Date:   Thu, 29 Aug 2019 15:26:43 -0400
From:   Willem de Bruijn <willemdebruijn.kernel@...il.com>
To:     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 Fri, Aug 23, 2019 at 3:11 PM Steve Zabele <zabele@...cast.net> wrote:
>
> Hi folks,
>
> Is there a way to find out where the SO_REUSEPORT bug reported a year ago in
> August (and apparently has been a bug with kernels later than 4.4) is being
> addressed?
>
> The bug characteristics, simple standalone test code demonstrating the bug,
> and an assessment of the likely location/cause of the bug within the kernel
> are all described here
>
> https://bugzilla.kernel.org/show_bug.cgi?id=200755
>
> I'm really hoping this gets fixed so we can move forward on updating our
> kernels/Ubuntu release from our aging 4.4/16.04 release
>
> Thanks!
>
> Steve
>
>
>
> -----Original Message-----
> From: Stephen Hemminger [mailto:stephen@...workplumber.org]
> Sent: Tuesday, July 16, 2019 10:03 AM
> To: Steve Zabele
> Cc: shum@...ndrew.org; vladimir116@...il.com; saifi.khan@...aSynergy.org;
> saifi.khan@...ikr.in; daniel@...earbox.net; on2k16nm@...il.com
> Subject: Re: Is bug 200755 in anyone's queue??
>
> On Tue, 16 Jul 2019 09:43:24 -0400
> "Steve Zabele" <zabele@...cast.net> wrote:
>
>
> > I came across bug report 200755 trying to figure out why some code I had
> > provided to customers a while ago no longer works with the current Linux
> > kernel. See
> >
> > https://bugzilla.kernel.org/show_bug.cgi?id=200755
> >
> > I've verified that, as reported, 'connect' no longer works for UDP.
> > Moreover, it appears it has been broken since the 4.5 kernel has been
> > released.
> >
> >
> >
> > It does also appear that the intended new feature of doing round robin
> > assignments to different UDP sockets opened with SO_REUSEPORT also does
> not
> > work as described.
> >
> >
> >
> > Since the original bug report was made nearly a year ago for the 4.14
> kernel
> > (and the bug is also still present in the 4.15 kernel) I'm curious if
> anyone
> > is on the hook to get this fixed any time soon.
> >
> >
> >
> > I'd rather not have to do my own demultiplexing using a single socket in
> > user space to work around what is clearly a (maybe not so recently
> > introduced) kernel bug if at all possible. My code had worked just fine on
> > 3.X kernels, and appears to work okay up through 4.4.
> >
>
> Kernel developers do not use bugzilla, I forward bug reports
> to netdev@...r.kernel.org (after filtering).

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).

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