[PATCH bpf-next v3 3/3] selftests/bpf: Tests for BTF_KIND_DECL_TAG dump in C format

[Eduard Zingerman <eddyz87@gmail.com>]

Covers the following cases:
- `__atribute__((btf_decl_tag("...")))` could be applied to structs
  and unions;
- decl tag applied to an empty struct is printed on a single line;
- decl tags with the same name could be applied to several structs;
- several decl tags could be applied to the same struct;
- attribute `packed` works fine with decl tags (it is a separate
  branch in `tools/lib/bpf/btf_dump.c:btf_dump_emit_attributes`;
- decl tag could be attached to typedef;
- decl tag could be attached to a struct field;
- decl tag could be attached to a struct field and a struct itself
  simultaneously;
- decl tag could be attached to a global variable;
- decl tag could be attached to a func proto parameter;
- btf__add_decl_tag could be interleaved with btf_dump__dump_type calls.

Signed-off-by: Eduard Zingerman <eddyz87@gmail.com>
---
 .../selftests/bpf/prog_tests/btf_dump.c       | 79 +++++++++++++++++++
 .../bpf/progs/btf_dump_test_case_decl_tag.c   | 65 +++++++++++++++
 2 files changed, 144 insertions(+)
 create mode 100644 tools/testing/selftests/bpf/progs/btf_dump_test_case_decl_tag.c

diff --git a/tools/testing/selftests/bpf/prog_tests/btf_dump.c b/tools/testing/selftests/bpf/prog_tests/btf_dump.c
index a0bdfc45660d..2e6135a555e3 100644
--- a/tools/testing/selftests/bpf/prog_tests/btf_dump.c
+++ b/tools/testing/selftests/bpf/prog_tests/btf_dump.c
@@ -23,6 +23,7 @@ static struct btf_dump_test_case {
 	{"btf_dump: bitfields", "btf_dump_test_case_bitfields", true},
 	{"btf_dump: multidim", "btf_dump_test_case_multidim", false},
 	{"btf_dump: namespacing", "btf_dump_test_case_namespacing", false},
+	{"btf_dump: decl_tag", "btf_dump_test_case_decl_tag", true},
 };
 
 static int btf_dump_all_types(const struct btf *btf, struct btf_dump *d)
@@ -345,6 +346,82 @@ static void test_btf_dump_incremental(void)
 	btf__free(btf);
 }
 
+static void test_btf_dump_func_proto_decl_tag(void)
+{
+	struct btf *btf = NULL;
+	struct btf_dump *d = NULL;
+	int id, err;
+
+	dump_buf_file = open_memstream(&dump_buf, &dump_buf_sz);
+	if (!ASSERT_OK_PTR(dump_buf_file, "dump_memstream"))
+		return;
+	btf = btf__new_empty();
+	if (!ASSERT_OK_PTR(btf, "new_empty"))
+		goto err_out;
+	d = btf_dump__new(btf, btf_dump_printf, dump_buf_file, NULL);
+	if (!ASSERT_OK(libbpf_get_error(d), "btf_dump__new"))
+		goto err_out;
+
+	/* First, BTF corresponding to the following C code:
+	 *
+	 * typedef void (*fn)(int a __btf_decl_tag("a_tag"));
+	 *
+	 */
+	id = btf__add_int(btf, "int", 4, BTF_INT_SIGNED);
+	ASSERT_EQ(id, 1, "int_id");
+	id = btf__add_func_proto(btf, 0);
+	ASSERT_EQ(id, 2, "func_proto_id");
+	err = btf__add_func_param(btf, "a", 1);
+	ASSERT_OK(err, "func_param_ok");
+	id = btf__add_decl_tag(btf, "a_tag", 2, 0);
+	ASSERT_EQ(id, 3, "decl_tag_a");
+	id = btf__add_ptr(btf, 2);
+	ASSERT_EQ(id, 4, "proto_ptr");
+	id = btf__add_typedef(btf, "fn", 4);
+	ASSERT_EQ(id, 5, "typedef");
+
+	err = btf_dump_all_types(btf, d);
+	ASSERT_OK(err, "btf_dump_all_types #1");
+	fflush(dump_buf_file);
+	dump_buf[dump_buf_sz] = 0; /* some libc implementations don't do this */
+
+	ASSERT_STREQ(dump_buf,
+		     "#if __has_attribute(btf_decl_tag)\n"
+		     "#define __btf_decl_tag(x) __attribute__((btf_decl_tag(x)))\n"
+		     "#else\n"
+		     "#define __btf_decl_tag(x)\n"
+		     "#endif\n"
+		     "\n"
+		     "typedef void (*fn)(int a __btf_decl_tag(\"a_tag\"));\n\n",
+		     "decl tags for fn");
+
+	/* Next, add BTF corresponding to the following C code:
+	 *
+	 * typedef int foo __btf_decl_tag("foo_tag");
+	 *
+	 * To verify that decl_tag's table is updated incrementally.
+	 */
+	id = btf__add_typedef(btf, "foo", 1);
+	ASSERT_EQ(id, 6, "typedef");
+	id = btf__add_decl_tag(btf, "foo_tag", 6, -1);
+	ASSERT_EQ(id, 7, "decl_tag_foo");
+
+	fseek(dump_buf_file, 0, SEEK_SET);
+	err = btf_dump_all_types(btf, d);
+	ASSERT_OK(err, "btf_dump_all_types #2");
+	fflush(dump_buf_file);
+	dump_buf[dump_buf_sz] = 0; /* some libc implementations don't do this */
+
+	ASSERT_STREQ(dump_buf,
+		     "typedef int foo __btf_decl_tag(\"foo_tag\");\n\n",
+		     "decl tags for foo");
+err_out:
+	fclose(dump_buf_file);
+	free(dump_buf);
+	btf_dump__free(d);
+	btf__free(btf);
+}
+
 #define STRSIZE				4096
 
