Re: [PATCH bpf-next v2 4/4] selftests/bpf: Add a test for arena fault-in under memory.max

Thank you for your contribution! Sashiko AI review found 1 potential issue(=
s) to consider:
- [Medium] The test passes on unexpected crashes because it only asserts th=
at the termination signal is not SIGSEGV, rather than strictly expecting SI=
GKILL from the memcg OOM killer.
--

--- Patch [4]: [PATCH bpf-next v2 4/4] selftests/bpf: Add a test for arena =
fault-in under memory.max ---
commit d5e6a0b924679b20cf7f391518ec1a560ae725e3
Author: Jiayuan Chen <[email protected]>

selftests/bpf: Add a test for arena fault-in under memory.max

This commit adds a BPF selftest for arena fault-in under a constrained memo=
ry
cgroup. It creates a child process to fault in an arena and verifies it is
killed by the memcg OOM killer rather than incorrectly dying from SIGSEGV.

> diff --git a/tools/testing/selftests/bpf/prog_tests/arena_memcg.c b/tools=
/testing/selftests/bpf/prog_tests/arena_memcg.c
> new file mode 100644
> index 0000000000000..ca039ebd3d674
> --- /dev/null
> +++ b/tools/testing/selftests/bpf/prog_tests/arena_memcg.c
[ ... ]
> +void serial_test_arena_memcg(void)
> +{
[ ... ]
> +	/*
> +	 * Faulting a valid arena address until memory.max is hit must not look
> +	 * like an invalid access. Without the fix the fault path allocated with
> +	 * the non-blocking allocator, turned its -ENOMEM into VM_FAULT_SIGSEGV,
> +	 * and the child died with SIGSEGV on a valid address; now it is handled
> +	 * by the memcg OOM path and the child is killed by SIGKILL instead.
> +	 */
> +	if (!ASSERT_TRUE(WIFSIGNALED(status), "child killed by signal"))
> +		goto out;
> +	if (!ASSERT_NEQ(WTERMSIG(status), SIGSEGV, "not killed by SIGSEGV"))
> +		dump_memcg(read_cgroup_file);

[Severity: Medium]
Could this mask other failures by accepting any signal other than SIGSEGV?

If the child process crashes due to an unrelated issue like a libbpf intern=
al
assertion (SIGABRT) or an invalid memory access (SIGBUS), this assertion wo=
uld
evaluate to true and the test would silently pass.

Should this explicitly expect SIGKILL from the memcg OOM killer using
ASSERT_EQ(WTERMSIG(status), SIGKILL)?

> +out:
> +	if (cgroup_fd >=3D 0)
> +		close(cgroup_fd);
> +	cleanup_cgroup_environment();
> +}

--=20
Sashiko AI review =C2=B7 https://sashiko.dev/#/patchset/20260805091720.1399=
[email protected]?part=3D4