Re: [PATCH] sched/fair: Fix integer underflow

[Pierre Gondois <pierre.gondois@arm.com>]

Hello Vincent,

On 9/13/24 18:14, Vincent Guittot wrote:
> Hi Pierre
> 
> On Fri, 13 Sept 2024 at 10:58, Pierre Gondois <pierre.gondois@arm.com> wrote:
>>
>> (struct sg_lb_stats).idle_cpus is of type 'unsigned int'.
>> (local->idle_cpus - busiest->idle_cpus) can underflow to UINT_MAX
>> for instance, and max_t(long, 0, UINT_MAX) will output UINT_MAX.
>>
>> Use lsub_positive() instead of max_t().
> 
> Have you faced the problem or this patch is based on code review ?
> 
> we have the below in sched_balance_find_src_group() that should ensure
> that we have local->idle_cpus > busiest->idle_cpus
> 
> if (busiest->group_weight > 1 &&
>      local->idle_cpus <= (busiest->idle_cpus + 1)) {
>      /*
>      * If the busiest group is not overloaded
>      * and there is no imbalance between this and busiest
>      * group wrt idle CPUs, it is balanced. The imbalance
>      * becomes significant if the diff is greater than 1
>      * otherwise we might end up to just move the imbalance
>      * on another group. Of course this applies only if
>      * there is more than 1 CPU per group.
>      */
>      goto out_balanced;
> }

It was with this setup:
- busiest->group_type == group_overloaded
so it might not have checked the value. I can check the exact path if desired,

Regards,
Pierre


> 
>>
>> Fixes: 0b0695f2b34a ("sched/fair: Rework load_balance()")
>> cc: stable@vger.kernel.org
>> Signed-off-by: Pierre Gondois <pierre.gondois@arm.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 9057584ec06d..6d9124499f52 100644
>> --- a/kernel/sched/fair.c
>> +++ b/kernel/sched/fair.c
>> @@ -10775,8 +10775,8 @@ static inline void calculate_imbalance(struct lb_env *env, struct sd_lb_stats *s
>>                           * idle CPUs.
>>                           */
>>                          env->migration_type = migrate_task;
>> -                       env->imbalance = max_t(long, 0,
>> -                                              (local->idle_cpus - busiest->idle_cpus));
>> +                       env->imbalance = local->idle_cpus;
>> +                       lsub_positive(&env->imbalance, busiest->idle_cpus);
>>                  }
>>
>>   #ifdef CONFIG_NUMA
>> --
>> 2.25.1
>>