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As far as I know, some signal is used to stop/continue and terminate/kill process. If I made the process ignore all the signal, what will happen? Will it be a zombie or something unable to be killed?

Gilles 'SO- stop being evil'
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CatDog
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2 Answers2

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There are signals that cannot be ignored. SIGKILL always kills a process without giving it a chance of reacting. SIGSTOP always suspends a process without giving it a chance of reacting. Some Unix variants may provide other non-ignorable, non-handleable signals; Linux only has the standard KILL and STOP.

The only way a process can remain behind after a SIGKILL is if it's in currently in a non-interruptible system call. The process can't run any more code, but it'll stay behind until the system call finishes or reaches an interruptible point. This is normally too short to observe, but you can occasionally see it (a lingering process in state D) due to buggy drivers, buggy hardware, or sometimes with network filesystems. See What if 'kill -9' does not work?

A unkillable process wouldn't be a “zombie” since a zombie is already dead, and the term “zombie process” in fact exists and means a process table entry corresponding to a process that's already dead. See How does linux handles zombie process

Gilles 'SO- stop being evil'
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First of all Zombie is a state where a pocess has released all its resources except PID and waits to be finally reaped i.e to be completely released.

Kernel Processes have Task_Interruptible state which means it will not respond to any delivered signals.

So finally to your question if you manage to mask all signals to avoid process management signals, they will be on their own and cannot be managed whenever necessary. This might also result in system overload.

Neo_Returns
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    This answer is misleading when it comes to the core point of the question: “if you manage to mask all processes” (I guess you meant “mask all signals”) — the core point of a correct answer to this question is that this is not possible. – Gilles 'SO- stop being evil' May 19 '18 at 09:58
  • Yes, I agree with you but the whole question is based on assumption to what if it can be done. And thanks for pointing out the silly mistake I have rectified it. – Neo_Returns May 19 '18 at 10:07