Re: [Patch bpf-next v2 2/9] sock: introduce sk_prot->update_proto()

From: Lorenz Bauer <lmb@cloudflare.com>
To: Cong Wang <xiyou.wangcong@gmail.com>
Cc: Networking <netdev@vger.kernel.org>, bpf <bpf@vger.kernel.org>,
	duanxiongchun@bytedance.com,
	Dongdong Wang <wangdongdong.6@bytedance.com>,
	Jiang Wang <jiang.wang@bytedance.com>,
	Cong Wang <cong.wang@bytedance.com>,
	John Fastabend <john.fastabend@gmail.com>,
	Daniel Borkmann <daniel@iogearbox.net>,
	Jakub Sitnicki <jakub@cloudflare.com>
Subject: Re: [Patch bpf-next v2 2/9] sock: introduce sk_prot->update_proto()
Date: Wed, 3 Mar 2021 09:35:13 +0000	[thread overview]
Message-ID: <CACAyw99BweMk-82f270=Vb=jDuec0q0N-6E8Rr8enaOGuZEDNQ@mail.gmail.com> (raw)
In-Reply-To: <CAM_iQpXqE9qJ=+zKA6H1Rq=KKgm8LZ=p=ZtvrrH+hfSrTg+zxw@mail.gmail.com>

On Tue, 2 Mar 2021 at 18:23, Cong Wang <xiyou.wangcong@gmail.com> wrote:
>
> > if the function returned a struct proto * like it does at the moment.
> > That way we keep sk->sk_prot manipulation confined to the sockmap code
> > and don't have to copy paste it into every proto.
>
> Well, TCP seems too special to do this, as it could call tcp_update_ulp()
> to update the proto.

I had a quick look, tcp_bpf_update_proto is the only caller of tcp_update_ulp,
which in turn is the only caller of icsk_ulp_ops->update, which in turn is only
implemented as tls_update in tls_main.c. Turns out that tls_update
has another one of these calls:

} else {
    /* Pairs with lockless read in sk_clone_lock(). */
    WRITE_ONCE(sk->sk_prot, p);
    sk->sk_write_space = write_space;
}

Maybe it looks familiar? :o) I think it would be a worthwhile change.

>
> >
> > > diff --git a/net/core/sock_map.c b/net/core/sock_map.c
> > > index 3bddd9dd2da2..13d2af5bb81c 100644
> > > --- a/net/core/sock_map.c
> > > +++ b/net/core/sock_map.c
> > > @@ -184,26 +184,10 @@ static void sock_map_unref(struct sock *sk, void *link_raw)
> > >
> > >  static int sock_map_init_proto(struct sock *sk, struct sk_psock *psock)
> > >  {
> > > -       struct proto *prot;
> > > -
> > > -       switch (sk->sk_type) {
> > > -       case SOCK_STREAM:
> > > -               prot = tcp_bpf_get_proto(sk, psock);
> > > -               break;
> > > -
> > > -       case SOCK_DGRAM:
> > > -               prot = udp_bpf_get_proto(sk, psock);
> > > -               break;
> > > -
> > > -       default:
> > > +       if (!sk->sk_prot->update_proto)
> > >                 return -EINVAL;
> > > -       }
> > > -
> > > -       if (IS_ERR(prot))
> > > -               return PTR_ERR(prot);
> > > -
> > > -       sk_psock_update_proto(sk, psock, prot);
> > > -       return 0;
> > > +       psock->saved_update_proto = sk->sk_prot->update_proto;
> > > +       return sk->sk_prot->update_proto(sk, false);
> >
> > I think reads / writes from sk_prot need READ_ONCE / WRITE_ONCE. We've
> > not been diligent about this so far, but I think it makes sense to be
> > careful in new code.
>
> Hmm, there are many places not using READ_ONCE/WRITE_ONCE,
> for a quick example:

I know! I'll defer to John and Jakub.

-- 
Lorenz Bauer  |  Systems Engineer
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