selftests/bpf: Extend map-in-map selftest to detect memory leaks

Add test validating that all inner maps are released properly after skeleton
is destroyed. To ensure determinism, trigger kernel-side synchronize_rcu()
before checking map existence by their IDs.
Signed-off-by: Andrii Nakryiko <andriin@fb.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Song Liu <songliubraving@fb.com>
Link: https://lore.kernel.org/bpf/20200729040913.2815687-2-andriin@fb.com

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

@@ -5,10 +5,60 @@
 
 #include "test_btf_map_in_map.skel.h"
 
+static int duration;
+
+static __u32 bpf_map_id(struct bpf_map *map)
+{
+	struct bpf_map_info info;
+	__u32 info_len = sizeof(info);
+	int err;
+
+	memset(&info, 0, info_len);
+	err = bpf_obj_get_info_by_fd(bpf_map__fd(map), &info, &info_len);
+	if (err)
+		return 0;
+	return info.id;
+}
+
+/*
+ * Trigger synchronize_rcu() in kernel.
+ *
+ * ARRAY_OF_MAPS/HASH_OF_MAPS lookup/update operations trigger synchronize_rcu()
+ * if looking up an existing non-NULL element or updating the map with a valid
+ * inner map FD. Use this fact to trigger synchronize_rcu(): create map-in-map,
+ * create a trivial ARRAY map, update map-in-map with ARRAY inner map. Then
+ * cleanup. At the end, at least one synchronize_rcu() would be called.
+ */
+static int kern_sync_rcu(void)
+{
+	int inner_map_fd, outer_map_fd, err, zero = 0;
+
+	inner_map_fd = bpf_create_map(BPF_MAP_TYPE_ARRAY, 4, 4, 1, 0);
+	if (CHECK(inner_map_fd < 0, "inner_map_create", "failed %d\n", -errno))
+		return -1;
+
+	outer_map_fd = bpf_create_map_in_map(BPF_MAP_TYPE_ARRAY_OF_MAPS, NULL,
+					     sizeof(int), inner_map_fd, 1, 0);
+	if (CHECK(outer_map_fd < 0, "outer_map_create", "failed %d\n", -errno)) {
+		close(inner_map_fd);
+		return -1;
+	}
+
+	err = bpf_map_update_elem(outer_map_fd, &zero, &inner_map_fd, 0);
+	if (err)
+		err = -errno;
+	CHECK(err, "outer_map_update", "failed %d\n", err);
+	close(inner_map_fd);
+	close(outer_map_fd);
+	return err;
+}
+
 void test_btf_map_in_map(void)
 {
-	int duration = 0, err, key = 0, val;
-	struct test_btf_map_in_map* skel;
+	int err, key = 0, val, i;
+	struct test_btf_map_in_map *skel;
+	int outer_arr_fd, outer_hash_fd;
+	int fd, map1_fd, map2_fd, map1_id, map2_id;
 
 	skel = test_btf_map_in_map__open_and_load();
 	if (CHECK(!skel, "skel_open", "failed to open&load skeleton\n"))
@@ -18,32 +68,78 @@ void test_btf_map_in_map(void)
 	if (CHECK(err, "skel_attach", "skeleton attach failed: %d\n", err))
 		goto cleanup;
 
+	map1_fd = bpf_map__fd(skel->maps.inner_map1);
+	map2_fd = bpf_map__fd(skel->maps.inner_map2);
+	outer_arr_fd = bpf_map__fd(skel->maps.outer_arr);
+	outer_hash_fd = bpf_map__fd(skel->maps.outer_hash);
+
 	/* inner1 = input, inner2 = input + 1 */
-	val = bpf_map__fd(skel->maps.inner_map1);
-	bpf_map_update_elem(bpf_map__fd(skel->maps.outer_arr), &key, &val, 0);
-	val = bpf_map__fd(skel->maps.inner_map2);
-	bpf_map_update_elem(bpf_map__fd(skel->maps.outer_hash), &key, &val, 0);
+	map1_fd = bpf_map__fd(skel->maps.inner_map1);
+	bpf_map_update_elem(outer_arr_fd, &key, &map1_fd, 0);
+	map2_fd = bpf_map__fd(skel->maps.inner_map2);
+	bpf_map_update_elem(outer_hash_fd, &key, &map2_fd, 0);
 	skel->bss->input = 1;
 	usleep(1);
 
