信号如何无法捕获（例如sigkill）？在Linux的引擎盖下发生了什么？

I understand there are catchable and uncatchable signals. However, it seems that both types of signals get sent to processes by the OS. What makes uncatchable signals uncatchable? Is it the signal handler that catches signals, and because no signal handler is written to handle an a particular signal (e.g. sigkill), that it becomes uncatchable? If that's true, can I conclude that it is possible to catch an uncatchable signal by writing a signal handler? I understand there are catchable and uncatchabl