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

[Alexei Starovoitov]

On Thu, Jul 6, 2023 at 8:33 AM Kuniyuki Iwashima <kuniyu@xxxxxxxxxx> wrote:
>
> From: Lorenz Bauer <lmb@xxxxxxxxxxxxx>
> Date: Thu, 6 Jul 2023 09:11:15 +0100
> > On Thu, Jul 6, 2023 at 1:41 AM Kuniyuki Iwashima <kuniyu@xxxxxxxxxx> wrote:
> > >
> > > Sorry for late reply.
> > >
> > > What we know about sk before inet6?_lookup_reuseport() are
> > >
> > >   (1) sk was full socket in bpf_sk_assign()
> > >   (2) sk had SOCK_RCU_FREE in bpf_sk_assign()
> > >   (3) sk was TCP_LISTEN here if TCP
> >
> > Are we looking at the same bpf_sk_assign? Confusingly there are two
> > very similarly named functions. The one we care about is:
> >
> > BPF_CALL_3(bpf_sk_assign, struct sk_buff *, skb, struct sock *, sk, u64, flags)
> > {
> >     if (!sk || flags != 0)
> >         return -EINVAL;
> >     if (!skb_at_tc_ingress(skb))
> >         return -EOPNOTSUPP;
> >     if (unlikely(dev_net(skb->dev) != sock_net(sk)))
> >         return -ENETUNREACH;
> >     if (sk_is_refcounted(sk) &&
> >         unlikely(!refcount_inc_not_zero(&sk->sk_refcnt)))
> >         return -ENOENT;
> >
> >     skb_orphan(skb);
> >     skb->sk = sk;
> >     skb->destructor = sock_pfree;
> >
> >     return 0;
> > }
> >
> > From this we can't tell what state the socket is in or whether it is
> > RCU freed or not.
>
> But we can in inet6?_steal_sock() by calling sk_is_refcounted() again
> via skb_steal_sock().
>
> In inet6?_steal_sock(), we call inet6?_lookup_reuseport() only for
> sk that was a TCP listener or UDP non-connected socket until just before
> the sk_state checks.  Then, we know *refcounted should be false for such
> sockets even before inet6?_lookup_reuseport().
>
> After the checks, sk might be poped out of the reuseport group before
> inet6?_lookup_reuseport() and reuse_sk might be NULL, but it's not
> related because *refcounted is a value for sk, not for reuse_sk.

I was about to apply v5 before I noticed this discussion on v4.
Sounds like v6 will be needed.
Next time please continue discussion in the latest version.