tools/bpf: implement libbpf btf__get_map_kv_tids() API function

Currently, to get map key/value type id's, the macro
  BPF_ANNOTATE_KV_PAIR(<map_name>, <key_type>, <value_type>)
needs to be defined in the bpf program for the
corresponding map.

During program/map loading time,
the local static function bpf_map_find_btf_info()
in libbpf.c is implemented to retrieve the key/value
type ids given the map name.

The patch refactored function bpf_map_find_btf_info()
to create an API btf__get_map_kv_tids() which includes
the bulk of implementation for the original function.
The API btf__get_map_kv_tids() can be used by bcc,
a JIT based bpf compilation system, which uses the
same BPF_ANNOTATE_KV_PAIR to record map key/value types.

Acked-by: Martin KaFai Lau <kafai@fb.com>
Signed-off-by: Yonghong Song <yhs@fb.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

tools/lib/bpf/btf.c

@@ -1,6 +1,7 @@
 // SPDX-License-Identifier: (LGPL-2.1 OR BSD-2-Clause)
 /* Copyright (c) 2018 Facebook */
 
+#include <stdio.h>
 #include <stdlib.h>
 #include <string.h>
 #include <unistd.h>
@@ -504,6 +505,78 @@ int btf__get_from_id(__u32 id, struct btf **btf)
 	return err;
 }
 
+int btf__get_map_kv_tids(const struct btf *btf, char *map_name,
+			 __u32 expected_key_size, __u32 expected_value_size,
+			 __u32 *key_type_id, __u32 *value_type_id)
+{
+	const struct btf_type *container_type;
+	const struct btf_member *key, *value;
+	const size_t max_name = 256;
+	char container_name[max_name];
+	__s64 key_size, value_size;
+	__s32 container_id;
+
+	if (snprintf(container_name, max_name, "____btf_map_%s", map_name) ==
+	    max_name) {
+		pr_warning("map:%s length of '____btf_map_%s' is too long\n",
+			   map_name, map_name);
+		return -EINVAL;
+	}
+
+	container_id = btf__find_by_name(btf, container_name);
+	if (container_id < 0) {
+		pr_warning("map:%s container_name:%s cannot be found in BTF. Missing BPF_ANNOTATE_KV_PAIR?\n",
+			   map_name, container_name);
+		return container_id;
+	}
+
+	container_type = btf__type_by_id(btf, container_id);
+	if (!container_type) {
+		pr_warning("map:%s cannot find BTF type for container_id:%u\n",
+			   map_name, container_id);
+		return -EINVAL;
+	}
+
+	if (BTF_INFO_KIND(container_type->info) != BTF_KIND_STRUCT ||
+	    BTF_INFO_VLEN(container_type->info) < 2) {
+		pr_warning("map:%s container_name:%s is an invalid container struct\n",
+			   map_name, container_name);
+		return -EINVAL;
+	}
+
+	key = (struct btf_member *)(container_type + 1);
+	value = key + 1;
+
+	key_size = btf__resolve_size(btf, key->type);
+	if (key_size < 0) {
+		pr_warning("map:%s invalid BTF key_type_size\n", map_name);
+		return key_size;
+	}
+
+	if (expected_key_size != key_size) {
+		pr_warning("map:%s btf_key_type_size:%u != map_def_key_size:%u\n",
+			   map_name, (__u32)key_size, expected_key_size);
+		return -EINVAL;
+	}
+
+	value_size = btf__resolve_size(btf, value->type);
+	if (value_size < 0) {
+		pr_warning("map:%s invalid BTF value_type_size\n", map_name);
+		return value_size;
+	}
+
+	if (expected_value_size != value_size) {
+		pr_warning("map:%s btf_value_type_size:%u != map_def_value_size:%u\n",
+			   map_name, (__u32)value_size, expected_value_size);
+		return -EINVAL;
+	}
+
+	*key_type_id = key->type;
+	*value_type_id = value->type;
+
+	return 0;
+}
+
 struct btf_ext_sec_copy_param {
 	__u32 off;
 	__u32 len;

tools/lib/bpf/btf.h

@@ -66,6 +66,10 @@ LIBBPF_API int btf__resolve_type(const struct btf *btf, __u32 type_id);
 LIBBPF_API int btf__fd(const struct btf *btf);
 LIBBPF_API const char *btf__name_by_offset(const struct btf *btf, __u32 offset);
 LIBBPF_API int btf__get_from_id(__u32 id, struct btf **btf);
+LIBBPF_API int btf__get_map_kv_tids(const struct btf *btf, char *map_name,
+				    __u32 expected_key_size,
+				    __u32 expected_value_size,
+				    __u32 *key_type_id, __u32 *value_type_id);
 
