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

Peter Zijlstra
2015-12-11T13:36:21Z

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@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.

I seems to have wrecked that in:

006cdf025a33 ("sched/fair: Optimize per entity utilization tracking")

maybe just make util_load u64 too?