Re: [PATCH v2 04/10] perf/arm_cspmu: Avoid placing cpumask var on stack

[Mark Rutland]

On Wed, Apr 03, 2024 at 08:51:03PM +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@xxxxxxx>
> 
> Signed-off-by: Dawei Li <dawei.li@xxxxxxxxxxxx>

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@xxxxxxx>

Mark.

> ---
>  drivers/perf/arm_cspmu/arm_cspmu.c | 8 +++-----
>  1 file changed, 3 insertions(+), 5 deletions(-)
> 
> diff --git a/drivers/perf/arm_cspmu/arm_cspmu.c b/drivers/perf/arm_cspmu/arm_cspmu.c
> index b9a252272f1e..fd1004251665 100644
> --- a/drivers/perf/arm_cspmu/arm_cspmu.c
> +++ b/drivers/perf/arm_cspmu/arm_cspmu.c
> @@ -1322,8 +1322,7 @@ static int arm_cspmu_cpu_online(unsigned int cpu, struct hlist_node *node)
>  
>  static int arm_cspmu_cpu_teardown(unsigned int cpu, struct hlist_node *node)
>  {
> -	int dst;
> -	struct cpumask online_supported;
> +	unsigned int dst;
>  
>  	struct arm_cspmu *cspmu =
>  		hlist_entry_safe(node, struct arm_cspmu, cpuhp_node);
> @@ -1333,9 +1332,8 @@ static int arm_cspmu_cpu_teardown(unsigned int cpu, struct hlist_node *node)
>  		return 0;
>  
>  	/* Choose a new CPU to migrate ownership of the PMU to */
> -	cpumask_and(&online_supported, &cspmu->associated_cpus,
> -		    cpu_online_mask);
> -	dst = cpumask_any_but(&online_supported, cpu);
> +	dst = cpumask_any_and_but(&cspmu->associated_cpus,
> +				  cpu_online_mask, cpu);
>  	if (dst >= nr_cpu_ids)
>  		return 0;
>  
> -- 
> 2.27.0
>