selftests/bpf: Add selftest for check_stack_max_depth bug

Use the bpf_timer_set_callback helper to mark timer_cb as an async
callback, and put a direct call to timer_cb in the main subprog.

As the check_stack_max_depth happens after the do_check pass, the order
does not matter. Without the previous fix, the test passes successfully.

Signed-off-by: Kumar Kartikeya Dwivedi <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Alexei Starovoitov <[e-mail]>

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

···
+// SPDX-License-Identifier: GPL-2.0
+#include <test_progs.h>
+
+#include "async_stack_depth.skel.h"
+
+void test_async_stack_depth(void)
+{
+	RUN_TESTS(async_stack_depth);
+}

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

···
+// SPDX-License-Identifier: GPL-2.0
+#include <vmlinux.h>
+#include <bpf/bpf_helpers.h>
+
+#include "bpf_misc.h"
+
+struct hmap_elem {
+	struct bpf_timer timer;
+};
+
+struct {
+	__uint(type, BPF_MAP_TYPE_HASH);
+	__uint(max_entries, 64);
+	__type(key, int);
+	__type(value, struct hmap_elem);
+} hmap SEC(".maps");
+
+__attribute__((noinline))
+static int timer_cb(void *map, int *key, struct bpf_timer *timer)
+{
+	volatile char buf[256] = {};
+	return buf[69];
+}
+
+SEC("tc")
+__failure __msg("combined stack size of 2 calls")
+int prog(struct __sk_buff *ctx)
+{
+	struct hmap_elem *elem;
+	volatile char buf[256] = {};
+
+	elem = bpf_map_lookup_elem(&hmap, &(int){0});
+	if (!elem)
+		return 0;
+
+	timer_cb(NULL, NULL, NULL);
+	return bpf_timer_set_callback(&elem->timer, timer_cb) + buf[0];
+}
+
+char _license[] SEC("license") = "GPL";