linux-kernel - Re: [RFC][PATCH 0/5] Fixes for abs() usage on 64bit values

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Date:	Tue, 15 Sep 2015 07:09:42 -0500
From:	Jeff Epler <jepler@...ythonic.net>
To:	Tejun Heo <tj@...nel.org>
Cc:	John Stultz <john.stultz@...aro.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Ingo Molnar <mingo@...nel.org>,
	"Steven Rostedt (Red Hat)" <rostedt@...dmis.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Masami Hiramatsu <masami.hiramatsu.pt@...achi.com>,
	Michal Nazarewicz <mina86@...a86.com>,
	Prarit Bhargava <prarit@...hat.com>,
	Richard Cochran <richardcochran@...il.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Theodore Ts'o <tytso@....edu>,
	Andreas Dilger <adilger.kernel@...ger.ca>,
	Dave Chinner <dchinner@...hat.com>,
	Joe Perches <joe@...ches.com>
Subject: Re: [RFC][PATCH 0/5] Fixes for abs() usage on 64bit values

On Mon, Sep 14, 2015 at 11:46:32PM -0400, Tejun Heo wrote:
> Hello,
> 
> On Mon, Sep 14, 2015 at 08:27:08PM -0700, John Stultz wrote:
> > Yea. The above make sense to me, but I suspect there's some very
> > subtle reason for the existing separated logic.
> > But I'd have to defer to akpm for hints on that.
> 
> Hmmm... people could be using it for calculating the distance between
> two unsigned values and in that case the original behavior would be
> the correct one.  e.g.
> 
>  unsigned diff, a, b;
>  diff = abs(a - b);

This kind of construct can overflow whether a and b are signed or
unsigned.  Only a two-argument function can correctly return the
absolute difference of two integers.

At my day job, we arranged for
abs(unsigned type) to be a compile-time error and supplied a two args
function absdiff for use in these situations -- absdiff(a,b) is the same
as abs(a-b) except it avoids overflow and returns an unsigned type.

(though I have no doubt that some instances of abs(a - b) still exist
where a and b are signed and the intermediate could still overflow...)

Jeff
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