netdev - Re: [Patch bpf-next v8 10/16] sock: introduce sk->sk_prot->psock_update_sk

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Date:   Tue, 06 Apr 2021 14:07:56 -0700
From:   John Fastabend <john.fastabend@...il.com>
To:     Cong Wang <xiyou.wangcong@...il.com>,
        Eric Dumazet <eric.dumazet@...il.com>
Cc:     Linux Kernel Network Developers <netdev@...r.kernel.org>,
        bpf <bpf@...r.kernel.org>, duanxiongchun@...edance.com,
        Dongdong Wang <wangdongdong.6@...edance.com>,
        Jiang Wang <jiang.wang@...edance.com>,
        Cong Wang <cong.wang@...edance.com>,
        John Fastabend <john.fastabend@...il.com>,
        Daniel Borkmann <daniel@...earbox.net>,
        Jakub Sitnicki <jakub@...udflare.com>,
        Lorenz Bauer <lmb@...udflare.com>
Subject: Re: [Patch bpf-next v8 10/16] sock: introduce
 sk->sk_prot->psock_update_sk_prot()

Cong Wang wrote:
> On Mon, Apr 5, 2021 at 1:25 AM Eric Dumazet <eric.dumazet@...il.com> wrote:
> >
> >
> >
> > On 3/31/21 4:32 AM, Cong Wang wrote:
> > > From: Cong Wang <cong.wang@...edance.com>
> > >
> > > Currently sockmap calls into each protocol to update the struct
> > > proto and replace it. This certainly won't work when the protocol
> > > is implemented as a module, for example, AF_UNIX.
> > >
> > > Introduce a new ops sk->sk_prot->psock_update_sk_prot(), so each
> > > protocol can implement its own way to replace the struct proto.
> > > This also helps get rid of symbol dependencies on CONFIG_INET.
> >
> > [...]
> >
> >
> > >
> > > -struct proto *tcp_bpf_get_proto(struct sock *sk, struct sk_psock *psock)
> > > +int tcp_bpf_update_proto(struct sock *sk, bool restore)
> > >  {
> > > +     struct sk_psock *psock = sk_psock(sk);
> >
> > I do not think RCU is held here ?
> >
> > sk_psock() is using rcu_dereference_sk_user_data()
> 
> Right, I just saw the syzbot report. But here we already have
> the writer lock of sk_callback_lock, hence RCU read lock here
> makes no sense to me. Probably we just have to tell RCU we
> already have sk_callback_lock.
> 
> Thanks.

I think you need to ensure its the psock we originally grabbed as
well. Otherwise how do we ensure the psock is not swapped from
another thread?