From mboxrd@z Thu Jan 1 00:00:00 1970 From: Stephen Hemminger Subject: [PATCH] udp: port starting location not random Date: Thu, 4 Oct 2012 14:08:28 -0700 Message-ID: <20121004140828.2d6f7bf9@nehalam.linuxnetplumber.net> Mime-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit Cc: netdev@vger.kernel.org To: Eric Dumazet , David Miller Return-path: Received: from mail.vyatta.com ([76.74.103.46]:41942 "EHLO mail.vyatta.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1757562Ab2JDVJF (ORCPT ); Thu, 4 Oct 2012 17:09:05 -0400 Sender: netdev-owner@vger.kernel.org List-ID: While working on VXLAN, noticed a bug in UDP introduced by: commit 9088c5609584684149f3fb5b065aa7f18dcb03ff Author: Eric Dumazet 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 --- 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 */