ptrace-call-userspace | system call to call a userspace method

I'm working on a project where I need to make a running program execute a function on demand. For this I am using ptrace. I know that this is possible because GDB does it.

Right now I am using an adapted version of the the code found on: https://github.com/eklitzke/ptrace-call-userspace This program displays how to call fprintf in a target program.

The program I am facing appears when the called function uses nanosleep(). If nanosleep() is called while inside the function called by the tracer, the tracee crashes with a SIGSEGV, but only after the sleep is concluded. If the function is called normally by the tracee itself, everything works properly.

I concluded that the problem is related to how the function is called, probably something to do with the tracee's stack or it's register values. I already checked that the stack is 16 byte aligned when entering the function for example.

Code of the tracer is present in the github above (difference is the called function and I also removed the arguments)

Code for the tracee is simple a dummy process that prints it's PID every second.

Code for the function that is called: