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

Dawei Li
2024-04-03T12:53:44Z

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>
---
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