Re: [PATCH bpf-next v6 2/2] selftests/bpf: Add test to verify freeing the special fields when update [lru_,]percpu_hash maps

[Yonghong Song <yonghong.song@linux.dev>]

On 11/5/25 7:14 AM, Leon Hwang wrote:
> Add test to verify that updating [lru_,]percpu_hash maps decrements
> refcount when BPF_KPTR_REF objects are involved.
>
> The tests perform the following steps:
>
> 1. Call update_elem() to insert an initial value.
> 2. Use bpf_refcount_acquire() to increment the refcount.
> 3. Store the node pointer in the map value.
> 4. Add the node to a linked list.
> 5. Probe-read the refcount and verify it is *2*.
> 6. Call update_elem() again to trigger refcount decrement.
> 7. Probe-read the refcount and verify it is *1*.
>
> Signed-off-by: Leon Hwang <leon.hwang@linux.dev>

LGTM with a few nits below.

Acked-by: Yonghong Song <yonghong.song@linux.dev>

> ---
>   .../bpf/prog_tests/refcounted_kptr.c          | 57 ++++++++++++++++++
>   .../selftests/bpf/progs/refcounted_kptr.c     | 60 +++++++++++++++++++
>   2 files changed, 117 insertions(+)
>
> diff --git a/tools/testing/selftests/bpf/prog_tests/refcounted_kptr.c b/tools/testing/selftests/bpf/prog_tests/refcounted_kptr.c
> index d6bd5e16e6372..086f679fa3f61 100644
> --- a/tools/testing/selftests/bpf/prog_tests/refcounted_kptr.c
> +++ b/tools/testing/selftests/bpf/prog_tests/refcounted_kptr.c
> @@ -44,3 +44,60 @@ void test_refcounted_kptr_wrong_owner(void)
>   	ASSERT_OK(opts.retval, "rbtree_wrong_owner_remove_fail_a2 retval");
>   	refcounted_kptr__destroy(skel);
>   }
> +
> +void test_percpu_hash_refcounted_kptr_refcount_leak(void)
> +{
> +	struct refcounted_kptr *skel;
> +	int cpu_nr, fd, err, key = 0;
> +	struct bpf_map *map;
> +	size_t values_sz;
> +	u64 *values;
> +	LIBBPF_OPTS(bpf_test_run_opts, opts,
> +		    .data_in = &pkt_v4,
> +		    .data_size_in = sizeof(pkt_v4),
> +		    .repeat = 1,
> +	);
> +
> +	cpu_nr = libbpf_num_possible_cpus();
> +	if (!ASSERT_GT(cpu_nr, 0, "libbpf_num_possible_cpus"))
> +		return;
> +
> +	values = calloc(cpu_nr, sizeof(u64));
> +	if (!ASSERT_OK_PTR(values, "calloc values"))
> +		return;
> +
> +	skel = refcounted_kptr__open_and_load();
> +	if (!ASSERT_OK_PTR(skel, "refcounted_kptr__open_and_load")) {
> +		free(values);
> +		return;
> +	}
> +
> +	values_sz = cpu_nr * sizeof(u64);
> +	memset(values, 0, values_sz);
> +
> +	map = skel->maps.percpu_hash;
> +	err = bpf_map__update_elem(map, &key, sizeof(key), values, values_sz, 0);
> +	if (!ASSERT_OK(err, "bpf_map__update_elem"))
> +		goto out;
> +
> +	fd = bpf_program__fd(skel->progs.percpu_hash_refcount_leak);
> +	err = bpf_prog_test_run_opts(fd, &opts);
> +	if (!ASSERT_OK(err, "bpf_prog_test_run_opts"))
> +		goto out;
> +	if (!ASSERT_EQ(opts.retval, 2, "opts.retval"))
> +		goto out;
> +
> +	err = bpf_map__update_elem(map, &key, sizeof(key), values, values_sz, 0);
> +	if (!ASSERT_OK(err, "bpf_map__update_elem"))
> +		goto out;
> +
> +	fd = bpf_program__fd(skel->progs.check_percpu_hash_refcount);
> +	err = bpf_prog_test_run_opts(fd, &opts);
> +	ASSERT_OK(err, "bpf_prog_test_run_opts");
> +	ASSERT_EQ(opts.retval, 1, "opts.retval");
> +
> +out:
> +	refcounted_kptr__destroy(skel);
> +	free(values);
> +}
> +

Empty line here.

> diff --git a/tools/testing/selftests/bpf/progs/refcounted_kptr.c b/tools/testing/selftests/bpf/progs/refcounted_kptr.c
> index 893a4fdb4b6e9..1aca85d86aebc 100644
> --- a/tools/testing/selftests/bpf/progs/refcounted_kptr.c
> +++ b/tools/testing/selftests/bpf/progs/refcounted_kptr.c
> @@ -568,4 +568,64 @@ int BPF_PROG(rbtree_sleepable_rcu_no_explicit_rcu_lock,
>   	return 0;
>   }
>   
> +private(kptr_ref) u64 ref;
> +
> +static int probe_read_refcount(void)
> +{
> +	u32 refcount;
> +
> +	bpf_probe_read_kernel(&refcount, sizeof(refcount), (void *) ref);
> +	return refcount;
> +}
> +
> +static int __insert_in_list(struct bpf_list_head *head, struct bpf_spin_lock *lock,
> +			    struct node_data __kptr **node)
> +{
> +	struct node_data *node_new, *node_ref, *node_old;
> +
> +	node_new = bpf_obj_new(typeof(*node_new));
> +	if (!node_new)
> +		return -1;
> +
> +	node_ref = bpf_refcount_acquire(node_new);
> +	node_old = bpf_kptr_xchg(node, node_new);

Change the above to node_old = bpf_kptr_xchg(node, node_node_ref); might 
be better for reasoning although node_ref/node_new are the same.

> +	if (node_old) {
> +		bpf_obj_drop(node_old);
> +		bpf_obj_drop(node_ref);
> +		return -2;
> +	}
> +
> +	bpf_spin_lock(lock);
> +	bpf_list_push_front(head, &node_ref->l);
> +	ref = (u64)(void *) &node_ref->ref;
> +	bpf_spin_unlock(lock);
> +	return probe_read_refcount();
> +}
> +
> +struct {
> +	__uint(type, BPF_MAP_TYPE_PERCPU_HASH);
> +	__type(key, int);
> +	__type(value, struct map_value);
> +	__uint(max_entries, 1);
> +} percpu_hash SEC(".maps");
> +
> +SEC("tc")
> +int percpu_hash_refcount_leak(void *ctx)
> +{
> +	struct map_value *v;
> +	int key = 0;
> +
> +	v = bpf_map_lookup_elem(&percpu_hash, &key);
> +	if (!v)
> +		return 0;
> +
> +	return __insert_in_list(&head, &lock, &v->node);
> +}
> +
> +SEC("tc")
> +int check_percpu_hash_refcount(void *ctx)
> +{
> +	return probe_read_refcount();
> +}
> +
>   char _license[] SEC("license") = "GPL";