linux-kernel - Re: [PATCH net v2] net: Update window_clamp if SOCK

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Date:   Mon, 9 Nov 2020 12:02:27 +0100
From:   Eric Dumazet <edumazet@...gle.com>
To:     Mao Wenan <wenan.mao@...ux.alibaba.com>
Cc:     David Miller <davem@...emloft.net>,
        Alexey Kuznetsov <kuznet@....inr.ac.ru>,
        Hideaki YOSHIFUJI <yoshfuji@...ux-ipv6.org>,
        Jakub Kicinski <kuba@...nel.org>,
        netdev <netdev@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        kernel-janitors@...r.kernel.org
Subject: Re: [PATCH net v2] net: Update window_clamp if SOCK_RCVBUF is set

On Mon, Nov 9, 2020 at 11:12 AM Mao Wenan <wenan.mao@...ux.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@...ux.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.