Re: [RFC PATCH net-next v2 1/2] tcp: Add net.ipv4.tcp_purge_receive_queue sysctl

[Leon Hwang <leon.hwang@linux.dev>]

On 2026/7/15 23:15, Eric Dumazet wrote:
> On Wed, Jul 15, 2026 at 4:54 PM Leon Hwang <leon.hwang@linux.dev> wrote:

[...]

>> diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c
>> index 61045a8886e4..4f1027173e95 100644
>> --- a/net/ipv4/tcp_input.c
>> +++ b/net/ipv4/tcp_input.c
>> @@ -4853,6 +4853,7 @@ void tcp_done_with_error(struct sock *sk, int err)
>>  /* When we get a reset we do this. */
>>  void tcp_reset(struct sock *sk, struct sk_buff *skb)
>>  {
>> +       const struct net *net = sock_net(sk);
>>         int err;
>>
>>         trace_tcp_receive_reset(sk);
>> @@ -4869,6 +4870,27 @@ void tcp_reset(struct sock *sk, struct sk_buff *skb)
>>                 err = ECONNREFUSED;
>>                 break;
>>         case TCP_CLOSE_WAIT:
>> +               /* RFC9293 3.10.7.4. Other States
>> +                *   Second, check the RST bit:
>> +                *     CLOSE-WAIT STATE
>> +                *
>> +                * If the RST bit is set, then any outstanding RECEIVEs and
>> +                * SEND should receive "reset" responses.  All segment queues
>> +                * should be flushed.  Users should also receive an unsolicited
>> +                * general "connection reset" signal.  Enter the CLOSED state,
>> +                * delete the TCB, and return.
>> +                *
>> +                * If net.ipv4.tcp_purge_receive_queue is enabled,
>> +                * sk_receive_queue will be flushed too.
>> +                */
>> +               if (unlikely(READ_ONCE(net->ipv4.sysctl_tcp_purge_receive_queue))) {
>> +                       struct tcp_sock *tp = tcp_sk(sk);
>> +
>> +                       skb_queue_purge(&sk->sk_receive_queue);
>> +                       WRITE_ONCE(tp->copied_seq, tp->rcv_nxt);
>> +                       WRITE_ONCE(tp->urg_data, 0);
>> +                       sk_set_peek_off(sk, -1);
>> +               }
>>                 err = EPIPE;
>>                 break;
>>         case TCP_CLOSE:
>> --
>> 2.55.0
>>
> 
> My thoughts are:
> 
> out_of_order_queue has been forgotten. skbs could be there and still
> 'block devmem'
> 
> WRITE_ONCE(tp->copied_seq, tp->rcv_nxt) is certainly wrong, because
> read() will return 0, instead of -1 (errno = EPIPE or ECONNRESET)
> So the application will not know a RST was received :/
> 
> I think that BSD and linux implementations have historically retained
> acknowledged,
> buffered receive data upon RST to allow applications to drain data
> already ACKed prior to the reset.
> 
> Adding a narrow sysctl specifically for CLOSE_WAIT creates
> inconsistent behavior across TCP states.


Got it. I won't pursue this sysctl approach in the future. Thanks for
the review.

Leon