Re: [Patch bpf-next] tcp: fix sock skb accounting in tcp_read_skb()

[Eric Dumazet <edumazet@xxxxxxxxxx>]

On Tue, Jul 26, 2022 at 6:47 PM Eric Dumazet <edumazet@xxxxxxxxxx> wrote:
>
> On Tue, Jul 26, 2022 at 6:14 PM Cong Wang <xiyou.wangcong@xxxxxxxxx> wrote:
>
> > If TCP really wants to queue a FIN with skb->len==0, then we have to
> > adjust the return value for recv_actor(), because we currently use 0 as
> > an error too (meaning no data is consumed):
> >
> >         if (sk_psock_verdict_apply(psock, skb, ret) < 0)
> >                 len = 0;  // here!
> > out:
> >         rcu_read_unlock();
> >         return len;
> >
> >
> > BTW, what is wrong if we simply drop it before queueing to
> > sk_receive_queue in TCP? Is it there just for collapse?
>
> Because an incoming segment can have payload and FIN.
>
> The consumer will need to consume the payload before FIN is considered/consumed,
> with the complication of MSG_PEEK ...
>
> Right after tcp_read_skb() removes the skb from sk_receive_queue,
> we need to update TCP state, regardless of recv_actor().
>
> Maybe like that:
>
> diff --git a/net/ipv4/tcp.c b/net/ipv4/tcp.c
> index ba2bdc81137490bd1748cde95789f8d2bff3ab0f..6e2c11cd921872e406baffc475c9870e147578a1
> 100644
> --- a/net/ipv4/tcp.c
> +++ b/net/ipv4/tcp.c
> @@ -1759,20 +1759,15 @@ int tcp_read_skb(struct sock *sk,
> skb_read_actor_t recv_actor)
>                 int used;
>
>                 __skb_unlink(skb, &sk->sk_receive_queue);
> +               seq = TCP_SKB_CB(skb)->end_seq;
>                 used = recv_actor(sk, skb);
>                 if (used <= 0) {
>                         if (!copied)
>                                 copied = used;
>                         break;
>                 }
> -               seq += used;
>                 copied += used;
>
> -               if (TCP_SKB_CB(skb)->tcp_flags & TCPHDR_FIN) {
> -                       consume_skb(skb);
> -                       ++seq;
> -                       break;
> -               }
>                 consume_skb(skb);
>                 break;
>         }

Or even better:

diff --git a/net/ipv4/tcp.c b/net/ipv4/tcp.c
index ba2bdc81137490bd1748cde95789f8d2bff3ab0f..66c187a2592c042565211565adb3f40a811dfd7d
100644
--- a/net/ipv4/tcp.c
+++ b/net/ipv4/tcp.c
@@ -1759,21 +1759,15 @@ int tcp_read_skb(struct sock *sk,
skb_read_actor_t recv_actor)
                int used;

                __skb_unlink(skb, &sk->sk_receive_queue);
+               seq = TCP_SKB_CB(skb)->end_seq;
                used = recv_actor(sk, skb);
+               consume_skb(skb);
                if (used <= 0) {
                        if (!copied)
                                copied = used;
                        break;
                }
-               seq += used;
                copied += used;
-
-               if (TCP_SKB_CB(skb)->tcp_flags & TCPHDR_FIN) {
-                       consume_skb(skb);
-                       ++seq;
-                       break;
-               }
-               consume_skb(skb);
                break;
        }
        WRITE_ONCE(tp->copied_seq, seq);