-	bpf_map_lookup_elem(bpf_map__fd(skel->maps.inner_map1), &key, &val);
+	bpf_map_lookup_elem(map1_fd, &key, &val);
 	CHECK(val != 1, "inner1", "got %d != exp %d\n", val, 1);
-	bpf_map_lookup_elem(bpf_map__fd(skel->maps.inner_map2), &key, &val);
+	bpf_map_lookup_elem(map2_fd, &key, &val);
 	CHECK(val != 2, "inner2", "got %d != exp %d\n", val, 2);
 
 	/* inner1 = input + 1, inner2 = input */
-	val = bpf_map__fd(skel->maps.inner_map2);
-	bpf_map_update_elem(bpf_map__fd(skel->maps.outer_arr), &key, &val, 0);
-	val = bpf_map__fd(skel->maps.inner_map1);
-	bpf_map_update_elem(bpf_map__fd(skel->maps.outer_hash), &key, &val, 0);
+	bpf_map_update_elem(outer_arr_fd, &key, &map2_fd, 0);
+	bpf_map_update_elem(outer_hash_fd, &key, &map1_fd, 0);
 	skel->bss->input = 3;
 	usleep(1);
 
-	bpf_map_lookup_elem(bpf_map__fd(skel->maps.inner_map1), &key, &val);
+	bpf_map_lookup_elem(map1_fd, &key, &val);
 	CHECK(val != 4, "inner1", "got %d != exp %d\n", val, 4);
-	bpf_map_lookup_elem(bpf_map__fd(skel->maps.inner_map2), &key, &val);
+	bpf_map_lookup_elem(map2_fd, &key, &val);
 	CHECK(val != 3, "inner2", "got %d != exp %d\n", val, 3);
 
+	for (i = 0; i < 5; i++) {
+		val = i % 2 ? map1_fd : map2_fd;
+		err = bpf_map_update_elem(outer_hash_fd, &key, &val, 0);
+		if (CHECK_FAIL(err)) {
+			printf("failed to update hash_of_maps on iter #%d\n", i);
+			goto cleanup;
+		}
+		err = bpf_map_update_elem(outer_arr_fd, &key, &val, 0);
+		if (CHECK_FAIL(err)) {
+			printf("failed to update hash_of_maps on iter #%d\n", i);
+			goto cleanup;
+		}
+	}
+
+	map1_id = bpf_map_id(skel->maps.inner_map1);
+	map2_id = bpf_map_id(skel->maps.inner_map2);
+	CHECK(map1_id == 0, "map1_id", "failed to get ID 1\n");
+	CHECK(map2_id == 0, "map2_id", "failed to get ID 2\n");
+
+	test_btf_map_in_map__destroy(skel);
+	skel = NULL;
+
+	/* we need to either wait for or force synchronize_rcu(), before
+	 * checking for "still exists" condition, otherwise map could still be
+	 * resolvable by ID, causing false positives.
+	 *
+	 * Older kernels (5.8 and earlier) freed map only after two
+	 * synchronize_rcu()s, so trigger two, to be entirely sure.
+	 */
+	CHECK(kern_sync_rcu(), "sync_rcu", "failed\n");
+	CHECK(kern_sync_rcu(), "sync_rcu", "failed\n");
+
+	fd = bpf_map_get_fd_by_id(map1_id);
+	if (CHECK(fd >= 0, "map1_leak", "inner_map1 leaked!\n")) {
+		close(fd);
+		goto cleanup;
+	}
+	fd = bpf_map_get_fd_by_id(map2_id);
+	if (CHECK(fd >= 0, "map2_leak", "inner_map2 leaked!\n")) {
+		close(fd);
+		goto cleanup;
+	}
+
 cleanup:
 	test_btf_map_in_map__destroy(skel);
 }