selftests/bpf: Add test for the packed enum member in struct/union

Add a simple test to the existing selftest program in order to make
sure that a packed enum member in struct unexceeds the struct_size.
Signed-off-by: Yoshiki Komachi <komachi.yoshiki@gmail.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/1583825550-18606-3-git-send-email-komachi.yoshiki@gmail.com

tools/testing/selftests/bpf/test_btf.c

@@ -1062,6 +1062,48 @@ static struct btf_raw_test raw_tests[] = {
 	.err_str = "Member exceeds struct_size",
 },
 
+/* Test member unexceeds the size of struct
+ *
+ * enum E {
+ *     E0,
+ *     E1,
+ * };
+ *
+ * struct A {
+ *     char m;
+ *     enum E __attribute__((packed)) n;
+ * };
+ */
+{
+	.descr = "size check test #5",
+	.raw_types = {
+		/* int */			/* [1] */
+		BTF_TYPE_INT_ENC(0, BTF_INT_SIGNED, 0, 32, sizeof(int)),
+		/* char */			/* [2] */
+		BTF_TYPE_INT_ENC(0, BTF_INT_SIGNED, 0, 8, 1),
+		/* enum E { */			/* [3] */
+		BTF_TYPE_ENC(NAME_TBD, BTF_INFO_ENC(BTF_KIND_ENUM, 0, 2), 1),
+		BTF_ENUM_ENC(NAME_TBD, 0),
+		BTF_ENUM_ENC(NAME_TBD, 1),
+		/* } */
+		/* struct A { */		/* [4] */
+		BTF_TYPE_ENC(NAME_TBD, BTF_INFO_ENC(BTF_KIND_STRUCT, 0, 2), 2),
+		BTF_MEMBER_ENC(NAME_TBD, 2, 0),	/* char m; */
+		BTF_MEMBER_ENC(NAME_TBD, 3, 8),/* enum E __attribute__((packed)) n; */
+		/* } */
+		BTF_END_RAW,
+	},
+	.str_sec = "\0E\0E0\0E1\0A\0m\0n",
+	.str_sec_size = sizeof("\0E\0E0\0E1\0A\0m\0n"),
+	.map_type = BPF_MAP_TYPE_ARRAY,
+	.map_name = "size_check5_map",
+	.key_size = sizeof(int),
+	.value_size = 2,
+	.key_type_id = 1,
+	.value_type_id = 4,
+	.max_entries = 4,
+},
+
 /* typedef const void * const_void_ptr;
  * struct A {
  *	const_void_ptr m;