linux-kernel - Re: [PATCH v2] mm,writeback: fix divide by zero in pos_ratio

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Message-ID: <20140430144903.GI4357@dhcp22.suse.cz>
Date:	Wed, 30 Apr 2014 16:49:03 +0200
From:	Michal Hocko <mhocko@...e.cz>
To:	Rik van Riel <riel@...hat.com>
Cc:	Masayoshi Mizuma <m.mizuma@...fujitsu.com>,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org,
	sandeen@...hat.com, akpm@...ux-foundation.org, jweiner@...hat.com,
	kosaki.motohiro@...fujitsu.com, fengguang.wu@...el.com,
	mpatlasov@...allels.com, Motohiro.Kosaki@...fujitsu.com
Subject: Re: [PATCH v2] mm,writeback: fix divide by zero in pos_ratio_polynom

On Wed 30-04-14 10:31:29, Rik van Riel wrote:
> On 04/30/2014 09:48 AM, Michal Hocko wrote:
> >On Wed 30-04-14 09:30:35, Rik van Riel wrote:
> >[...]
> >>Subject: mm,writeback: fix divide by zero in pos_ratio_polynom
> >>
> >>It is possible for "limit - setpoint + 1" to equal zero, leading to a
> >>divide by zero error. Blindly adding 1 to "limit - setpoint" is not
> >>working, so we need to actually test the divisor before calling div64.
> >>
> >>Signed-off-by: Rik van Riel <riel@...hat.com>
> >>---
> >>  mm/page-writeback.c | 13 +++++++++++--
> >>  1 file changed, 11 insertions(+), 2 deletions(-)
> >>
> >>diff --git a/mm/page-writeback.c b/mm/page-writeback.c
> >>index ef41349..f98a297 100644
> >>--- a/mm/page-writeback.c
> >>+++ b/mm/page-writeback.c
> >>@@ -597,11 +597,16 @@ static inline long long pos_ratio_polynom(unsigned long setpoint,
> >>  					  unsigned long dirty,
> >>  					  unsigned long limit)
> >>  {
> >>+	unsigned long divisor;
> >>  	long long pos_ratio;
> >>  	long x;
> >>
> >>+	divisor = limit - setpoint;
> >>+	if (!divisor)
> >>+		divisor = 1;	/* Avoid div-by-zero */
> >>+
> >
> >This is still prone to u64 -> s32 issue, isn't it?
> >What was the original problem anyway? Was it really setpoint > limit or
> >rather the overflow?
> 
> Thinking about it some more, is it possible that
> limit and/or setpoint are larger than 32 bits, but
> the difference between them is not?
> 
> In that case, truncating both to 32 bits before
> doing the subtraction would be troublesome, and
> it would be better to do a cast in the comparison:
> 
> if (!(s32)divisor)
> 	divisor = 1;

How is that any different than defining divisor as 32b directly?
 

-- 
Michal Hocko
SUSE Labs
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