linux-kernel - Re: [PATCH 2/9] perf/arm-cmn: Avoid explicit cpumask var allocation from stack

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Date: Fri, 5 Apr 2024 15:30:51 +0100
From: Robin Murphy <robin.murphy@....com>
To: Dawei Li <dawei.li@...ngroup.cn>, will@...nel.org, mark.rutland@....com
Cc: 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 2/9] perf/arm-cmn: Avoid explicit cpumask var allocation
 from stack

On 2024-04-02 11:56 am, 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.
> 
> Use *cpumask_var API(s) to address it.

I think the temporary mask may simply be redundant anyway. It seems like 
I may have misunderstood, and cpumask_of_node() actually only covers 
online CPUs already.

Thanks,
Robin.

> Signed-off-by: Dawei Li <dawei.li@...ngroup.cn>
> ---
>   drivers/perf/arm-cmn.c | 13 +++++++++----
>   1 file changed, 9 insertions(+), 4 deletions(-)
> 
> diff --git a/drivers/perf/arm-cmn.c b/drivers/perf/arm-cmn.c
> index 7ef9c7e4836b..7278fd72d3da 100644
> --- a/drivers/perf/arm-cmn.c
> +++ b/drivers/perf/arm-cmn.c
> @@ -1949,21 +1949,26 @@ static int arm_cmn_pmu_offline_cpu(unsigned int cpu, struct hlist_node *cpuhp_no
>   {
>   	struct arm_cmn *cmn;
>   	unsigned int target;
> +	cpumask_var_t mask;
>   	int node;
> -	cpumask_t mask;
>   
>   	cmn = hlist_entry_safe(cpuhp_node, struct arm_cmn, cpuhp_node);
>   	if (cpu != cmn->cpu)
>   		return 0;
>   
> +	if (!alloc_cpumask_var(&mask, GFP_KERNEL))
> +		return 0;
> +
>   	node = dev_to_node(cmn->dev);
> -	if (cpumask_and(&mask, cpumask_of_node(node), cpu_online_mask) &&
> -	    cpumask_andnot(&mask, &mask, cpumask_of(cpu)))
> -		target = cpumask_any(&mask);
> +	if (cpumask_and(mask, cpumask_of_node(node), cpu_online_mask) &&
> +	    cpumask_andnot(mask, mask, cpumask_of(cpu)))
> +		target = cpumask_any(mask);
>   	else
>   		target = cpumask_any_but(cpu_online_mask, cpu);
>   	if (target < nr_cpu_ids)
>   		arm_cmn_migrate(cmn, target);
> +
> +	free_cpumask_var(mask);
>   	return 0;
>   }
>