 static void btf_dump_snprintf(void *ctx, const char *fmt, va_list args)
@@ -963,6 +1040,8 @@ void test_btf_dump() {
 	}
 	if (test__start_subtest("btf_dump: incremental"))
 		test_btf_dump_incremental();
+	if (test__start_subtest("btf_dump: func arg decl_tag"))
+		test_btf_dump_func_proto_decl_tag();
 
 	btf = libbpf_find_kernel_btf();
 	if (!ASSERT_OK_PTR(btf, "no kernel BTF found"))
diff --git a/tools/testing/selftests/bpf/progs/btf_dump_test_case_decl_tag.c b/tools/testing/selftests/bpf/progs/btf_dump_test_case_decl_tag.c
new file mode 100644
index 000000000000..1205351e9a68
--- /dev/null
+++ b/tools/testing/selftests/bpf/progs/btf_dump_test_case_decl_tag.c
@@ -0,0 +1,65 @@
+// SPDX-License-Identifier: (LGPL-2.1 OR BSD-2-Clause)
+
+/*
+ * BTF-to-C dumper test for __atribute__((btf_decl_tag("..."))).
+ */
+
+#define SEC(x) __attribute__((section(x)))
+
+/* ----- START-EXPECTED-OUTPUT ----- */
+#if __has_attribute(btf_decl_tag)
+#define __btf_decl_tag(x) __attribute__((btf_decl_tag(x)))
+#else
+#define __btf_decl_tag(x)
+#endif
+
+struct empty_with_tag {} __btf_decl_tag("a");
+
+struct one_tag {
+	int x;
+} __btf_decl_tag("b");
+
+struct same_tag {
+	int x;
+} __btf_decl_tag("b");
+
+struct two_tags {
+	int x;
+} __btf_decl_tag("a") __btf_decl_tag("b");
+
+struct packed {
+	int x;
+	short y;
+} __attribute__((packed)) __btf_decl_tag("another_name");
+
+typedef int td_with_tag __btf_decl_tag("td");
+
+struct tags_on_fields {
+	int x __btf_decl_tag("t1");
+	int y;
+	int z __btf_decl_tag("t2") __btf_decl_tag("t3");
+};
+
+struct tag_on_field_and_struct {
+	int x __btf_decl_tag("t1");
+} __btf_decl_tag("t2");
+
+struct root_struct {
+	struct empty_with_tag a;
+	struct one_tag b;
+	struct same_tag c;
+	struct two_tags d;
+	struct packed e;
+	td_with_tag f;
+	struct tags_on_fields g;
+	struct tag_on_field_and_struct h;
+};
+
+SEC(".data") int global_var __btf_decl_tag("var_tag") = (int)777;
+
+/* ------ END-EXPECTED-OUTPUT ------ */
+
+int f(struct root_struct *s)
+{
+	return 0;
+}
-- 
2.34.1