Re: [PATCH v2] sched: Fix out-of-bound access in uclamp

[Quentin Perret]

On Friday 30 Apr 2021 at 09:45:32 (+0200), Vincent Guittot wrote:
> On Thu, 29 Apr 2021 at 17:27, Quentin Perret <qperret@xxxxxxxxxx> wrote:
> >
> > Util-clamp places tasks in different buckets based on their clamp values
> > for performance reasons. However, the size of buckets is currently
> > computed using a rounding division, which can lead to an off-by-one
> > error in some configurations.
> >
> > For instance, with 20 buckets, the bucket size will be 1024/20=51. A
> > task with a clamp of 1024 will be mapped to bucket id 1024/51=20. Sadly,
> > correct indexes are in range [0,19], hence leading to an out of bound
> > memory access.
> >
> > Fix the math to compute the bucket size.
> >
> > Fixes: 69842cba9ace ("sched/uclamp: Add CPU's clamp buckets refcounting")
> > Suggested-by: Qais Yousef <qais.yousef@xxxxxxx>
> > Signed-off-by: Quentin Perret <qperret@xxxxxxxxxx>
> >
> > ---
> >
> > Changes in v2:
> >  - replaced the DIV_ROUND_UP(a,b) with a/b+1 (Dietmar)
> 
> Doesn't this create unfairness between buckets ?
> 
> If we take your example above of 20 buckets, delta is now 52. Then we
> expect the last bucket to get the range [972-1024] but values lower
> than 988 will go in the idx 18.

Well, that's just the limitation of integer arithmetics isn't it?

> And the more bucket you will have, the
> worse it will be

Sure, but 20 is a hard limit, and if we ever need more than that then
maybe we should think about getting rid of the buckets altogether.

> Your problem comes from the fact that we use 1025 values instead of
> 1024.

I don't understand what you mean here. Right now we'll assign bucket id
20 for any clamp in the range [1020-1024], so I don't think we can
special case 1024.

> Wouldn't it be easier to have a special condition for
> SCHED_CAPACITY_SCALE value

As per the above, I don't see how that'll work.

Thanks,
Quentin