Re: [bpf PATCH 2/2] bpf, sockmap: fix incorrect fwd_alloc accounting

[John Fastabend <john.fastabend@gmail.com>]

Cong Wang wrote:
> On Wed, Mar 24, 2021 at 2:00 PM John Fastabend <john.fastabend@gmail.com> wrote:
> >
> > Incorrect accounting fwd_alloc can result in a warning when the socket
> > is torn down,
> >

[...]

> > To resolve lets only account for sockets on the ingress queue that are
> > still associated with the current socket. On the redirect case we will
> > check memory limits per 6fa9201a89898, but will omit fwd_alloc accounting
> > until skb is actually enqueued. When the skb is sent via skb_send_sock_locked
> > or received with sk_psock_skb_ingress memory will be claimed on psock_other.
                     ^^^^^^^^^^^^^^^^^^^^
> 
> You mean sk_psock_skb_ingress(), right?

Yes.

[...]

> > @@ -880,12 +876,13 @@ static void sk_psock_strp_read(struct strparser *strp, struct sk_buff *skb)
> >                 kfree_skb(skb);
> >                 goto out;
> >         }
> > -       skb_set_owner_r(skb, sk);
> >         prog = READ_ONCE(psock->progs.skb_verdict);
> >         if (likely(prog)) {
> > +               skb->sk = psock->sk;
> 
> Why is skb_orphan() not needed here?

These come from strparser which do not have skb->sk set.

> 
> Nit: You can just use 'sk' here, so "skb->sk = sk".

Sure that is a bit nicer, will respin with this.

> 
> 
> >                 tcp_skb_bpf_redirect_clear(skb);
> >                 ret = sk_psock_bpf_run(psock, prog, skb);
> >                 ret = sk_psock_map_verd(ret, tcp_skb_bpf_redirect_fetch(skb));
> > +               skb->sk = NULL;
> 
> Why do you want to set it to NULL here?

So we don't cause the stack to throw other errors later if we
were to call skb_orphan for example. Various places in the skb
helpers expect both skb->sk and skb->destructor to be set together
and here we are just using it as a mechanism to feed the sk into
the BPF program side. The above skb_set_owner_r for example
would likely BUG().

> 
> Thanks.