linux-kernel - Re: [PATCH v1 1/1] perf/smmuv3: Don't cast parameter in bit operations

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Date:   Wed, 9 Feb 2022 21:45:54 +0000
From:   Robin Murphy <robin.murphy@....com>
To:     Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        Will Deacon <will@...nel.org>,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org
Cc:     Mark Rutland <mark.rutland@....com>
Subject: Re: [PATCH v1 1/1] perf/smmuv3: Don't cast parameter in bit
 operations

On 2022-02-09 18:47, Andy Shevchenko wrote:
> While in this particular case it would not be a (critical) issue,
> the pattern itself is bad and error prone in case somebody blindly
> copies to their code.
> 
> Don't cast parameter to unsigned long pointer in the bit operations.
> Instead copy to a local variable on stack of a proper type and use.

Seems reasonable, in fact in my build it appears to save a spill and 
reload from the stack so the code actually gets a teeny bit better. Not 
sure a cleanup warrants a "Fixes" tag, but either way,

Reviewed-by: Robin Murphy <robin.murphy@....com>

> Note, new compilers might warn on this line for potential outbound access.
> 
> Fixes: 7d839b4b9e00 ("perf/smmuv3: Add arm64 smmuv3 pmu driver")
> Signed-off-by: Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
> ---
>   drivers/perf/arm_smmuv3_pmu.c | 4 +++-
>   1 file changed, 3 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/perf/arm_smmuv3_pmu.c b/drivers/perf/arm_smmuv3_pmu.c
> index c49108a72865..00d4c45a8017 100644
> --- a/drivers/perf/arm_smmuv3_pmu.c
> +++ b/drivers/perf/arm_smmuv3_pmu.c
> @@ -654,6 +654,7 @@ static int smmu_pmu_offline_cpu(unsigned int cpu, struct hlist_node *node)
>   static irqreturn_t smmu_pmu_handle_irq(int irq_num, void *data)
>   {
>   	struct smmu_pmu *smmu_pmu = data;
> +	DECLARE_BITMAP(ovs, BITS_PER_TYPE(u64));
>   	u64 ovsr;
>   	unsigned int idx;
>   
> @@ -663,7 +664,8 @@ static irqreturn_t smmu_pmu_handle_irq(int irq_num, void *data)
>   
>   	writeq(ovsr, smmu_pmu->reloc_base + SMMU_PMCG_OVSCLR0);
>   
> -	for_each_set_bit(idx, (unsigned long *)&ovsr, smmu_pmu->num_counters) {
> +	bitmap_from_u64(ovs, ovsr);
> +	for_each_set_bit(idx, ovs, smmu_pmu->num_counters) {
>   		struct perf_event *event = smmu_pmu->events[idx];
>   		struct hw_perf_event *hwc;
>