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Subject: Re: [PATCH net v2] net: Update window_clamp if SOCK_RCVBUF is set
To:     Eric Dumazet <edumazet@google.com>
Cc:     David Miller <davem@davemloft.net>,
        Alexey Kuznetsov <kuznet@ms2.inr.ac.ru>,
        Hideaki YOSHIFUJI <yoshfuji@linux-ipv6.org>,
        Jakub Kicinski <kuba@kernel.org>,
        netdev <netdev@vger.kernel.org>,
        LKML <linux-kernel@vger.kernel.org>,
        kernel-janitors@vger.kernel.org
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From:   Mao Wenan <wenan.mao@linux.alibaba.com>
Message-ID: <302ad089-9e82-1856-0652-b92251c14e37@linux.alibaba.com>
Date:   Tue, 10 Nov 2020 00:26:40 +0800
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在 2020/11/9 下午10:01, Eric Dumazet 写道:
> On Mon, Nov 9, 2020 at 12:41 PM Eric Dumazet <edumazet@google.com> wrote:
>>
>> Packetdrill test would be :
>>
>> // Force syncookies
>> `sysctl -q net.ipv4.tcp_syncookies=2`
>>
>>      0 socket(..., SOCK_STREAM, IPPROTO_TCP) = 3
>>     +0 setsockopt(3, SOL_SOCKET, SO_REUSEADDR, [1], 4) = 0
>>     +0 setsockopt(3, SOL_SOCKET, SO_RCVBUF, [2048], 4) = 0
>>     +0 bind(3, ..., ...) = 0
>>     +0 listen(3, 1) = 0
>>
>> +0 < S 0:0(0) win 32792 <mss 1000,sackOK,TS val 100 ecr 0,nop,wscale 7>
>>     +0 > S. 0:0(0) ack 1 <mss 1460,sackOK,TS val 4000 ecr 100,nop,wscale 0>
>>    +.1 < . 1:1(0) ack 1 win 1024 <nop,nop,TS val 200 ecr 4000>
>>     +0 accept(3, ..., ...) = 4
>> +0 %{ assert tcpi_snd_wscale == 0, tcpi_snd_wscale }%
>>
> 
> Also, please add to your next submission an appropriate Fixes: tag :
> 
> Fixes: e88c64f0a425 ("tcp: allow effective reduction of TCP's
> rcv-buffer via setsockopt")

OK, thanks, I can reproduce wscale=0 with your packetdrill, and I will 
send v3 with the fixes tag.

> 
>> On Mon, Nov 9, 2020 at 12:02 PM Eric Dumazet <edumazet@google.com> wrote:
>>>
>>> On Mon, Nov 9, 2020 at 11:12 AM Mao Wenan <wenan.mao@linux.alibaba.com> wrote:
>>>>
>>>>
>>>>
>>>> 在 2020/11/9 下午5:56, Eric Dumazet 写道:
>>>>> On Mon, Nov 9, 2020 at 10:33 AM Mao Wenan <wenan.mao@linux.alibaba.com> wrote:
>>>>>>
>>>>>> When net.ipv4.tcp_syncookies=1 and syn flood is happened,
>>>>>> cookie_v4_check or cookie_v6_check tries to redo what
>>>>>> tcp_v4_send_synack or tcp_v6_send_synack did,
>>>>>> rsk_window_clamp will be changed if SOCK_RCVBUF is set,
>>>>>> which will make rcv_wscale is different, the client
>>>>>> still operates with initial window scale and can overshot
>>>>>> granted window, the client use the initial scale but local
>>>>>> server use new scale to advertise window value, and session
>>>>>> work abnormally.
>>>>>
>>>>> What is not working exactly ?
>>>>>
>>>>> Sending a 'big wscale' should not really matter, unless perhaps there
>>>>> is a buggy stack at the remote end ?
>>>> 1)in tcp_v4_send_synack, if SO_RCVBUF is set and
>>>> tcp_full_space(sk)=65535, pass req->rsk_window_clamp=65535 to
>>>> tcp_select_initial_window, rcv_wscale will be zero, and send to client,
>>>> the client consider wscale is 0;
>>>> 2)when ack is back from client, if there is no this patch,
>>>> req->rsk_window_clamp is 0, and pass to tcp_select_initial_window,
>>>> wscale will be 7, this new rcv_wscale is no way to advertise to client.
>>>> 3)if server send rcv_wind to client with window=63, it consider the real
>>>> window is 63*2^7=8064, but client consider the server window is only
>>>> 63*2^0=63, it can't send big packet to server, and the send-q of client
>>>> is full.
>>>>
>>>
>>> I see, please change your patches so that tcp_full_space() is used _once_
>>>
>>> listener sk_rcvbuf can change under us.
>>>
>>> I really have no idea how window can be set to 63, so please send us
>>> the packetdrill test once you have it.