selftests/bpf: Test new btf__add_btf() API

Add a test that validates that btf__add_btf() API is correctly copying
all the types from the source BTF into destination BTF object and
adjusts type IDs and string offsets properly.
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/bpf/20211006051107.17921-4-andrii@kernel.org

tools/testing/selftests/bpf/prog_tests/btf_write.c

@@ -342,8 +342,94 @@ static void test_btf_add()
 	btf__free(btf);
 }
 
+static void test_btf_add_btf()
+{
+	struct btf *btf1 = NULL, *btf2 = NULL;
+	int id;
+
+	btf1 = btf__new_empty();
+	if (!ASSERT_OK_PTR(btf1, "btf1"))
+		return;
+
+	btf2 = btf__new_empty();
+	if (!ASSERT_OK_PTR(btf2, "btf2"))
+		goto cleanup;
+
+	gen_btf(btf1);
+	gen_btf(btf2);
+
+	id = btf__add_btf(btf1, btf2);
+	if (!ASSERT_EQ(id, 20, "id"))
+		goto cleanup;
+
+	VALIDATE_RAW_BTF(
+		btf1,
+		"[1] INT 'int' size=4 bits_offset=0 nr_bits=32 encoding=SIGNED",
+		"[2] PTR '(anon)' type_id=1",
+		"[3] CONST '(anon)' type_id=5",
+		"[4] VOLATILE '(anon)' type_id=3",
+		"[5] RESTRICT '(anon)' type_id=4",
+		"[6] ARRAY '(anon)' type_id=2 index_type_id=1 nr_elems=10",
+		"[7] STRUCT 's1' size=8 vlen=2\n"
+		"\t'f1' type_id=1 bits_offset=0\n"
+		"\t'f2' type_id=1 bits_offset=32 bitfield_size=16",
+		"[8] UNION 'u1' size=8 vlen=1\n"
+		"\t'f1' type_id=1 bits_offset=0 bitfield_size=16",
+		"[9] ENUM 'e1' size=4 vlen=2\n"
+		"\t'v1' val=1\n"
+		"\t'v2' val=2",
+		"[10] FWD 'struct_fwd' fwd_kind=struct",
+		"[11] FWD 'union_fwd' fwd_kind=union",
+		"[12] ENUM 'enum_fwd' size=4 vlen=0",
+		"[13] TYPEDEF 'typedef1' type_id=1",
+		"[14] FUNC 'func1' type_id=15 linkage=global",
+		"[15] FUNC_PROTO '(anon)' ret_type_id=1 vlen=2\n"
+		"\t'p1' type_id=1\n"
+		"\t'p2' type_id=2",
+		"[16] VAR 'var1' type_id=1, linkage=global-alloc",
+		"[17] DATASEC 'datasec1' size=12 vlen=1\n"
+		"\ttype_id=1 offset=4 size=8",
+		"[18] TAG 'tag1' type_id=16 component_idx=-1",
+		"[19] TAG 'tag2' type_id=14 component_idx=1",
+
+		/* types appended from the second BTF */
+		"[20] INT 'int' size=4 bits_offset=0 nr_bits=32 encoding=SIGNED",
+		"[21] PTR '(anon)' type_id=20",
+		"[22] CONST '(anon)' type_id=24",
+		"[23] VOLATILE '(anon)' type_id=22",
+		"[24] RESTRICT '(anon)' type_id=23",
+		"[25] ARRAY '(anon)' type_id=21 index_type_id=20 nr_elems=10",
+		"[26] STRUCT 's1' size=8 vlen=2\n"
+		"\t'f1' type_id=20 bits_offset=0\n"
+		"\t'f2' type_id=20 bits_offset=32 bitfield_size=16",
+		"[27] UNION 'u1' size=8 vlen=1\n"
+		"\t'f1' type_id=20 bits_offset=0 bitfield_size=16",
+		"[28] ENUM 'e1' size=4 vlen=2\n"
+		"\t'v1' val=1\n"
+		"\t'v2' val=2",
+		"[29] FWD 'struct_fwd' fwd_kind=struct",
+		"[30] FWD 'union_fwd' fwd_kind=union",
+		"[31] ENUM 'enum_fwd' size=4 vlen=0",
+		"[32] TYPEDEF 'typedef1' type_id=20",
+		"[33] FUNC 'func1' type_id=34 linkage=global",
+		"[34] FUNC_PROTO '(anon)' ret_type_id=20 vlen=2\n"
+		"\t'p1' type_id=20\n"
+		"\t'p2' type_id=21",
+		"[35] VAR 'var1' type_id=20, linkage=global-alloc",
+		"[36] DATASEC 'datasec1' size=12 vlen=1\n"
+		"\ttype_id=20 offset=4 size=8",
+		"[37] TAG 'tag1' type_id=35 component_idx=-1",
+		"[38] TAG 'tag2' type_id=33 component_idx=1");
+
+cleanup:
+	btf__free(btf1);
+	btf__free(btf2);
+}
+
 void test_btf_write()
 {
 	if (test__start_subtest("btf_add"))
 		test_btf_add();
+	if (test__start_subtest("btf_add_btf"))
+		test_btf_add_btf();
 }