[PATCH bpf-next] selftests/bpf: fix off-by-one in bpf_cpumask_populate related selftest

[Matt Bobrowski <mattbobrowski@google.com>]

The test_populate test uses >= instead of > when checking if the
runtime nr_cpus exceeds the bit capacity of a cpumask_t.

On a system where the physical CPU core count perfectly matches the
CONFIG_NR_CPUS upper bound (e.g. nr_cpus = 512 and CONFIG_NR_CPUS =
512), the condition nr_cpus >= CPUMASK_TEST_MASKLEN * 8 evaluates to
true (512 >= 512). This incorrectly causes the test to fail with an
error value of 3.

A 512-bit cpumask_t provides enough bits (indices 0 through 511) to
represent 512 CPUs. The subsequent bpf_for(i, 0, nr_cpus) loop
iterates up to nr_cpus - 1 (511), which perfectly aligns with the
maximum valid index of the bitmask.

Change the condition to nr_cpus > CPUMASK_TEST_MASKLEN * 8 to fix the
false positive failure on these systems.

Fixes: 918ba2636d4e ("selftests: bpf: add bpf_cpumask_populate selftests")
Signed-off-by: Matt Bobrowski <mattbobrowski@google.com>
---
 tools/testing/selftests/bpf/progs/cpumask_success.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/tools/testing/selftests/bpf/progs/cpumask_success.c b/tools/testing/selftests/bpf/progs/cpumask_success.c
index 0e04c31b91c0..774706e7b058 100644
--- a/tools/testing/selftests/bpf/progs/cpumask_success.c
+++ b/tools/testing/selftests/bpf/progs/cpumask_success.c
@@ -866,7 +866,7 @@ int BPF_PROG(test_populate, struct task_struct *task, u64 clone_flags)
 	 * access NR_CPUS, the upper bound for nr_cpus, so we infer
 	 * it from the size of cpumask_t.
 	 */
-	if (nr_cpus < 0 || nr_cpus >= CPUMASK_TEST_MASKLEN * 8) {
+	if (nr_cpus < 0 || nr_cpus > CPUMASK_TEST_MASKLEN * 8) {
 		err = 3;
 		goto out;
 	}
-- 
2.54.0.rc1.513.gad8abe7a5a-goog