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From:   Eric Dumazet <edumazet@google.com>
Date:   Mon, 9 Nov 2020 12:02:27 +0100
Message-ID: <CANn89i+oS75TVKBDOBrr7Ff55Uctq4_HUcM_05Ed8kUL1HkHLw@mail.gmail.com>
Subject: Re: [PATCH net v2] net: Update window_clamp if SOCK_RCVBUF is set
To:     Mao Wenan <wenan.mao@linux.alibaba.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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On Mon, Nov 9, 2020 at 11:12 AM Mao Wenan <wenan.mao@linux.alibaba.com> wro=
te:
>
>
>
> =E5=9C=A8 2020/11/9 =E4=B8=8B=E5=8D=885:56, Eric Dumazet =E5=86=99=E9=81=
=93:
> > On Mon, Nov 9, 2020 at 10:33 AM Mao Wenan <wenan.mao@linux.alibaba.com>=
 wrote:
> >>
> >> When net.ipv4.tcp_syncookies=3D1 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)=3D65535, pass req->rsk_window_clamp=3D65535 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=3D63, it consider the rea=
l
> window is 63*2^7=3D8064, but client consider the server window is only
> 63*2^0=3D63, 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.