netdev - Re: [Patch bpf-next] tcp: fix sock skb accounting in tcp_read

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Date:   Tue, 26 Jul 2022 18:47:55 +0200
From:   Eric Dumazet <edumazet@...gle.com>
To:     Cong Wang <xiyou.wangcong@...il.com>
Cc:     netdev <netdev@...r.kernel.org>, bpf <bpf@...r.kernel.org>,
        Cong Wang <cong.wang@...edance.com>,
        syzbot <syzbot+a0e6f8738b58f7654417@...kaller.appspotmail.com>,
        Stanislav Fomichev <sdf@...gle.com>,
        John Fastabend <john.fastabend@...il.com>
Subject: Re: [Patch bpf-next] tcp: fix sock skb accounting in tcp_read_skb()

On Tue, Jul 26, 2022 at 6:14 PM Cong Wang <xiyou.wangcong@...il.com> 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;
        }