How to ignore SIGKILL or force a process into 'D' sleep state?

Question

I am trying to figure out how to get a process to ignore SIGKILL. The way I understand it, this isn't normally possible. My idea is to get a process into the 'D' state permanently. I want to do this for testing purposes (the corner case isn't really reproducible). I'm not sure this is possible programatically (I don't want to go damage hardware). I'm working in C++ and Python, but any language should be fine. I have root access.

I don't have any code to show because I don't know how to get started with this, or if it's even possible. Could I possibly set up a bad NFS and try reading from it?

Apologies in advance if this is a duplicate question; I didn't find anyone else trying to induce the D state.

Many thanks.

Answer 1

To get a process into the "D" state (uninterruptible sleep), you have to write kernel code which does that, and then call that code from user space via a system call.

In the Linux kernel, this is done by setting the current task state to uninterruptible, and invoking the scheduler:

set_current_state(TASK_UNINTERRUPTIBLE);
schedule();

Of course, these actions are normally wrapped with additional preparations so that the task has a way to wake up, such as registering on some wait queue or whatever.

Device drivers for low-latency devices such as mass storage use uninterruptible sleeps to simplify their logic. It should only be used when there is a sure-fire way that the process will wake up almost no matter what happens.

Kernel code to do little thing like performing an uninterruptible sleep can be put into a tiny module (start with a minimal driver skeleton) whose initialization function performs the code and then returns nonzero. You can then run the code using insmod, e.g.

 insmod my_uninterruptible_sleep_mod.ko

there is no need to rmmod because the function fails, and so the module is unloaded immediately.

Answer 2

It is not possible to ignore SIGKILL or handle it in any way.

From man sigaction:

The sa_mask field specified in act is not allowed to block SIGKILL or SIGSTOP. Any attempt to do so will be silently ignored.