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

[Peter Zijlstra <peterz@infradead.org>]

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@virtuozzo.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.