linux-kernel - Re: [PATCH] sched/fair: fix mul overflow on 32-bit systems

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Date:	Fri, 11 Dec 2015 09:58:03 -0800
From:	bsegall@...gle.com
To:	Andrey Ryabinin <aryabinin@...tuozzo.com>
Cc:	Peter Zijlstra <peterz@...radead.org>, <mingo@...hat.com>,
	<linux-kernel@...r.kernel.org>, <yuyang.du@...el.com>,
	Morten Rasmussen <morten.rasmussen@....com>,
	Paul Turner <pjt@...gle.com>
Subject: Re: [PATCH] sched/fair: fix mul overflow on 32-bit systems

Andrey Ryabinin <aryabinin@...tuozzo.com> writes:

> On 12/11/2015 04:36 PM, Peter Zijlstra wrote:
>> On Fri, Dec 11, 2015 at 02:25:51PM +0100, Peter Zijlstra wrote:
>>> On Fri, Dec 11, 2015 at 03:55:18PM +0300, Andrey Ryabinin wrote:
>>>> Make 'r' 64-bit type to avoid overflow in 'r * LOAD_AVG_MAX'
>>>> on 32-bit systems:
>>>> 	UBSAN: Undefined behaviour in kernel/sched/fair.c:2785:18
>>>> 	signed integer overflow:
>>>> 	87950 * 47742 cannot be represented in type 'int'
>>>>
>>>> Fixes: 9d89c257dfb9 ("sched/fair: Rewrite runnable load and utilization average tracking")
>>>> Signed-off-by: Andrey Ryabinin <aryabinin@...tuozzo.com>
>>>> ---
>>>>  kernel/sched/fair.c | 4 ++--
>>>>  1 file changed, 2 insertions(+), 2 deletions(-)
>>>>
>>>> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
>>>> index e3266eb..733f0b8 100644
>>>> --- a/kernel/sched/fair.c
>>>> +++ b/kernel/sched/fair.c
>>>> @@ -2780,14 +2780,14 @@ static inline int update_cfs_rq_load_avg(u64 now, struct cfs_rq *cfs_rq)
>>>>  	int decayed, removed = 0;
>>>>  
>>>>  	if (atomic_long_read(&cfs_rq->removed_load_avg)) {
>>>> -		long r = atomic_long_xchg(&cfs_rq->removed_load_avg, 0);
>>>> +		s64 r = atomic_long_xchg(&cfs_rq->removed_load_avg, 0);
>>>>  		sa->load_avg = max_t(long, sa->load_avg - r, 0);
>>>>  		sa->load_sum = max_t(s64, sa->load_sum - r * LOAD_AVG_MAX, 0);
>>>
>>> This makes sense, because sched_avg::load_sum is u64.
>>>
>>>>  		removed = 1;
>>>>  	}
>>>>  
>>>>  	if (atomic_long_read(&cfs_rq->removed_util_avg)) {
>>>> -		long r = atomic_long_xchg(&cfs_rq->removed_util_avg, 0);
>>>> +		s64 r = atomic_long_xchg(&cfs_rq->removed_util_avg, 0);
>>>>  		sa->util_avg = max_t(long, sa->util_avg - r, 0);
>>>>  		sa->util_sum = max_t(s32, sa->util_sum - r * LOAD_AVG_MAX, 0);
>>>>  	}
>>>
>>> However sched_avg::util_sum is u32, so this is still wrecked.
>> 
>> I seems to have wrecked that in:
>> 
>>   006cdf025a33 ("sched/fair: Optimize per entity utilization tracking")
>> 
>> maybe just make util_load u64 too?
>> 
>
> Is there any guarantee that the final result of expression 'util_sum - r * LOAD_AVG_MAX' always can be represented by s32?
>
> If yes, than we could just do this:
> 	max_t(s32, (u64)sa->util_sum - r * LOAD_AVG_MAX, 0)

Well, the (original) intent was that util_sum was always 32-bit, since
it is at most LOAD_AVG_MAX times a fixed point math fraction from
arch_scale_cpu_capacity(), which is probably 4096 or 1024 or something
like that (and LOAD_AVG_MAX is < 2**16, so it's not even close to
overflow).

I'm not 100% sure this hasn't been broken, but unless it has been,
the second hunk is completely unnecessary (the first hunk looks correct
and necessary).
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