linux-kernel - Re: [PATCH] sched/cpupri: fix the task priority BUG

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Date:   Tue, 6 Apr 2021 13:42:27 +0200
From:   Dietmar Eggemann <dietmar.eggemann@....com>
To:     Zeng Tao <prime.zeng@...ilicon.com>, mingo@...hat.com,
        peterz@...radead.org, juri.lelli@...hat.com,
        vincent.guittot@...aro.org
Cc:     linuxarm@...wei.com, Steven Rostedt <rostedt@...dmis.org>,
        Ben Segall <bsegall@...gle.com>, Mel Gorman <mgorman@...e.de>,
        Daniel Bristot de Oliveira <bristot@...hat.com>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] sched/cpupri: fix the task priority BUG_ON checks

On 26/03/2021 04:15, Zeng Tao wrote:
> The BUG_ON checks are intended to check if the task priotity is valid,
> but in the function convert_prio, if the task priority is not valid, it
> will be converted to an uninitialized stack variable. Fix it by moving
> the BUG_ON checks to the default branch of convert_prio.
> 
> Fixes: 934fc3314b39 ("sched/cpupri: Remap CPUPRI_NORMAL to MAX_RT_PRIO-1")
> Signed-off-by: Zeng Tao <prime.zeng@...ilicon.com>
> ---
>  kernel/sched/cpupri.c | 6 ++----
>  1 file changed, 2 insertions(+), 4 deletions(-)
> 
> diff --git a/kernel/sched/cpupri.c b/kernel/sched/cpupri.c
> index ec9be78..c5a0e6e 100644
> --- a/kernel/sched/cpupri.c
> +++ b/kernel/sched/cpupri.c
> @@ -60,6 +60,8 @@ static int convert_prio(int prio)
>  	case MAX_RT_PRIO:
>  		cpupri = CPUPRI_HIGHER;		/* 100 */
>  		break;
> +	default:
> +		BUG();
>  	}
>  
>  	return cpupri;
> @@ -148,8 +150,6 @@ int cpupri_find_fitness(struct cpupri *cp, struct task_struct *p,
>  	int task_pri = convert_prio(p->prio);
>  	int idx, cpu;
>  
> -	BUG_ON(task_pri >= CPUPRI_NR_PRIORITIES);
> -
>  	for (idx = 0; idx < task_pri; idx++) {
>  
>  		if (!__cpupri_find(cp, p, lowest_mask, idx))
> @@ -215,8 +215,6 @@ void cpupri_set(struct cpupri *cp, int cpu, int newpri)
>  
>  	newpri = convert_prio(newpri);
>  
> -	BUG_ON(newpri >= CPUPRI_NR_PRIORITIES);
> -
>  	if (newpri == oldpri)
>  		return;
>  
> 

LGTM. Currently, convert_prio() is never called with prio <
CPUPRI_INVALID (-1) or prio > MAX_RT_PRIO (100) but in case it would
then this would be caught much easier with this patch.

Reviewed-by: Dietmar Eggemann <dietmar.eggemann@....com>