selftests/bpf: Skip btf_tag test if btf_tag attribute not supported

Commit c240ba28 ("selftests/bpf: Add a test with a bpf
program with btf_tag attributes") added btf_tag selftest
to test BTF_KIND_TAG generation from C source code, and to
test kernel validation of generated BTF types.
But if an old clang (clang 13 or earlier) is used, the
following compiler warning may be seen:
  progs/tag.c:23:20: warning: unknown attribute 'btf_tag' ignored
and the test itself is marked OK. The compiler warning is bad
and the test itself shouldn't be marked OK.

This patch added the check for btf_tag attribute support.
If btf_tag is not supported by the clang, the attribute will
not be used in the code and the test will be marked as skipped.
For example, with clang 13:
  ./test_progs -t btf_tag
  #21 btf_tag:SKIP
  Summary: 1/0 PASSED, 1 SKIPPED, 0 FAILED

The selftests/README.rst is updated to clarify when the btf_tag
test may be skipped.
Signed-off-by: Yonghong Song <yhs@fb.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/20210915061036.2577971-1-yhs@fb.com

tools/testing/selftests/bpf/README.rst

@@ -201,6 +201,20 @@ Without it, the error from compiling bpf selftests looks like:
 
 __ https://reviews.llvm.org/D93563
 
+btf_tag test and Clang version
+==============================
+
+The btf_tag selftest require LLVM support to recognize the btf_tag attribute.
+It was introduced in `Clang 14`__.
+
+Without it, the btf_tag selftest will be skipped and you will observe:
+
+.. code-block:: console
+
+  #<test_num> btf_tag:SKIP
+
+__ https://reviews.llvm.org/D106614
+
 Clang dependencies for static linking tests
 ===========================================
 

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

@@ -10,5 +10,11 @@ void test_btf_tag(void)
 	skel = tag__open_and_load();
 	if (!ASSERT_OK_PTR(skel, "btf_tag"))
 		return;
+
+	if (skel->rodata->skip_tests) {
+		printf("%s:SKIP: btf_tag attribute not supported", __func__);
+		test__skip();
+	}
+
 	tag__destroy(skel);
 }

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

@@ -4,8 +4,19 @@
 #include <bpf/bpf_helpers.h>
 #include <bpf/bpf_tracing.h>
 
+#ifndef __has_attribute
+#define __has_attribute(x) 0
+#endif
+
+#if __has_attribute(btf_tag)
 #define __tag1 __attribute__((btf_tag("tag1")))
 #define __tag2 __attribute__((btf_tag("tag2")))
+volatile const bool skip_tests __tag1 __tag2 = false;
+#else
+#define __tag1
+#define __tag2
+volatile const bool skip_tests = true;
+#endif
 
 struct key_t {
 	int a;
@@ -20,7 +31,6 @@ struct {
 	__type(value, __u64);
 } hashmap1 SEC(".maps");
 
-__u32 total __tag1 __tag2 = 0;
 
 static __noinline int foo(int x __tag1 __tag2) __tag1 __tag2
 {