From: Eric Dumazet <eric.dumazet@gmail.com>
To: Stephen Hemminger <shemminger@vyatta.com>
Cc: David Miller <davem@davemloft.net>,
dada1@cosmosbay.com, netdev@vger.kernel.org
Subject: Re: [PATCH] udp: port starting location not random
Date: Thu, 04 Oct 2012 23:45:53 +0200 [thread overview]
Message-ID: <1349387153.21172.1.camel@edumazet-glaptop> (raw)
In-Reply-To: <20121004142859.08ab0385@nehalam.linuxnetplumber.net>
On Thu, 2012-10-04 at 14:28 -0700, Stephen Hemminger wrote:
> 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.
Hey, I hope you understand random32() does allocate a 32bit value, not a
15bit one...
If really we had such a bug, I am pretty sure we would have noticed.
next prev parent reply other threads:[~2012-10-04 21:45 UTC|newest]
Thread overview: 9+ messages / expand[flat|nested] mbox.gz Atom feed top
2012-10-04 21:08 [PATCH] udp: port starting location not random Stephen Hemminger
2012-10-04 21:12 ` David Miller
2012-10-04 21:28 ` Stephen Hemminger
2012-10-04 21:45 ` Eric Dumazet [this message]
2012-10-04 21:50 ` David Miller
2012-10-04 21:57 ` Eric Dumazet
2012-10-04 22:06 ` Stephen Hemminger
2012-10-04 21:59 ` Stephen Hemminger
[not found] <20121004.170103.1493867833710136820.davem@davemloft.net>
[not found] ` <b400428e-ffe4-42ee-b656-99b0314d64f4@tahiti.vyatta.com>
2012-10-04 21:12 ` David Miller
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