netdev - Re: tcp: disallow bind() to reuse addr/port regression in 2.6.38

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Message-Id: <201104022046.11701.cyril.bonte@free.fr>
Date:	Sat, 2 Apr 2011 20:46:11 +0200
From:	Cyril Bonté <cyril.bonte@...e.fr>
To:	Eric Dumazet <eric.dumazet@...il.com>
Cc:	netdev@...r.kernel.org, Daniel Baluta <daniel.baluta@...il.com>,
	Gaspar Chilingarov <gasparch@...il.com>,
	Charles Duffy <charles@...is.net>, Willy Tarreau <w@....eu>
Subject: Re: tcp: disallow bind() to reuse addr/port regression in 2.6.38

Le samedi 2 avril 2011 20:10:48, Eric Dumazet a écrit :
> Le samedi 02 avril 2011 à 20:01 +0200, Cyril Bonté a écrit :
> (...)
> > > 		if (shutdown(listenfd, SHUT_WR) == 0 &&
> > 		
> > 		    listen(listenfd, 1024) == 0 &&
> > 		    shutdown(listenfd, SHUT_RD) == 0) {
> > 			
> > 			printf("shutdown OK\n");
> > 		
> > 		}
> > 	
> > 	}
> > 	exit(0);
> > 
> > }
> 
> Wow, not clear what this is doing....
> 
> for sure the listen() call is not needed ?
> 
> And the shutdown(listenfd, SHUT_WR) is clearly useless too.

Well, I'm not the best one to explain that part but from what i read in the 
comments of this part of code, both listen and SHUT_WR are used to detect 
errors on various OS (OpenBSD, Solaris, ...).

> I feel you only needed the shutdown(listenfd, SHUT_RD) call.
> 
> Why haproxy needs to setup a second listening socket on same port ?

I simplified the test case, which is far from what haproxy do (just forgot to 
explain the real behaviour).
To reload the configuration, a new haproxy process is launched, sending a 
signal to the previous one and asking it to free the ports for a while (the 
shutdown part in the test). The new process then tries to bind the ports, 
which worked until 2.6.38 (if an error occurs, a new signal is sent to the 
previous process to listen to its sockets again).

-- 
Cyril Bonté
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