linux-kernel - Re: [PATCH v3] mm,writeback: fix divide by zero in pos_ratio

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Message-ID: <53614F3C.8020009@redhat.com>
Date:	Wed, 30 Apr 2014 15:30:04 -0400
From:	Rik van Riel <riel@...hat.com>
To:	Andrew Morton <akpm@...ux-foundation.org>
CC:	Michal Hocko <mhocko@...e.cz>,
	Masayoshi Mizuma <m.mizuma@...fujitsu.com>,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org,
	sandeen@...hat.com, jweiner@...hat.com,
	kosaki.motohiro@...fujitsu.com, fengguang.wu@...el.com,
	mpatlasov@...allels.com, Motohiro.Kosaki@...fujitsu.com
Subject: Re: [PATCH v3] mm,writeback: fix divide by zero in pos_ratio_polynom

On 04/30/2014 03:00 PM, Andrew Morton wrote:
> On Wed, 30 Apr 2014 10:41:14 -0400 Rik van Riel <riel@...hat.com> wrote:
>
>> 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.
>>
>> ...
>>
>> --- a/mm/page-writeback.c
>> +++ b/mm/page-writeback.c
>> @@ -598,10 +598,15 @@ static inline long long pos_ratio_polynom(unsigned long setpoint,
>>   					  unsigned long limit)
>>   {
>>   	long long pos_ratio;
>> +	long divisor;
>>   	long x;
>>
>> +	divisor = limit - setpoint;
>> +	if (!(s32)divisor)
>> +		divisor = 1;	/* Avoid div-by-zero */
>> +
>>   	x = div_s64(((s64)setpoint - (s64)dirty) << RATELIMIT_CALC_SHIFT,
>> -		    limit - setpoint + 1);
>> +		    (s32)divisor);
>
> Doesn't this just paper over the bug one time in four billion?  The
> other 3999999999 times, pos_ratio_polynom() returns an incorect result?
>
> If it is indeed the case that pos_ratio_polynom() callers are
> legitimately passing a setpoint which is more than 2^32 less than limit
> then it would be better to handle that input correctly.

The easy way would be by calling div64_s64 and div64_u64,
which are 64 bit all the way through.

Any objections?

The inlined bits seem to be stubs calling the _rem variants
of the functions, and discarding the remainder.

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