selftests/bpf: Add a test with a bpf program with btf_tag attributes

Add a bpf program with btf_tag attributes. The program is
loaded successfully with the kernel. With the command
  bpftool btf dump file ./tag.o
the following dump shows that tags are properly encoded:
  [8] STRUCT 'key_t' size=12 vlen=3
          'a' type_id=2 bits_offset=0
          'b' type_id=2 bits_offset=32
          'c' type_id=2 bits_offset=64
  [9] TAG 'tag1' type_id=8 component_id=-1
  [10] TAG 'tag2' type_id=8 component_id=-1
  [11] TAG 'tag1' type_id=8 component_id=1
  [12] TAG 'tag2' type_id=8 component_id=1
  ...
  [21] FUNC_PROTO '(anon)' ret_type_id=2 vlen=1
          'x' type_id=2
  [22] FUNC 'foo' type_id=21 linkage=static
  [23] TAG 'tag1' type_id=22 component_id=0
  [24] TAG 'tag2' type_id=22 component_id=0
  [25] TAG 'tag1' type_id=22 component_id=-1
  [26] TAG 'tag2' type_id=22 component_id=-1
  ...
  [29] VAR 'total' type_id=27, linkage=global
  [30] TAG 'tag1' type_id=29 component_id=-1
  [31] TAG 'tag2' type_id=29 component_id=-1

If an old clang compiler, which does not support btf_tag attribute,
is used, these btf_tag attributes will be silently ignored.
Signed-off-by: Yonghong Song <yhs@fb.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20210914223058.248949-1-yhs@fb.com

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

+// SPDX-License-Identifier: GPL-2.0
+/* Copyright (c) 2021 Facebook */
+#include <test_progs.h>
+#include "tag.skel.h"
+
+void test_btf_tag(void)
+{
+	struct tag *skel;
+
+	skel = tag__open_and_load();
+	if (!ASSERT_OK_PTR(skel, "btf_tag"))
+		return;
+	tag__destroy(skel);
+}

tools/testing/selftests/bpf/progs/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>
+
+#define __tag1 __attribute__((btf_tag("tag1")))
+#define __tag2 __attribute__((btf_tag("tag2")))
+
+struct key_t {
+	int a;
+	int b __tag1 __tag2;
+	int c;
+} __tag1 __tag2;
+
+struct {
+	__uint(type, BPF_MAP_TYPE_HASH);
+	__uint(max_entries, 3);
+	__type(key, struct key_t);
+	__type(value, __u64);
+} hashmap1 SEC(".maps");
+
+__u32 total __tag1 __tag2 = 0;
+
+static __noinline int foo(int x __tag1 __tag2) __tag1 __tag2
+{
+	struct key_t key;
+	__u64 val = 1;
+
+	key.a = key.b = key.c = x;
+	bpf_map_update_elem(&hashmap1, &key, &val, 0);
+	return 0;
+}
+
+SEC("fentry/bpf_fentry_test1")
+int BPF_PROG(sub, int x)
+{
+	return foo(x);
+}