From mboxrd@z Thu Jan  1 00:00:00 1970
From: Stephen Hemminger <shemminger@vyatta.com>
Subject: Re: [PATCH] udp: port starting location not random
Date: Thu, 4 Oct 2012 14:28:59 -0700
Message-ID: <20121004142859.08ab0385@nehalam.linuxnetplumber.net>
References: <20121004140828.2d6f7bf9@nehalam.linuxnetplumber.net>
	<20121004.171246.1480276635491836767.davem@davemloft.net>
Mime-Version: 1.0
Content-Type: text/plain; charset=US-ASCII
Content-Transfer-Encoding: 7bit
Cc: dada1@cosmosbay.com, netdev@vger.kernel.org
To: David Miller <davem@davemloft.net>
Return-path: <netdev-owner@vger.kernel.org>
Received: from mail.vyatta.com ([76.74.103.46]:58655 "EHLO mail.vyatta.com"
	rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP
	id S1946556Ab2JDV3g (ORCPT <rfc822;netdev@vger.kernel.org>);
	Thu, 4 Oct 2012 17:29:36 -0400
In-Reply-To: <20121004.171246.1480276635491836767.davem@davemloft.net>
Sender: netdev-owner@vger.kernel.org
List-ID: <netdev.vger.kernel.org>

On Thu, 04 Oct 2012 17:12:46 -0400 (EDT)
David Miller <davem@davemloft.net> wrote:

> From: Stephen Hemminger <shemminger@vyatta.com>
> Date: Thu, 4 Oct 2012 14:08:28 -0700
> 
> > While working on VXLAN, noticed a bug in UDP introduced by:
> > 
> > commit 9088c5609584684149f3fb5b065aa7f18dcb03ff
> > Author: Eric Dumazet <dada1@cosmosbay.com>
> > Date:   Wed Oct 8 11:44:17 2008 -0700
> > 
> >     udp: Improve port randomization
> >     
> > 
> > The logic for choosing where to start for port randomization incorrectly
> > calculates the starting port number. It is always ends up using
> > the low end of the range independent of the value of random.
> > This causes all UDP port searches to start at the same port.
> > 
> > Doing the following fixes it but at the cost of doing a real divide.
> > 
> > Signed-off-by: Stephen Hemminger <shemminger@vyatta.com>
> > 
> > ---
> > Resend, previous send was not going to netdev.
> > 
> > Not sure if worth fixing for stable, because only has performance impact
> > and some application might be depending on current broken behaviour.
> > 
> > 
> > 
> > --- a/net/ipv4/udp.c	2012-10-01 17:06:53.107427436 -0700
> > +++ b/net/ipv4/udp.c	2012-10-04 13:43:21.278960379 -0700
> > @@ -216,7 +216,7 @@ int udp_lib_get_port(struct sock *sk, un
> >  		remaining = (high - low) + 1;
> >  
> >  		rand = net_random();
> > -		first = (((u64)rand * remaining) >> 32) + low;
> > +		first = rand % remaining + low;
> 
> Try replacing "remaining" with "(remaining << (64 - 16))" in
> the expression instead.

The standalone program gets same result.