netdev - Re: [RFC PATCH] tcp: accept RST for rcv_nxt

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Date:   Fri, 13 Jan 2017 12:28:19 -0500
From:   Jason Baron <jbaron@...mai.com>
To:     Eric Dumazet <eric.dumazet@...il.com>
Cc:     netdev@...r.kernel.org
Subject: 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:
>
>>  
>> +/* 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
>
>> +{
>> +	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

>> +			(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
>