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

[Vincent Guittot]

Hi Pierre

On Fri, 13 Sept 2024 at 10:58, Pierre Gondois <pierre.gondois@xxxxxxx> 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;
}

>
> Fixes: 0b0695f2b34a ("sched/fair: Rework load_balance()")
> cc: stable@xxxxxxxxxxxxxxx
> Signed-off-by: Pierre Gondois <pierre.gondois@xxxxxxx>
> ---
>  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
>