linux-kernel - Re: [BACKPORT 4.4.y 18/25] tcp/dccp: drop SYN packets if accept queue is full

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Date:   Tue, 26 Mar 2019 10:21:09 +0900
From:   Greg KH <gregkh@...uxfoundation.org>
To:     Arnd Bergmann <arnd@...db.de>
Cc:     stable@...r.kernel.org, "David S. Miller" <davem@...emloft.net>,
        Gerrit Renker <gerrit@....abdn.ac.uk>,
        Eric Dumazet <edumazet@...gle.com>,
        Alexey Kuznetsov <kuznet@....inr.ac.ru>,
        Hideaki YOSHIFUJI <yoshfuji@...ux-ipv6.org>,
        Neal Cardwell <ncardwell@...gle.com>,
        Yuchung Cheng <ycheng@...gle.com>,
        Wei Wang <weiwan@...gle.com>,
        Ilya Lesokhin <ilyal@...lanox.com>,
        Priyaranjan Jha <priyarjha@...gle.com>,
        Soheil Hassas Yeganeh <soheil@...gle.com>,
        Yafang Shao <laoar.shao@...il.com>, netdev@...r.kernel.org,
        linux-kernel@...r.kernel.org, dccp@...r.kernel.org
Subject: Re: [BACKPORT 4.4.y 18/25] tcp/dccp: drop SYN packets if accept
 queue is full

On Fri, Mar 22, 2019 at 04:44:09PM +0100, Arnd Bergmann wrote:
> From: Eric Dumazet <edumazet@...gle.com>
> 
> Per listen(fd, backlog) rules, there is really no point accepting a SYN,
> sending a SYNACK, and dropping the following ACK packet if accept queue
> is full, because application is not draining accept queue fast enough.
> 
> This behavior is fooling TCP clients that believe they established a
> flow, while there is nothing at server side. They might then send about
> 10 MSS (if using IW10) that will be dropped anyway while server is under
> stress.
> 
> Signed-off-by: Eric Dumazet <edumazet@...gle.com>
> Acked-by: Neal Cardwell <ncardwell@...gle.com>
> Acked-by: Yuchung Cheng <ycheng@...gle.com>
> Signed-off-by: David S. Miller <davem@...emloft.net>
> (cherry picked from commit 5ea8ea2cb7f1d0db15762c9b0bb9e7330425a071)

Also queued up for 4.9.y