Messages in this thread | | | From | bsegall@google ... | Subject | Re: [PATCH] sched/fair: fix mul overflow on 32-bit systems | Date | Fri, 11 Dec 2015 09:58:03 -0800 |
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Andrey Ryabinin <aryabinin@virtuozzo.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@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? >> > > 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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