutive datagrams arriving
> at the same socket is not unexpected.
> 
> I suspect that this quick hack might "work". It seemed to on the
> supplied .c file:
> 
>                   score = compute_score(sk, net, saddr, sport,
>                                         daddr, hnum, dif, sdif);
>                   if (score > badness) {
>   -                       if (sk->sk_reuseport) {
>   +                       if (sk->sk_reuseport && !sk->sk_state !=
> TCP_ESTABLISHED) {
> 
> But a more robust approach, that also works on existing kernels, is to
> swap the default distribution algorithm with a custom BPF based one (
> SO_ATTACH_REUSEPORT_EBPF).
> 

Yes, I suspect that reuseport could still be used by to load-balance incoming packets
targetting the same 4-tuple.

So all sockets would have the same score, and we would select the first socket in
the list (if not applying reuseport hashing)