Re: [PATCH v2 06/10] perf/dwc_pcie: Avoid placing cpumask var on stack

[Mark Rutland]

On Wed, Apr 03, 2024 at 08:51:05PM +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/dwc_pcie_pmu.c | 10 ++++------
>  1 file changed, 4 insertions(+), 6 deletions(-)
> 
> diff --git a/drivers/perf/dwc_pcie_pmu.c b/drivers/perf/dwc_pcie_pmu.c
> index 957058ad0099..c5e328f23841 100644
> --- a/drivers/perf/dwc_pcie_pmu.c
> +++ b/drivers/perf/dwc_pcie_pmu.c
> @@ -690,9 +690,8 @@ static int dwc_pcie_pmu_offline_cpu(unsigned int cpu, struct hlist_node *cpuhp_n
>  {
>  	struct dwc_pcie_pmu *pcie_pmu;
>  	struct pci_dev *pdev;
> -	int node;
> -	cpumask_t mask;
>  	unsigned int target;
> +	int node;
>  
>  	pcie_pmu = hlist_entry_safe(cpuhp_node, struct dwc_pcie_pmu, cpuhp_node);
>  	/* Nothing to do if this CPU doesn't own the PMU */
> @@ -702,10 +701,9 @@ static int dwc_pcie_pmu_offline_cpu(unsigned int cpu, struct hlist_node *cpuhp_n
>  	pcie_pmu->on_cpu = -1;
>  	pdev = pcie_pmu->pdev;
>  	node = dev_to_node(&pdev->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) {
> -- 
> 2.27.0
>