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

From: John Fastabend <[email-masked]>
Date: Tue, 30 May 2023 11:43:16 -0700

John Fastabend wrote:
> Eric Dumazet wrote:
> > On Tue, May 23, 2023 at 4:56 AM John Fastabend <[email-masked]> 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 <[email-masked]>
> > > Signed-off-by: John Fastabend <[email-masked]>
> > > ---
> > >  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.