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Date:   Tue, 10 Mar 2020 16:41:22 +0900
From:   Kuniyuki Iwashima <kuniyu@...zon.co.jp>
To:     <eric.dumazet@...il.com>
CC:     <davem@...emloft.net>, <edumazet@...gle.com>, <kuni1840@...il.com>,
        <kuniyu@...zon.co.jp>, <kuznet@....inr.ac.ru>,
        <netdev@...r.kernel.org>, <osa-contribution-log@...zon.com>,
        <yoshfuji@...ux-ipv6.org>
Subject: Re: [PATCH v4 net-next 2/5] tcp: bind(0) remove the SO_REUSEADDR restriction when ephemeral ports are exhausted.

From:   Eric Dumazet <eric.dumazet@...il.com>
Date:   Mon, 9 Mar 2020 21:04:24 -0700
> On 3/8/20 11:16 AM, Kuniyuki Iwashima wrote:
> > Commit aacd9289af8b82f5fb01bcdd53d0e3406d1333c7 ("tcp: bind() use stronger
> > condition for bind_conflict") introduced a restriction to forbid to bind
> > SO_REUSEADDR enabled sockets to the same (addr, port) tuple in order to
> > assign ports dispersedly so that we can connect to the same remote host.
> > 
> > The change results in accelerating port depletion so that we fail to bind
> > sockets to the same local port even if we want to connect to the different
> > remote hosts.
> > 
> > You can reproduce this issue by following instructions below.
> >   1. # sysctl -w net.ipv4.ip_local_port_range="32768 32768"
> >   2. set SO_REUSEADDR to two sockets.
> >   3. bind two sockets to (localhost, 0) and the latter fails.
> > 
> > Therefore, when ephemeral ports are exhausted, bind(0) should fallback to
> > the legacy behaviour to enable the SO_REUSEADDR option and make it possible
> > to connect to different remote (addr, port) tuples.
> 
> Sadly this commit tries hard to support obsolete SO_REUSEADDR for active connections,
> which makes little sense now we have more powerful IP_BIND_ADDRESS_NO_PORT
> 
> SO_REUSEADDR only really makes sense for a listener, because you want a
> server to be able to restart after core dump, while prior sockets are still
> kept in TIME_WAIT state.
> 
> Same for SO_REUSEPORT : it only made sense for sharded listeners in linux kernel.
> 
> Trying to allocate a sport at bind() time, without knowing the destination address/port
> is really not something that can be fixed.
> 
> Your patches might allow a 2x increase, while IP_BIND_ADDRESS_NO_PORT
> basically allows for 1000x increase of the possible combinations.
> 
> 
> 
> > 
> > This patch allows us to bind SO_REUSEADDR enabled sockets to the same
> > (addr, port) only when all ephemeral ports are exhausted.
> > 
> > The only notable thing is that if all sockets bound to the same port have
> > both SO_REUSEADDR and SO_REUSEPORT enabled, we can bind sockets to an
> > ephemeral port and also do listen().
> > 
> > Fixes: aacd9289af8b ("tcp: bind() use stronger condition for bind_conflict")
> 
> I disagree with this Fixes: tag  : I do not want this patch in stable kernels,
> particularly if you put the sysctl patch as a followup without a Fixes: tag.
> 
> Please reorder your patch to first introduce the sysctl, then this one.
> 
> Or squash the two patches.

I'm sorry, I will remove the tag and squash the patches.

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