[RFC bpf-next 04/12] selftests/bpf: Tests for BTF_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`.

Signed-off-by: Eduard Zingerman <eddyz87@gmail.com>
---
 .../selftests/bpf/prog_tests/btf_dump.c       |  1 +
 .../bpf/progs/btf_dump_test_case_decl_tag.c   | 39 +++++++++++++++++++
 2 files changed, 40 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 24da335482d4..5f6ce7f1a801 100644
--- a/tools/testing/selftests/bpf/prog_tests/btf_dump.c
+++ b/tools/testing/selftests/bpf/prog_tests/btf_dump.c
@@ -21,6 +21,7 @@ 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, void *ctx)
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..470bbbb530dc
--- /dev/null
+++ b/tools/testing/selftests/bpf/progs/btf_dump_test_case_decl_tag.c
@@ -0,0 +1,39 @@
+// SPDX-License-Identifier: (LGPL-2.1 OR BSD-2-Clause)
+
+/*
+ * BTF-to-C dumper test for __atribute__((btf_decl_tag("..."))).
+ */
+/* ----- START-EXPECTED-OUTPUT ----- */
+struct empty_with_tag {} __attribute__((btf_decl_tag("a")));
+
+struct one_tag {
+	int x;
+} __attribute__((btf_decl_tag("b")));
+
+struct same_tag {
+	int x;
+} __attribute__((btf_decl_tag("b")));
+
+struct two_tags {
+	int x;
+} __attribute__((btf_decl_tag("a"))) __attribute__((btf_decl_tag("b")));
+
+struct packed {
+	int x;
+	short y;
+} __attribute__((packed)) __attribute__((btf_decl_tag("another_name")));
+
+struct root_struct {
+	struct empty_with_tag a;
+	struct one_tag b;
+	struct same_tag c;
+	struct two_tags d;
+	struct packed e;
+};
+
+/* ------ END-EXPECTED-OUTPUT ------ */
+
+int f(struct root_struct *s)
+{
+	return 0;
+}
-- 
2.34.1