From: Hui Zhu <zhuhui@kylinos.cn>

Add a new selftest, `test_memcg_ops_hierarchies`, to validate the
behavior of attaching `memcg_bpf_ops` in a nested cgroup hierarchy,
specifically testing the `BPF_F_ALLOW_OVERRIDE` flag.

The test case performs the following steps:
1. Creates a three-level deep cgroup hierarchy: `/cg`, `/cg/cg`, and
   `/cg/cg/cg`.
2. Attaches a BPF struct_ops to the top-level cgroup (`/cg`) with the
   `BPF_F_ALLOW_OVERRIDE` flag.
3. Successfully attaches a new struct_ops to the middle cgroup
   (`/cg/cg`) without the flag, overriding the inherited one.
4. Asserts that attaching another struct_ops to the deepest cgroup
   (`/cg/cg/cg`) fails with -EBUSY, because its parent did not specify
   `BPF_F_ALLOW_OVERRIDE`.

This test ensures that the attachment logic correctly enforces the
override rules across a cgroup subtree.

Signed-off-by: Geliang Tang <geliang@kernel.org>
Signed-off-by: Hui Zhu <zhuhui@kylinos.cn>
---
 .../selftests/bpf/prog_tests/memcg_ops.c      | 70 +++++++++++++++++++
 1 file changed, 70 insertions(+)

diff --git a/tools/testing/selftests/bpf/prog_tests/memcg_ops.c b/tools/testing/selftests/bpf/prog_tests/memcg_ops.c
index a019ba2387bc..08ac97752ac9 100644
--- a/tools/testing/selftests/bpf/prog_tests/memcg_ops.c
+++ b/tools/testing/selftests/bpf/prog_tests/memcg_ops.c
@@ -539,3 +539,73 @@ void test_memcg_ops_below_min_over_high(void)
 	close(low_cgroup_fd);
 	cleanup_cgroup_environment();
 }
+
+void test_memcg_ops_hierarchies(void)
+{
+	int ret, first = -1, second = -1, third = -1;
+	struct memcg_ops *skel;
+	struct bpf_map *map;
+	struct bpf_link *link1 = NULL, *link2 = NULL, *link3 = NULL;
+	DECLARE_LIBBPF_OPTS(bpf_struct_ops_opts, opts);
+
+	ret = setup_cgroup_environment();
+	if (!ASSERT_OK(ret, "setup_cgroup_environment"))
+		goto cleanup;
+
+	first = create_and_get_cgroup("/cg");
+	if (!ASSERT_GE(first, 0, "create_and_get_cgroup /cg"))
+		goto cleanup;
+	ret = enable_controllers("/cg", "memory");
+	if (!ASSERT_OK(ret, "enable_controllers"))
+		goto cleanup;
+
+	second = create_and_get_cgroup("/cg/cg");
+	if (!ASSERT_GE(second, 0, "create_and_get_cgroup /cg/cg"))
+		goto cleanup;
+	ret = enable_controllers("/cg/cg", "memory");
+	if (!ASSERT_OK(ret, "enable_controllers"))
+		goto cleanup;
+
+	third = create_and_get_cgroup("/cg/cg/cg");
+	if (!ASSERT_GE(third, 0, "create_and_get_cgroup /cg/cg/cg"))
+		goto cleanup;
+	ret = enable_controllers("/cg/cg/cg", "memory");
+	if (!ASSERT_OK(ret, "enable_controllers"))
+		goto cleanup;
+
+	skel = memcg_ops__open_and_load();
+	if (!ASSERT_OK_PTR(skel, "memcg_ops__open_and_load"))
+		goto cleanup;
+
+	map = bpf_object__find_map_by_name(skel->obj, "low_mcg_ops");
+	if (!ASSERT_OK_PTR(map, "bpf_object__find_map_by_name low_mcg_ops"))
+		goto cleanup;
+
+	opts.relative_fd = first;
+	opts.flags = BPF_F_ALLOW_OVERRIDE;
+	link1 = bpf_map__attach_struct_ops_opts(map, &opts);
+	if (!ASSERT_OK_PTR(link1, "bpf_map__attach_struct_ops_opts"))
+		goto cleanup;
+
+	opts.relative_fd = second;
+	opts.flags = 0;
+	link2 = bpf_map__attach_struct_ops_opts(map, &opts);
+	if (!ASSERT_OK_PTR(link2, "bpf_map__attach_struct_ops_opts"))
+		goto cleanup;
+
+	opts.relative_fd = third;
+	opts.flags = 0;
+	link3 = bpf_map__attach_struct_ops_opts(map, &opts);
+	if (!ASSERT_ERR_PTR(link3, "bpf_map__attach_struct_ops_opts"))
+		goto cleanup;
+
+cleanup:
+	bpf_link__destroy(link1);
+	bpf_link__destroy(link2);
+	bpf_link__destroy(link3);
+	memcg_ops__detach(skel);
+	close(first);
+	close(second);
+	close(third);
+	cleanup_cgroup_environment();
+}
-- 
2.43.0

> diff --git a/tools/testing/selftests/bpf/prog_tests/memcg_ops.c b/tools/testing/selftests/bpf/prog_tests/memcg_ops.c
> index a019ba2387bc..08ac97752ac9 100644
> --- a/tools/testing/selftests/bpf/prog_tests/memcg_ops.c
> +++ b/tools/testing/selftests/bpf/prog_tests/memcg_ops.c
> @@ -539,3 +539,73 @@ void test_memcg_ops_below_min_over_high(void)

[ ... ]

> +void test_memcg_ops_hierarchies(void)
> +{
> +	int ret, first = -1, second = -1, third = -1;
> +	struct memcg_ops *skel;
                          ^^^^

Should skel be initialized to NULL here? The other tests in this file
(test_memcg_ops_over_high, test_memcg_ops_below_low_over_high, and
test_memcg_ops_below_min_over_high) all initialize their skeleton pointers
to NULL.

If any of the early checks fail (e.g. setup_cgroup_environment() or
create_and_get_cgroup()), the code jumps to cleanup where memcg_ops__detach()
is called. The skeleton __detach() function dereferences its argument without
checking for NULL, which would cause a crash with an uninitialized pointer.

> +	struct bpf_map *map;
> +	struct bpf_link *link1 = NULL, *link2 = NULL, *link3 = NULL;
> +	DECLARE_LIBBPF_OPTS(bpf_struct_ops_opts, opts);
> +
> +	ret = setup_cgroup_environment();
> +	if (!ASSERT_OK(ret, "setup_cgroup_environment"))
> +		goto cleanup;

[ ... ]

> +cleanup:
> +	bpf_link__destroy(link1);
> +	bpf_link__destroy(link2);
> +	bpf_link__destroy(link3);
> +	memcg_ops__detach(skel);
                         ^^^^

This dereferences skel without checking if it was ever assigned.

> +	close(first);
> +	close(second);
> +	close(third);
> +	cleanup_cgroup_environment();
> +}


---
AI reviewed your patch. Please fix the bug or email reply why it's not a bug.
See: https://github.com/kernel-patches/vmtest/blob/master/ci/claude/README.md

CI run summary: https://github.com/kernel-patches/bpf/actions/runs/21352241149