On Tue, Jul 26, 2022 at 6:49 PM Eric Dumazet <edumazet@xxxxxxxxxx> wrote:
>
> 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);
Note that this code will still not behave properly if we have in
receive queues two skbs of 1000 bytes of payload like:
seq 1:1001
seq 501:1501
tcp_recvmsg() would copy 1000 bytes from the first skb, then 500 bytes
from second skb.
