Re: [RFC PATCH] tcp: accept RST for rcv_nxt - 1 after receiving a FIN

On 01/11/2017 10:48 AM, Eric Dumazet wrote:

On Thu, 2017-01-05 at 16:33 -0500, Jason Baron wrote:

quoted

 
+/* Accept RST for rcv_nxt - 1 after a FIN.
+ * When tcp connections are abruptly terminated from Mac OSX (via ^C), a
+ * FIN is sent followed by a RST packet. The RST is sent with the same
+ * sequence number as the FIN, and thus according to RFC 5961 a challenge
+ * ACK should be sent. However, Mac OSX does not reply to the challenge ACK
+ * with a RST on the closed socket, hence accept this class of RSTs.
+ */
+static bool tcp_reset_check(struct sock *sk, struct sk_buff *skb)

const struct sock *sk, const struct sk_buff *skb

quoted

+{
+	struct tcp_sock *tp = tcp_sk(sk);
+
+	return unlikely((TCP_SKB_CB(skb)->seq == (tp->rcv_nxt - 1)) &&
+			(TCP_SKB_CB(skb)->end_seq == (tp->rcv_nxt - 1))	&&

Why is the test on end_seq needed ?

Hi,

(Re-sending - seems like my reply was lost)

I wanted to define this condition as narrowly as I could. I'm ok
dropping it -
I'm not sure its going to make much difference in practice. So to that end,
dropping this extra check makes sense.

I posted this as RFC because RFC 5961, I don't think says anything about
accepting rcv_nxt - 1 in this case, so I was wondering what people
thought...

Thanks,

-Jason

quoted

+			(sk->sk_state == TCP_CLOSE_WAIT ||
+			 sk->sk_state == TCP_LAST_ACK ||
+			 sk->sk_state == TCP_CLOSING));
+}

Testing many states can be done more efficiently :

   (1 << sk->sk_state) & (TCPF_CLOSE_WAIT | TCPF_LAST_ACK |
                          TCPF_CLOSING)

Thanks