linux-kernel - Re: [PATCH bpf-next v4 6/7] bpf, net: Support SO_REUSEPORT sockets with bpf_sk

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Message-ID: <CAN+4W8hLXYZuNFG+=J-FWLXWhbwT5TrHjMg5VzjQhv2NBo5VaA@mail.gmail.com>
Date:   Mon, 3 Jul 2023 10:57:23 +0100
From:   Lorenz Bauer <lmb@...valent.com>
To:     Kuniyuki Iwashima <kuniyu@...zon.com>
Cc:     andrii@...nel.org, ast@...nel.org, bpf@...r.kernel.org,
        daniel@...earbox.net, davem@...emloft.net, dsahern@...nel.org,
        edumazet@...gle.com, haoluo@...gle.com, hemanthmalla@...il.com,
        joe@...ium.io, joe@...d.net.nz, john.fastabend@...il.com,
        jolsa@...nel.org, kpsingh@...nel.org, kuba@...nel.org,
        linux-kernel@...r.kernel.org, linux-kselftest@...r.kernel.org,
        martin.lau@...ux.dev, mykolal@...com, netdev@...r.kernel.org,
        pabeni@...hat.com, sdf@...gle.com, shuah@...nel.org,
        song@...nel.org, willemdebruijn.kernel@...il.com, yhs@...com
Subject: Re: [PATCH bpf-next v4 6/7] bpf, net: Support SO_REUSEPORT sockets
 with bpf_sk_assign

On Wed, Jun 28, 2023 at 7:54 PM Kuniyuki Iwashima <kuniyu@...zon.com> wrote:

> > +     reuse_sk = inet6_lookup_reuseport(net, sk, skb, doff,
> > +                                       saddr, sport, daddr, ntohs(dport),
> > +                                       ehashfn);
> > +     if (!reuse_sk || reuse_sk == sk)
> > +             return sk;
> > +
> > +     /* We've chosen a new reuseport sock which is never refcounted. This
> > +      * implies that sk also isn't refcounted.
> > +      */
> > +     WARN_ON_ONCE(*refcounted);
>
> One more nit.
>
> WARN_ON_ONCE() should be tested before inet6?_lookup_reuseport() not to
> miss the !reuse_sk case.

I was just pondering that as well, but I came to the opposite
conclusion. In the !reuse_sk case we don't really know anything about
sk, except that it isn't part of a reuseport group. How can we be sure
that it's not refcounted?