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

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Message-ID: <20121004140828.2d6f7bf9@nehalam.linuxnetplumber.net>
Date:	Thu, 4 Oct 2012 14:08:28 -0700
From:	Stephen Hemminger <shemminger@...tta.com>
To:	Eric Dumazet <dada1@...mosbay.com>,
	David Miller <davem@...emloft.net>
Cc:	netdev@...r.kernel.org
Subject: [PATCH] udp: port starting location not random

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;
 		/*
 		 * force rand to be an odd multiple of UDP_HTABLE_SIZE
 		 */
--
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