netdev - Re: [PATCH net-next] tcp: be less liberal in tsecr received while in SYN

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Message-ID: <f970e46e-7153-4000-beef-f2d621998a8e@kernel.org>
Date: Tue, 25 Feb 2025 17:56:11 +0100
From: Matthieu Baerts <matttbe@...nel.org>
To: Eric Dumazet <edumazet@...gle.com>
Cc: Paolo Abeni <pabeni@...hat.com>, Kuniyuki Iwashima <kuniyu@...zon.com>,
 Simon Horman <horms@...nel.org>, Florian Westphal <fw@...len.de>,
 netdev@...r.kernel.org, eric.dumazet@...il.com,
 Jakub Kicinski <kuba@...nel.org>, Yong-Hao Zou <yonghaoz1994@...il.com>,
 "David S . Miller" <davem@...emloft.net>,
 Neal Cardwell <ncardwell@...gle.com>
Subject: Re: [PATCH net-next] tcp: be less liberal in tsecr received while in
 SYN_RECV state

Hi Eric,

On 25/02/2025 11:51, Eric Dumazet wrote:
> On Tue, Feb 25, 2025 at 11:48 AM Matthieu Baerts <matttbe@...nel.org> wrote:
>>
>> On 25/02/2025 11:42, Eric Dumazet wrote:
>>> On Tue, Feb 25, 2025 at 11:39 AM Eric Dumazet <edumazet@...gle.com> wrote:
>>>>
>>>
>>>>
>>>> Yes, this would be it :
>>>>
>>>> diff --git a/net/ipv4/tcp_timer.c b/net/ipv4/tcp_timer.c
>>>> index 728bce01ccd3ddb1f374fa96b86434a415dbe2cb..3555567ba4fb1ccd5c5921e39d11ff08f1d0cafd
>>>> 100644
>>>> --- a/net/ipv4/tcp_timer.c
>>>> +++ b/net/ipv4/tcp_timer.c
>>>> @@ -477,8 +477,8 @@ static void tcp_fastopen_synack_timer(struct sock
>>>> *sk, struct request_sock *req)
>>>>          * regular retransmit because if the child socket has been accepted
>>>>          * it's not good to give up too easily.
>>>>          */
>>>> -       inet_rtx_syn_ack(sk, req);
>>>>         req->num_timeout++;
>>>> +       inet_rtx_syn_ack(sk, req);
>>>>         tcp_update_rto_stats(sk);
>>>>         if (!tp->retrans_stamp)
>>>>                 tp->retrans_stamp = tcp_time_stamp_ts(tp);
>>>
>>> Obviously, I need to refine the patch and send a V2 later.
>>
>> Sorry, I still have the issue with this modification. I also checked
>> with the previous patch, just to be sure, but the problem is still there
>> as well.
> 
> I said "req->num_timeout" is not updated where I thought it was.

I think that in case of SYN+ACK retransmission, req->num_timeout is
incremented after tcp_synack_options():

  reqsk_timer_handler()
  --> inet_rtx_syn_ack()
    --> tcp_rtx_synack()
      --> tcp_v6_send_synack()
        --> tcp_make_synack()
          --> tcp_synack_options()
  then: req->num_timeout++

> Look at all the places were req->num_timeout or req->num_retrans are
> set/changed.... this will give you some indications.

I'm probably missing something obvious, but if the goal is to set
snt_tsval_first only the first time, why can we not simply set

  tcp_rsk(req)->snt_tsval_first = 0;

in tcp_conn_request(), and only set it to tsval in tcp_synack_options()
when it is 0? Something like that:


> diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c
> index 217a8747a79b..26b3daa5efd2 100644
> --- a/net/ipv4/tcp_input.c
> +++ b/net/ipv4/tcp_input.c
> @@ -7249,6 +7249,7 @@ int tcp_conn_request(struct request_sock_ops *rsk_ops,
>         tcp_rsk(req)->af_specific = af_ops;
>         tcp_rsk(req)->ts_off = 0;
>         tcp_rsk(req)->req_usec_ts = false;
> +       tcp_rsk(req)->snt_tsval_first = 0;
>  #if IS_ENABLED(CONFIG_MPTCP)
>         tcp_rsk(req)->is_mptcp = 0;
>  #endif
> diff --git a/net/ipv4/tcp_output.c b/net/ipv4/tcp_output.c
> index 485ca131091e..020c624532d7 100644
> --- a/net/ipv4/tcp_output.c
> +++ b/net/ipv4/tcp_output.c
> @@ -943,7 +943,7 @@ static unsigned int tcp_synack_options(const struct sock *sk,
>                 opts->options |= OPTION_TS;
>                 opts->tsval = tcp_skb_timestamp_ts(tcp_rsk(req)->req_usec_ts, skb) +
>                               tcp_rsk(req)->ts_off;
> -               if (!req->num_timeout)
> +               if (!tcp_rsk(req)->snt_tsval_first)
>                         tcp_rsk(req)->snt_tsval_first = opts->tsval;
>                 WRITE_ONCE(tcp_rsk(req)->snt_tsval_last, opts->tsval);
>                 opts->tsecr = READ_ONCE(req->ts_recent)


Or is the goal to update this field as long as the timeout didn't fire?
In this case maybe req->num_timeout should be updated before calling
inet_rtx_syn_ack() in reqsk_timer_handler(), no?

> Do not worry, I will make sure V2 is fine.

I don't doubt about that, thank you! :)

>> (In the v2, do you mind also removing the underscore from the MIB entry
>> name (TcpExtTSECR_Rejected) please? It looks like that's the only MIB
>> entry with an underscore.)
> 
> ok

Cheers,
Matt
-- 
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