selftests/bpf: Add assert for user stacks in test_task_stack

This is a follow up to:
commit b8e3a87 ("bpf: Add crosstask check to __bpf_get_stack").

This test ensures that the task iterator only gets a single
user stack (for the current task).

Signed-off-by: Jordan Rome <linux@jordanrome.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Acked-by: Stanislav Fomichev <sdf@google.com>
Link: https://lore.kernel.org/bpf/20231112023010.144675-1-linux@jordanrome.com

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

@@ -332,6 +332,8 @@ static void test_task_stack(void)
 	do_dummy_read(skel->progs.dump_task_stack);
 	do_dummy_read(skel->progs.get_task_user_stacks);
 
+	ASSERT_EQ(skel->bss->num_user_stacks, 1, "num_user_stacks");
+
 	bpf_iter_task_stack__destroy(skel);
 }
 

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

@@ -35,6 +35,8 @@ int dump_task_stack(struct bpf_iter__task *ctx)
 	return 0;
 }
 
+int num_user_stacks = 0;
+
 SEC("iter/task")
 int get_task_user_stacks(struct bpf_iter__task *ctx)
 {
@@ -51,6 +53,9 @@ int get_task_user_stacks(struct bpf_iter__task *ctx)
 	if (res <= 0)
 		return 0;
 
+	/* Only one task, the current one, should succeed */
+	++num_user_stacks;
+
 	buf_sz += res;
 
 	/* If the verifier doesn't refine bpf_get_task_stack res, and instead