selftests/bpf: Add a C test for btf_type_tag

The following is the main btf_type_tag usage in the
C test:
  #define __tag1 __attribute__((btf_type_tag("tag1")))
  #define __tag2 __attribute__((btf_type_tag("tag2")))
  struct btf_type_tag_test {
       int __tag1 * __tag1 __tag2 *p;
  } g;

The bpftool raw dump with related types:
  [4] INT 'int' size=4 bits_offset=0 nr_bits=32 encoding=SIGNED
  [11] STRUCT 'btf_type_tag_test' size=8 vlen=1
          'p' type_id=14 bits_offset=0
  [12] TYPE_TAG 'tag1' type_id=16
  [13] TYPE_TAG 'tag2' type_id=12
  [14] PTR '(anon)' type_id=13
  [15] TYPE_TAG 'tag1' type_id=4
  [16] PTR '(anon)' type_id=15
  [17] VAR 'g' type_id=11, linkage=global

With format C dump, we have
  struct btf_type_tag_test {
        int __attribute__((btf_type_tag("tag1"))) * __attribute__((btf_type_tag("tag1"))) __attribute__((btf_type_tag("tag2"))) *p;
  };
The result C code is identical to the original definition except macro's are gone.
Signed-off-by: Yonghong Song <yhs@fb.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/20211112012646.1508231-1-yhs@fb.com

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

@@ -3,6 +3,12 @@
 #include <test_progs.h>
 #include "btf_decl_tag.skel.h"
 
+/* struct btf_type_tag_test is referenced in btf_type_tag.skel.h */
+struct btf_type_tag_test {
+        int **p;
+};
+#include "btf_type_tag.skel.h"
+
 static void test_btf_decl_tag(void)
 {
 	struct btf_decl_tag *skel;
@@ -19,8 +25,26 @@ static void test_btf_decl_tag(void)
 	btf_decl_tag__destroy(skel);
 }
 
+static void test_btf_type_tag(void)
+{
+	struct btf_type_tag *skel;
+
+	skel = btf_type_tag__open_and_load();
+	if (!ASSERT_OK_PTR(skel, "btf_type_tag"))
+		return;
+
+	if (skel->rodata->skip_tests) {
+		printf("%s:SKIP: btf_type_tag attribute not supported", __func__);
+		test__skip();
+	}
+
+	btf_type_tag__destroy(skel);
+}
+
 void test_btf_tag(void)
 {
 	if (test__start_subtest("btf_decl_tag"))
 		test_btf_decl_tag();
+	if (test__start_subtest("btf_type_tag"))
+		test_btf_type_tag();
 }

tools/testing/selftests/bpf/progs/btf_type_tag.c

+// SPDX-License-Identifier: GPL-2.0
+/* Copyright (c) 2021 Facebook */
+#include "vmlinux.h"
+#include <bpf/bpf_helpers.h>
+#include <bpf/bpf_tracing.h>
+
+#if __has_attribute(btf_type_tag)
+#define __tag1 __attribute__((btf_type_tag("tag1")))
+#define __tag2 __attribute__((btf_type_tag("tag2")))
+volatile const bool skip_tests = false;
+#else
+#define __tag1
+#define __tag2
+volatile const bool skip_tests = true;
+#endif
+
+struct btf_type_tag_test {
+	int __tag1 * __tag1 __tag2 *p;
+} g;
+
+SEC("fentry/bpf_fentry_test1")
+int BPF_PROG(sub, int x)
+{
+  return 0;
+}