linux-kernel - Re: [PATCH v2 09/10] perf/qcom_l2: Avoid placing cpumask var on stack

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Date: Wed, 3 Apr 2024 15:36:30 +0100
From: Mark Rutland <mark.rutland@....com>
To: Dawei Li <dawei.li@...ngroup.cn>
Cc: will@...nel.org, yury.norov@...il.com, linux@...musvillemoes.dk,
	xueshuai@...ux.alibaba.com, renyu.zj@...ux.alibaba.com,
	yangyicong@...ilicon.com, jonathan.cameron@...wei.com,
	andersson@...nel.org, konrad.dybcio@...aro.org,
	linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
	linux-arm-msm@...r.kernel.org
Subject: Re: [PATCH v2 09/10] perf/qcom_l2: Avoid placing cpumask var on stack

On Wed, Apr 03, 2024 at 08:51:08PM +0800, Dawei Li wrote:
> For CONFIG_CPUMASK_OFFSTACK=y kernel, explicit allocation of cpumask
> variable on stack is not recommended since it can cause potential stack
> overflow.
> 
> Instead, kernel code should always use *cpumask_var API(s) to allocate
> cpumask var in config-neutral way, leaving allocation strategy to
> CONFIG_CPUMASK_OFFSTACK.
> 
> But dynamic allocation in cpuhp's teardown callback is somewhat problematic
> for if allocation fails(which is unlikely but still possible):
> - If -ENOMEM is returned to caller, kernel crashes for non-bringup
>   teardown;
> - If callback pretends nothing happened and returns 0 to caller, it may
>   trap system into an in-consisitent/compromised state;
> 
> Use newly-introduced cpumask_any_and_but() to address all issues above.
> It eliminates usage of temporary cpumask var in generic way, no matter how
> the cpumask var is allocated.
> 
> Suggested-by: Mark Rutland <mark.rutland@....com>
> Signed-off-by: Dawei Li <dawei.li@...ngroup.cn>

The logic looks good to me, but I'd like the commit message updated the same as
per my comment on patch 2.

With that commit message:

Reviewed-by: Mark Rutland <mark.rutland@....com>

Mark.

> ---
>  drivers/perf/qcom_l2_pmu.c | 8 +++-----
>  1 file changed, 3 insertions(+), 5 deletions(-)
> 
> diff --git a/drivers/perf/qcom_l2_pmu.c b/drivers/perf/qcom_l2_pmu.c
> index 148df5ae8ef8..b5a44dc1dc3a 100644
> --- a/drivers/perf/qcom_l2_pmu.c
> +++ b/drivers/perf/qcom_l2_pmu.c
> @@ -801,9 +801,8 @@ static int l2cache_pmu_online_cpu(unsigned int cpu, struct hlist_node *node)
>  
>  static int l2cache_pmu_offline_cpu(unsigned int cpu, struct hlist_node *node)
>  {
> -	struct cluster_pmu *cluster;
>  	struct l2cache_pmu *l2cache_pmu;
> -	cpumask_t cluster_online_cpus;
> +	struct cluster_pmu *cluster;
>  	unsigned int target;
>  
>  	l2cache_pmu = hlist_entry_safe(node, struct l2cache_pmu, node);
> @@ -820,9 +819,8 @@ static int l2cache_pmu_offline_cpu(unsigned int cpu, struct hlist_node *node)
>  	cluster->on_cpu = -1;
>  
>  	/* Any other CPU for this cluster which is still online */
> -	cpumask_and(&cluster_online_cpus, &cluster->cluster_cpus,
> -		    cpu_online_mask);
> -	target = cpumask_any_but(&cluster_online_cpus, cpu);
> +	target = cpumask_any_and_but(&cluster->cluster_cpus,
> +				     cpu_online_mask, cpu);
>  	if (target >= nr_cpu_ids) {
>  		disable_irq(cluster->irq);
>  		return 0;
> -- 
> 2.27.0
>