netdev - Re: [bpf PATCH 2/2] bpf, sockmap: fix incorrect fwd

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Message-ID: <605bf9718f2fc_64fde2082b@john-XPS-13-9370.notmuch>
Date:   Wed, 24 Mar 2021 19:46:09 -0700
From:   John Fastabend <john.fastabend@...il.com>
To:     Cong Wang <xiyou.wangcong@...il.com>,
        John Fastabend <john.fastabend@...il.com>
Cc:     Andrii Nakryiko <andrii@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Alexei Starovoitov <ast@...com>, bpf <bpf@...r.kernel.org>,
        Linux Kernel Network Developers <netdev@...r.kernel.org>,
        Lorenz Bauer <lmb@...udflare.com>
Subject: Re: [bpf PATCH 2/2] bpf, sockmap: fix incorrect fwd_alloc accounting

Cong Wang wrote:
> On Wed, Mar 24, 2021 at 2:00 PM John Fastabend <john.fastabend@...il.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.