netdev - Re: [PATCH bpf v10 07/14] bpf: sockmap, wake up polling after data copy

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Date: Tue, 30 May 2023 20:51:27 +0200
From: Eric Dumazet <edumazet@...gle.com>
To: John Fastabend <john.fastabend@...il.com>
Cc: jakub@...udflare.com, daniel@...earbox.net, bpf@...r.kernel.org, 
	netdev@...r.kernel.org, ast@...nel.org, andrii@...nel.org, will@...valent.com
Subject: Re: [PATCH bpf v10 07/14] bpf: sockmap, wake up polling after data copy

On Tue, May 30, 2023 at 8:43 PM John Fastabend <john.fastabend@...il.com> wrote:
>
> John Fastabend wrote:
> > Eric Dumazet wrote:
> > > On Tue, May 23, 2023 at 4:56 AM John Fastabend <john.fastabend@...il.com> wrote:
> > > >
> > > > When TCP stack has data ready to read sk_data_ready() is called. Sockmap
> > > > overwrites this with its own handler to call into BPF verdict program.
> > > > But, the original TCP socket had sock_def_readable that would additionally
> > > > wake up any user space waiters with sk_wake_async().
> > > >
> > > > Sockmap saved the callback when the socket was created so call the saved
> > > > data ready callback and then we can wake up any epoll() logic waiting
> > > > on the read.
> > > >
> > > > Note we call on 'copied >= 0' to account for returning 0 when a FIN is
> > > > received because we need to wake up user for this as well so they
> > > > can do the recvmsg() -> 0 and detect the shutdown.
> > > >
> > > > Fixes: 04919bed948dc ("tcp: Introduce tcp_read_skb()")
> > > > Reviewed-by: Jakub Sitnicki <jakub@...udflare.com>
> > > > Signed-off-by: John Fastabend <john.fastabend@...il.com>
> > > > ---
> > > >  net/core/skmsg.c | 11 ++++++++++-
> > > >  1 file changed, 10 insertions(+), 1 deletion(-)
> > > >
> > > > diff --git a/net/core/skmsg.c b/net/core/skmsg.c
> > > > index bcd45a99a3db..08be5f409fb8 100644
> > > > --- a/net/core/skmsg.c
> > > > +++ b/net/core/skmsg.c
> > > > @@ -1199,12 +1199,21 @@ static int sk_psock_verdict_recv(struct sock *sk, struct sk_buff *skb)
> > > >  static void sk_psock_verdict_data_ready(struct sock *sk)
> > > >  {
> > > >         struct socket *sock = sk->sk_socket;
> > > > +       int copied;
> > > >
> > > >         trace_sk_data_ready(sk);
> > > >
> > > >         if (unlikely(!sock || !sock->ops || !sock->ops->read_skb))
> > > >                 return;
> > > > -       sock->ops->read_skb(sk, sk_psock_verdict_recv);
> > > > +       copied = sock->ops->read_skb(sk, sk_psock_verdict_recv);
> > > > +       if (copied >= 0) {
> > > > +               struct sk_psock *psock;
> > > > +
> > > > +               rcu_read_lock();
> > > > +               psock = sk_psock(sk);
> > > > +               psock->saved_data_ready(sk);
> > > > +               rcu_read_unlock();
> > > > +       }
> > > >  }
> > > >
> > > >  void sk_psock_start_verdict(struct sock *sk, struct sk_psock *psock)
> > > > --
> > > > 2.33.0
> > > >
> > >
> > > It seems psock could be NULL here, right ?
> > >
> > > What do you think if I submit the following fix ?
> > >
> > > diff --git a/net/core/skmsg.c b/net/core/skmsg.c
> > > index a9060e1f0e4378fa47cfd375b4729b5b0a9f54ec..a29508e1ff3568583263b9307f7b1a0e814ba76d
> > > 100644
> > > --- a/net/core/skmsg.c
> > > +++ b/net/core/skmsg.c
> > > @@ -1210,7 +1210,8 @@ static void sk_psock_verdict_data_ready(struct sock *sk)
> > >
> > >                 rcu_read_lock();
> > >                 psock = sk_psock(sk);
> > > -               psock->saved_data_ready(sk);
> > > +               if (psock)
> > > +                       psock->saved_data_ready(sk);
> > >                 rcu_read_unlock();
> > >         }
> > >  }
> >
> > Yes please do presumably this is plausible if user delete map entry while
> > data is being sent and we get a race. We don't have any tests for this
> > in our CI though because we never delete socks after adding them and
> > rely on the sock close. This shouldn't happen in that path because of the
> > data_ready is blocked on SOCK_DEAD flag iirc.
> >
> > I'll think if we can add some stress test to add map update/delete in
> > a tight loop with live socket sending/receiving traffic.
> >
> > Thanks
>
> I can also submit it if its easier just let me know.

 I will, this is based on a syzbot report I will also release,
thanks !