[PATCH bpf v2] selftests/bpf: Exercise double release in cpumask failure test

[Tianyi Chen <diannaaav@gmail.com>]

From: Tianyi Chen <hi@tychen.cc>

create_cpumask() can return NULL, but test_alloc_double_release does
not check its result before releasing it. The expected NULL-pointer
diagnostic allows the test to pass when the first release is rejected,
without checking the second release at all. Removing the second call
still produces the same expected rejection.

Return when creation fails and expect the release-ownership diagnostic.
This makes the verifier accept the first release and reject the second
use of the invalidated reference.

Fixes: 7b6abcfa15cd ("selftests/bpf: Add selftest suite for cpumask kfuncs")
Assisted-by: LLM
Signed-off-by: Tianyi Chen <hi@tychen.cc>
---
Changes in v2:
- Rebase onto current bpf/master; the test logic is unchanged.
- Keep validation details outside the commit description.

Validation: all 36 cpumask subtests passed, including
test_alloc_double_release, in an x86-64 KVM guest running the rebuilt
bpf/master kernel (Linux 7.3.0-rc2), with LLVM 20-built selftests.
There were no skips or failures.

The v1 CI PR expired after repeated "Patch is empty" reports without
conflicting hunks. This patch applies cleanly to the current tree and is
sent in a new thread with git format-patch and git send-email.

v1: https://lore.kernel.org/r/20260906143934.848632-1-hi@tychen.cc

 tools/testing/selftests/bpf/progs/cpumask_failure.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/tools/testing/selftests/bpf/progs/cpumask_failure.c b/tools/testing/selftests/bpf/progs/cpumask_failure.c
index 4628feb53d86..e98493bd5758 100644
--- a/tools/testing/selftests/bpf/progs/cpumask_failure.c
+++ b/tools/testing/selftests/bpf/progs/cpumask_failure.c
@@ -45,12 +45,14 @@ int BPF_PROG(test_alloc_no_release, struct task_struct *task, u64 clone_flags)
 }
 
 SEC("tp_btf/task_newtask")
-__failure __msg("NULL pointer passed to trusted R1")
+__failure __msg("release kfunc bpf_cpumask_release expects referenced PTR_TO_BTF_ID passed to R1")
 int BPF_PROG(test_alloc_double_release, struct task_struct *task, u64 clone_flags)
 {
 	struct bpf_cpumask *cpumask;
 
 	cpumask = create_cpumask();
+	if (!cpumask)
+		return 0;
 
 	/* cpumask is released twice. */
 	bpf_cpumask_release(cpumask);

base-commit: 15071f2a1263e82150c77eeb1e94dbfc31950a8e
-- 
2.55.0