libbpf: Fix BPF_MAP_TYPE_PERF_EVENT_ARRAY auto-pinning

When a BPF map of type BPF_MAP_TYPE_PERF_EVENT_ARRAY doesn't have the
max_entries parameter set, the map will be created with max_entries set
to the number of available CPUs. When we try to reuse such a pinned map,
map_is_reuse_compat will return false, as max_entries in the map
definition differs from max_entries of the existing map, causing the
following error:

  libbpf: couldn't reuse pinned map at '/sys/fs/bpf/m_logging': parameter mismatch

Fix this by overwriting max_entries in the map definition. For this to
work, we need to do this in bpf_object__create_maps, before calling
bpf_object__reuse_map.

Fixes: 57a00f41644f ("libbpf: Add auto-pinning of maps when loading BPF objects")
Signed-off-by: Stijn Tintel <stijn@linux-ipv6.be>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Song Liu <songliubraving@fb.com>
Link: https://lore.kernel.org/bpf/20220225152355.315204-1-stijn@linux-ipv6.be

src/libbpf.c

@@ -4859,7 +4859,6 @@ static int bpf_object__create_map(struct bpf_object *obj, struct bpf_map *map, b
 	LIBBPF_OPTS(bpf_map_create_opts, create_attr);
 	struct bpf_map_def *def = &map->def;
 	const char *map_name = NULL;
-	__u32 max_entries;
 	int err = 0;
 
 	if (kernel_supports(obj, FEAT_PROG_NAME))
@@ -4869,21 +4868,6 @@ static int bpf_object__create_map(struct bpf_object *obj, struct bpf_map *map, b
 	create_attr.numa_node = map->numa_node;
 	create_attr.map_extra = map->map_extra;
 
-	if (def->type == BPF_MAP_TYPE_PERF_EVENT_ARRAY && !def->max_entries) {
-		int nr_cpus;
-
-		nr_cpus = libbpf_num_possible_cpus();
-		if (nr_cpus < 0) {
-			pr_warn("map '%s': failed to determine number of system CPUs: %d\n",
-				map->name, nr_cpus);
-			return nr_cpus;
-		}
-		pr_debug("map '%s': setting size to %d\n", map->name, nr_cpus);
-		max_entries = nr_cpus;
-	} else {
-		max_entries = def->max_entries;
-	}
-
 	if (bpf_map__is_struct_ops(map))
 		create_attr.btf_vmlinux_value_type_id = map->btf_vmlinux_value_type_id;
 
@@ -4933,7 +4917,7 @@ static int bpf_object__create_map(struct bpf_object *obj, struct bpf_map *map, b
 
 	if (obj->gen_loader) {
 		bpf_gen__map_create(obj->gen_loader, def->type, map_name,
-				    def->key_size, def->value_size, max_entries,
+				    def->key_size, def->value_size, def->max_entries,
 				    &create_attr, is_inner ? -1 : map - obj->maps);
 		/* Pretend to have valid FD to pass various fd >= 0 checks.
 		 * This fd == 0 will not be used with any syscall and will be reset to -1 eventually.
@@ -4942,7 +4926,7 @@ static int bpf_object__create_map(struct bpf_object *obj, struct bpf_map *map, b
 	} else {
 		map->fd = bpf_map_create(def->type, map_name,
 					 def->key_size, def->value_size,
-					 max_entries, &create_attr);
+					 def->max_entries, &create_attr);
 	}
 	if (map->fd < 0 && (create_attr.btf_key_type_id ||
 			    create_attr.btf_value_type_id)) {
@@ -4959,7 +4943,7 @@ static int bpf_object__create_map(struct bpf_object *obj, struct bpf_map *map, b
 		map->btf_value_type_id = 0;
 		map->fd = bpf_map_create(def->type, map_name,
 					 def->key_size, def->value_size,
-					 max_entries, &create_attr);
+					 def->max_entries, &create_attr);
 	}
 
 	err = map->fd < 0 ? -errno : 0;
@@ -5063,6 +5047,24 @@ static int bpf_object_init_prog_arrays(struct bpf_object *obj)
 	return 0;
 }
 
+static int map_set_def_max_entries(struct bpf_map *map)
+{
+	if (map->def.type == BPF_MAP_TYPE_PERF_EVENT_ARRAY && !map->def.max_entries) {
+		int nr_cpus;
+
+		nr_cpus = libbpf_num_possible_cpus();
+		if (nr_cpus < 0) {
+			pr_warn("map '%s': failed to determine number of system CPUs: %d\n",
+				map->name, nr_cpus);
+			return nr_cpus;
+		}
+		pr_debug("map '%s': setting size to %d\n", map->name, nr_cpus);
+		map->def.max_entries = nr_cpus;
+	}
+
+	return 0;
+}
+
 static int
 bpf_object__create_maps(struct bpf_object *obj)
 {
@@ -5095,6 +5097,10 @@ bpf_object__create_maps(struct bpf_object *obj)
 			continue;
 		}
 
+		err = map_set_def_max_entries(map);
+		if (err)
+			goto err_out;
+
 		retried = false;
 retry:
 		if (map->pin_path) {