 LIBBPF_API struct btf_ext *btf_ext__new(__u8 *data, __u32 size);
 LIBBPF_API void btf_ext__free(struct btf_ext *btf_ext);

tools/lib/bpf/libbpf.c

@@ -1056,72 +1056,18 @@ bpf_program__collect_reloc(struct bpf_program *prog, GElf_Shdr *shdr,
 
 static int bpf_map_find_btf_info(struct bpf_map *map, const struct btf *btf)
 {
-	const struct btf_type *container_type;
-	const struct btf_member *key, *value;
 	struct bpf_map_def *def = &map->def;
-	const size_t max_name = 256;
+	__u32 key_type_id, value_type_id;
-	char container_name[max_name];
+	int ret;
-	__s64 key_size, value_size;
-	__s32 container_id;
 
-	if (snprintf(container_name, max_name, "____btf_map_%s", map->name) ==
+	ret = btf__get_map_kv_tids(btf, map->name, def->key_size,
-	    max_name) {
+				   def->value_size, &key_type_id,
-		pr_warning("map:%s length of '____btf_map_%s' is too long\n",
+				   &value_type_id);
-			   map->name, map->name);
+	if (ret)
-		return -EINVAL;
+		return ret;
-	}
 
-	container_id = btf__find_by_name(btf, container_name);
+	map->btf_key_type_id = key_type_id;
-	if (container_id < 0) {
+	map->btf_value_type_id = value_type_id;
-		pr_debug("map:%s container_name:%s cannot be found in BTF. Missing BPF_ANNOTATE_KV_PAIR?\n",
-			 map->name, container_name);
-		return container_id;
-	}
-
-	container_type = btf__type_by_id(btf, container_id);
-	if (!container_type) {
-		pr_warning("map:%s cannot find BTF type for container_id:%u\n",
-			   map->name, container_id);
-		return -EINVAL;
-	}
-
-	if (BTF_INFO_KIND(container_type->info) != BTF_KIND_STRUCT ||
-	    BTF_INFO_VLEN(container_type->info) < 2) {
-		pr_warning("map:%s container_name:%s is an invalid container struct\n",
-			   map->name, container_name);
-		return -EINVAL;
-	}
-
-	key = (struct btf_member *)(container_type + 1);
-	value = key + 1;
-
-	key_size = btf__resolve_size(btf, key->type);
-	if (key_size < 0) {
-		pr_warning("map:%s invalid BTF key_type_size\n",
-			   map->name);
-		return key_size;
-	}
-
-	if (def->key_size != key_size) {
-		pr_warning("map:%s btf_key_type_size:%u != map_def_key_size:%u\n",
-			   map->name, (__u32)key_size, def->key_size);
-		return -EINVAL;
-	}
-
-	value_size = btf__resolve_size(btf, value->type);
-	if (value_size < 0) {
-		pr_warning("map:%s invalid BTF value_type_size\n", map->name);
-		return value_size;
-	}
-
-	if (def->value_size != value_size) {
-		pr_warning("map:%s btf_value_type_size:%u != map_def_value_size:%u\n",
-			   map->name, (__u32)value_size, def->value_size);
-		return -EINVAL;
-	}
-
-	map->btf_key_type_id = key->type;
-	map->btf_value_type_id = value->type;
 
 	return 0;
 }

tools/lib/bpf/libbpf.map

@@ -133,6 +133,7 @@ LIBBPF_0.0.2 {
 		bpf_map_lookup_elem_flags;
 		bpf_object__find_map_fd_by_name;
 		bpf_get_link_xdp_id;
+		btf__get_map_kv_tids;
 		btf_ext__free;
 		btf_ext__func_info_rec_size;
 		btf_ext__line_info_rec_size;