From mboxrd@z Thu Jan 1 00:00:00 1970 From: Jesper Dangaard Brouer Subject: Re: [net-next PATCH] net: codel: Avoid undefined behavior from signed overflow Date: Thu, 31 Oct 2013 15:15:51 +0100 Message-ID: <20131031151551.675ab908@redhat.com> References: <20131030172341.19203.93490.stgit@dragon> <1383156104.4857.49.camel@edumazet-glaptop.roam.corp.google.com> Mime-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit Cc: Jesper Dangaard Brouer , netdev@vger.kernel.org, "Paul E. McKenney" , Dave Taht To: Eric Dumazet Return-path: Received: from mx1.redhat.com ([209.132.183.28]:62046 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1754302Ab3JaOQE (ORCPT ); Thu, 31 Oct 2013 10:16:04 -0400 In-Reply-To: <1383156104.4857.49.camel@edumazet-glaptop.roam.corp.google.com> Sender: netdev-owner@vger.kernel.org List-ID: On Wed, 30 Oct 2013 11:01:44 -0700 Eric Dumazet wrote: > On Wed, 2013-10-30 at 18:23 +0100, Jesper Dangaard Brouer wrote: > > 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) > > > > I see nothing enforcing an unsigned subtraction as claimed in your > changelog. > > a / b could be signed. > > Paul commit 5a581b367b5 was OK because of existing typecheck(unsigned > long, ....) Okay, I'll cook up another patch, after work. Adding all the typecheck() stuff, just bloats the code. Would it be better/okay just to do?: (s32)((u32)(a) - (u32)(b)) > 0) -- Best regards, Jesper Dangaard Brouer MSc.CS, Sr. Network Kernel Developer at Red Hat Author of http://www.iptv-analyzer.org LinkedIn: http://www.linkedin.com/in/brouer