From: Paul Chaignon <paul.chaignon@gmail.com>
To: Matt Bobrowski <mattbobrowski@google.com>
Cc: bpf@vger.kernel.org, Alexei Starovoitov <ast@kernel.org>,
Daniel Borkmann <daniel@iogearbox.net>,
Andrii Nakryiko <andrii@kernel.org>,
Martin KaFai Lau <martin.lau@linux.dev>,
Eduard Zingerman <eddyz87@gmail.com>, Song Liu <song@kernel.org>,
Yonghong Song <yonghong.song@linux.dev>,
Jiri Olsa <jolsa@kernel.org>,
Emil Tsalapatis <emil@etsalapatis.com>
Subject: Re: [PATCH bpf-next] selftests/bpf: fix off-by-one in bpf_cpumask_populate related selftest
Date: Mon, 20 Apr 2026 14:00:04 +0200 [thread overview]
Message-ID: <aeYVRJhCs3lSLJAS@mail.gmail.com> (raw)
In-Reply-To: <20260420093734.2400330-1-mattbobrowski@google.com>
On Mon, Apr 20, 2026 at 09:37:34AM +0000, Matt Bobrowski wrote:
> 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>
Acked-by: Paul Chaignon <paul.chaignon@gmail.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
>
>
next prev parent reply other threads:[~2026-04-20 12:00 UTC|newest]
Thread overview: 3+ messages / expand[flat|nested] mbox.gz Atom feed top
2026-04-20 9:37 [PATCH bpf-next] selftests/bpf: fix off-by-one in bpf_cpumask_populate related selftest Matt Bobrowski
2026-04-20 12:00 ` Paul Chaignon [this message]
2026-04-20 14:30 ` patchwork-bot+netdevbpf
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