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

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Date: Tue, 30 May 2023 08:30:41 +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 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();
        }
 }