libbpf: Add base BTF accessor

Add ability to get base BTF. It can be also used to check if BTF is split BTF.
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/20201202065244.530571-3-andrii@kernel.org

tools/lib/bpf/btf.c

@@ -432,6 +432,11 @@ __u32 btf__get_nr_types(const struct btf *btf)
 	return btf->start_id + btf->nr_types - 1;
 }
 
+const struct btf *btf__base_btf(const struct btf *btf)
+{
+	return btf->base_btf;
+}
+
 /* internal helper returning non-const pointer to a type */
 static struct btf_type *btf_type_by_id(struct btf *btf, __u32 type_id)
 {

tools/lib/bpf/btf.h

@@ -51,6 +51,7 @@ LIBBPF_API __s32 btf__find_by_name(const struct btf *btf,
 LIBBPF_API __s32 btf__find_by_name_kind(const struct btf *btf,
 					const char *type_name, __u32 kind);
 LIBBPF_API __u32 btf__get_nr_types(const struct btf *btf);
+LIBBPF_API const struct btf *btf__base_btf(const struct btf *btf);
 LIBBPF_API const struct btf_type *btf__type_by_id(const struct btf *btf,
 						  __u32 id);
 LIBBPF_API size_t btf__pointer_size(const struct btf *btf);

tools/lib/bpf/libbpf.map

@@ -340,6 +340,7 @@ LIBBPF_0.2.0 {
 
 LIBBPF_0.3.0 {
 	global:
+		btf__base_btf;
 		btf__parse_elf_split;
 		btf__parse_raw_split;
 		btf__parse_split;