netdev - Re: [PATCH] udp: port starting location not random

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Date:	Thu, 4 Oct 2012 14:28:59 -0700
From:	Stephen Hemminger <shemminger@...tta.com>
To:	David Miller <davem@...emloft.net>
Cc:	dada1@...mosbay.com, netdev@...r.kernel.org
Subject: Re: [PATCH] udp: port starting location not random

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

> From: Stephen Hemminger <shemminger@...tta.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@...mosbay.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@...tta.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.

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