From mboxrd@z Thu Jan 1 00:00:00 1970 From: Jesper Dangaard Brouer Subject: [net-next PATCH] net: codel: Avoid undefined behavior from signed overflow Date: Wed, 30 Oct 2013 18:23:42 +0100 Message-ID: <20131030172341.19203.93490.stgit@dragon> Mime-Version: 1.0 Content-Type: text/plain; charset="utf-8" Content-Transfer-Encoding: 7bit Cc: Eric Dumazet , "Paul E. McKenney" , Dave Taht , Jesper Dangaard Brouer To: netdev@vger.kernel.org Return-path: Received: from mx1.redhat.com ([209.132.183.28]:6869 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1750865Ab3J3RSR (ORCPT ); Wed, 30 Oct 2013 13:18:17 -0400 Sender: netdev-owner@vger.kernel.org List-ID: From: Jesper Dangaard Brouer As described in commit 5a581b367 (jiffies: Avoid undefined behavior from signed overflow), according to the C standard 3.4.3p3, overflow of a signed integer results in undefined behavior. To fix this, do as the above commit, and do an unsigned subtraction, and interpreting the result as a signed two's-complement number. This is based on the theory from RFC 1982 and is nicely described in wikipedia here: https://en.wikipedia.org/wiki/Serial_number_arithmetic#General_Solution A side-note, I have seen practical issues with the previous logic when dealing with 16-bit, on a 64-bit machine (gcc version 4.4.5). This were 32-bit, which I have not observed issues with. Cc: Paul E. McKenney Signed-off-by: Jesper Dangaard Brouer --- include/net/codel.h | 8 ++++---- 1 files changed, 4 insertions(+), 4 deletions(-) diff --git a/include/net/codel.h b/include/net/codel.h index 389cf62..700fcdf 100644 --- a/include/net/codel.h +++ b/include/net/codel.h @@ -72,10 +72,10 @@ static inline codel_time_t codel_get_time(void) return ns >> CODEL_SHIFT; } -#define codel_time_after(a, b) ((s32)(a) - (s32)(b) > 0) -#define codel_time_after_eq(a, b) ((s32)(a) - (s32)(b) >= 0) -#define codel_time_before(a, b) ((s32)(a) - (s32)(b) < 0) -#define codel_time_before_eq(a, b) ((s32)(a) - (s32)(b) <= 0) +#define codel_time_after(a, b) ((s32)((a) - (b)) > 0) +#define codel_time_after_eq(a, b) ((s32)((a) - (b)) >= 0) +#define codel_time_before(a, b) ((s32)((a) - (b)) < 0) +#define codel_time_before_eq(a, b) ((s32)((a) - (b)) <= 0) /* Qdiscs using codel plugin must use codel_skb_cb in their own cb[] */ struct codel_skb_cb {