This is cute, but why not just do this with gdb? All-stop mode is on by default so it will stop all threads when you attach, and then you can simply invoke the exit_group() syscall.

    tty1$ ./program
    
    tty2$ gdb attach $(ps aux | grep program | head -1 | awk '{ print $2 }')
    Attaching to process 708267
    [New LWP 708268]
    [New LWP 708269]
    [New LWP 708270]
    [New LWP 708271]
    [Thread debugging using libthread_db enabled]
    Using host libthread_db library "/lib/x86_64-linux-gnu/libthread_db.so.1".
    0x00007f00b1633117 in ?? () from /lib/x86_64-linux-gnu/libc.so.6
    
    (gdb) p (long)syscall(231, 0)
    [Thread 0x7f00565e6640 (LWP 708271) exited]
    [Thread 0x7f005a5e8640 (LWP 708270) exited]
    [Thread 0x7f005ade9640 (LWP 708269) exited]
    [Thread 0x7f005d5ee640 (LWP 708268) exited]
    [Inferior 1 (process 708267) exited normally]
    The program being debugged exited while in a function called from GDB.
    Evaluation of the expression containing the function
    (syscall) will be abandoned.
    (gdb) quit
    
    tty1$ echo $?
    0

> However, what if the need arises to forcefully terminate a process, disguising it as a successful exit? ... Since the parent process would detect the non-zero exit code, there was a high likelihood of making things worse.

I'm still trying to imagine what situation that is. I still cannot think about any real case where this is something you need.

If there is, I guess this is a very rare case? As it was already commented, I guess just gdb then?

But the technical details on how they do it are interesting nevertheless.

how can we build a tool to stop a process forcefully but make it look like a successful termination? As far as I know, Linux doesn’t provide an API to perform such a kill.

Interestingly, Windows does:

https://learn.microsoft.com/en-us/windows/win32/api/processt...

Using gdb is probably easier.