selftests/bpf: Add reproducer for decl_tag in func_proto argument

It should trigger a WARN_ON_ONCE in btf_type_id_size:

  RIP: 0010:btf_type_id_size+0x8bd/0x940 kernel/bpf/btf.c:1952
  btf_func_proto_check kernel/bpf/btf.c:4506 [inline]
  btf_check_all_types kernel/bpf/btf.c:4734 [inline]
  btf_parse_type_sec+0x1175/0x1980 kernel/bpf/btf.c:4763
  btf_parse kernel/bpf/btf.c:5042 [inline]
  btf_new_fd+0x65a/0xb00 kernel/bpf/btf.c:6709
  bpf_btf_load+0x6f/0x90 kernel/bpf/syscall.c:4342
  __sys_bpf+0x50a/0x6c0 kernel/bpf/syscall.c:5034
  __do_sys_bpf kernel/bpf/syscall.c:5093 [inline]
  __se_sys_bpf kernel/bpf/syscall.c:5091 [inline]
  __x64_sys_bpf+0x7c/0x90 kernel/bpf/syscall.c:5091
  do_syscall_64+0x54/0x70 arch/x86/entry/common.c:48

Signed-off-by: Stanislav Fomichev <sdf@google.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Yonghong Song <yhs@fb.com>
Link: https://lore.kernel.org/bpf/20221123035422.872531-1-sdf@google.com

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

@@ -3948,6 +3948,20 @@ static struct btf_raw_test raw_tests[] = {
 	.btf_load_err = true,
 	.err_str = "Invalid return type",
 },
+{
+	.descr = "decl_tag test #17, func proto, argument",
+	.raw_types = {
+		BTF_TYPE_ENC(NAME_TBD, BTF_INFO_ENC(BTF_KIND_DECL_TAG, 0, 0), 4), (-1),	/* [1] */
+		BTF_TYPE_ENC(0, BTF_INFO_ENC(BTF_KIND_PTR, 0, 0), 0), /* [2] */
+		BTF_FUNC_PROTO_ENC(0, 1),			/* [3] */
+			BTF_FUNC_PROTO_ARG_ENC(NAME_TBD, 1),
+		BTF_VAR_ENC(NAME_TBD, 2, 0),			/* [4] */
+		BTF_END_RAW,
+	},
+	BTF_STR_SEC("\0local\0tag1\0var"),
+	.btf_load_err = true,
+	.err_str = "Invalid arg#1",
+},
 {
 	.descr = "type_tag test #1",
 	.raw_types = {