bpf: Add elem_id pointer as argument to __btf_resolve_size

If the resolved type is array, make btf_resolve_size return also
ID of the elem type. It will be needed in following changes.
Signed-off-by: Jiri Olsa <jolsa@kernel.org>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: Andrii Nakryiko <andriin@fb.com>
Link: https://lore.kernel.org/bpf/20200825192124.710397-5-jolsa@kernel.org

kernel/bpf/btf.c

@@ -1079,6 +1079,7 @@ static const struct resolve_vertex *env_stack_peak(struct btf_verifier_env *env)
  * *type_size: (x * y * sizeof(u32)).  Hence, *type_size always
  *             corresponds to the return type.
  * *elem_type: u32
+ * *elem_id: id of u32
  * *total_nelems: (x * y).  Hence, individual elem size is
  *                (*type_size / *total_nelems)
  *
@@ -1086,15 +1087,16 @@ static const struct resolve_vertex *env_stack_peak(struct btf_verifier_env *env)
  * return type: type "struct X"
  * *type_size: sizeof(struct X)
  * *elem_type: same as return type ("struct X")
+ * *elem_id: 0
  * *total_nelems: 1
  */
 static const struct btf_type *
 __btf_resolve_size(const struct btf *btf, const struct btf_type *type,
 		   u32 *type_size, const struct btf_type **elem_type,
-		   u32 *total_nelems)
+		   u32 *elem_id, u32 *total_nelems)
 {
 	const struct btf_type *array_type = NULL;
-	const struct btf_array *array;
+	const struct btf_array *array = NULL;
 	u32 i, size, nelems = 1;
 
 	for (i = 0; i < MAX_RESOLVE_DEPTH; i++) {
@@ -1146,6 +1148,8 @@ __btf_resolve_size(const struct btf *btf, const struct btf_type *type,
 		*total_nelems = nelems;
 	if (elem_type)
 		*elem_type = type;
+	if (elem_id)
+		*elem_id = array ? array->type : 0;
 
 	return array_type ? : type;
 }
@@ -3984,7 +3988,7 @@ int btf_struct_access(struct bpf_verifier_log *log,
 		mname = __btf_name_by_offset(btf_vmlinux, member->name_off);
 
 		mtype = __btf_resolve_size(btf_vmlinux, mtype, &msize,
-					   &elem_type, &total_nelems);
+					   &elem_type, NULL, &total_nelems);
 		if (IS_ERR(mtype)) {
 			bpf_log(log, "field %s doesn't have size\n", mname);
 			return -EFAULT;