Re: [PATCH v2 03/10] perf/arm-cmn: Avoid placing cpumask var on stack

[Mark Rutland <mark.rutland@arm.com>]

On Wed, Apr 03, 2024 at 08:51:02PM +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@arm.com>
> Signed-off-by: Dawei Li <dawei.li@shingroup.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@arm.com>

Mark.

> ---
>  drivers/perf/arm-cmn.c | 10 +++++-----
>  1 file changed, 5 insertions(+), 5 deletions(-)
> 
> diff --git a/drivers/perf/arm-cmn.c b/drivers/perf/arm-cmn.c
> index 7ef9c7e4836b..6bfb0c4a1287 100644
> --- a/drivers/perf/arm-cmn.c
> +++ b/drivers/perf/arm-cmn.c
> @@ -1950,20 +1950,20 @@ static int arm_cmn_pmu_offline_cpu(unsigned int cpu, struct hlist_node *cpuhp_no
>  	struct arm_cmn *cmn;
>  	unsigned int target;
>  	int node;
> -	cpumask_t mask;
>  
>  	cmn = hlist_entry_safe(cpuhp_node, struct arm_cmn, cpuhp_node);
>  	if (cpu != cmn->cpu)
>  		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);
> -	else
> +
> +	target = cpumask_any_and_but(cpumask_of_node(node), cpu_online_mask, cpu);
> +	if (target >= nr_cpu_ids)
>  		target = cpumask_any_but(cpu_online_mask, cpu);
> +
>  	if (target < nr_cpu_ids)
>  		arm_cmn_migrate(cmn, target);
> +
>  	return 0;
>  }
>  
> -- 
> 2.27.0
>