selftests/bpf: Add test for abnormal cnt during multi-uprobe attachment

If an abnormally huge cnt is used for multi-uprobes attachment, the
following warning will be reported:

  ------------[ cut here ]------------
  WARNING: CPU: 7 PID: 406 at mm/util.c:632 kvmalloc_node+0xd9/0xe0
  Modules linked in: bpf_testmod(O)
  CPU: 7 PID: 406 Comm: test_progs Tainted: G ...... 6.7.0-rc3+ #32
  Hardware name: QEMU Standard PC (i440FX + PIIX, 1996) ......
  RIP: 0010:kvmalloc_node+0xd9/0xe0
  ......
  Call Trace:
   <TASK>
   ? __warn+0x89/0x150
   ? kvmalloc_node+0xd9/0xe0
   bpf_uprobe_multi_link_attach+0x14a/0x480
   __sys_bpf+0x14a9/0x2bc0
   do_syscall_64+0x36/0xb0
   entry_SYSCALL_64_after_hwframe+0x6e/0x76
   ......
   </TASK>
  ---[ end trace 0000000000000000 ]---

So add a test to ensure the warning is fixed.
Signed-off-by: Hou Tao <houtao1@huawei.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Jiri Olsa <jolsa@kernel.org>
Acked-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20231215100708.2265609-4-houtao@huaweicloud.com

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

@@ -234,6 +234,34 @@ static void test_attach_api_syms(void)
 	test_attach_api("/proc/self/exe", NULL, &opts);
 }
 
+static void test_attach_api_fails(void)
+{
+	LIBBPF_OPTS(bpf_link_create_opts, opts);
+	const char *path = "/proc/self/exe";
+	struct uprobe_multi *skel = NULL;
+	unsigned long offset = 0;
+	int link_fd = -1;
+
+	skel = uprobe_multi__open_and_load();
+	if (!ASSERT_OK_PTR(skel, "uprobe_multi__open_and_load"))
+		goto cleanup;
+
+	/* abnormal cnt */
+	opts.uprobe_multi.path = path;
+	opts.uprobe_multi.offsets = &offset;
+	opts.uprobe_multi.cnt = INT_MAX;
+	link_fd = bpf_link_create(bpf_program__fd(skel->progs.uprobe), 0,
+				  BPF_TRACE_UPROBE_MULTI, &opts);
+	if (!ASSERT_ERR(link_fd, "link_fd"))
+		goto cleanup;
+	if (!ASSERT_EQ(link_fd, -E2BIG, "big cnt"))
+		goto cleanup;
+cleanup:
+	if (link_fd >= 0)
+		close(link_fd);
+	uprobe_multi__destroy(skel);
+}
+
 static void __test_link_api(struct child *child)
 {
 	int prog_fd, link1_fd = -1, link2_fd = -1, link3_fd = -1, link4_fd = -1;
@@ -311,7 +339,7 @@ static void __test_link_api(struct child *child)
 	free(offsets);
 }
 
-void test_link_api(void)
+static void test_link_api(void)
 {
 	struct child *child;
 
@@ -412,4 +440,6 @@ void test_uprobe_multi_test(void)
 		test_bench_attach_uprobe();
 	if (test__start_subtest("bench_usdt"))
 		test_bench_attach_usdt();
+	if (test__start_subtest("attach_api_fails"))
+		test_attach_api_fails();
 }