Re: [PATCH v2] mm,writeback: fix divide by zero in pos_ratio_polynom

Rik van Riel
2014-04-30T15:31:17Z

On 04/30/2014 10:49 AM, Michal Hocko wrote:
> 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@redhat.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?

For unsigned, it probably doesn't make a difference.

For signed int vs unsigned long, I wonder if there is a
corner case where casting the second to the first before
doing the "limit - setpoint" calculation can lead to a
different outcome...