selftests/bpf: Add test to verify freeing the special fields in pcpu maps

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:
. Call update_elem() to insert an initial value.
. Use bpf_refcount_acquire() to increment the refcount.
. Store the node pointer in the map value.
. Add the node to a linked list.
. Probe-read the refcount and verify it is *2*.
. Call update_elem() again to trigger refcount decrement.
. Probe-read the refcount and verify it is *1*.

Signed-off-by: Leon Hwang <leon.hwang@linux.dev>
Acked-by: Yonghong Song <yonghong.song@linux.dev>
Link: https://lore.kernel.org/r/20251105151407.12723-3-leon.hwang@linux.dev
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

tools/testing/selftests/bpf/prog_tests/refcounted_kptr.c

@@ -44,3 +44,59 @@ 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);
+}

tools/testing/selftests/bpf/progs/refcounted_kptr.c

@@ -568,4 +568,64 @@ err_out:
 	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);
+	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";