Is it possible to add a system call via a LKM?

Question

I'd like to add a new system call via an LKM, but I'm not sure how to do this. That is, I know that if I want to add a completely new system call, I can look through the sys_call_table and find a sys_ni_syscall and just replace it, but I was curious if it was possible to actually add to the sys_call_table. I realize it's probably not possible, given that it's a fixed size array, but I was wondering if there were any other clever ways to add system calls without overriding an unused system call number.

Answer 1

Check The Linux Documentation Project website for "The Linux Kernel Module Programming Guide" (http://www.tldp.org/LDP/lkmpg/2.6/html/index.html). Specifically, look here for System Calls: http://www.tldp.org/LDP/lkmpg/2.6/html/x978.html. That should give you a start, at least.

Answer 2

Here's an example
linux system calls

edit:
The example above shows howto implement a system call, as far as implementing one from a loadable module; AFAIK, that's not possible, unless you where to overwrite an existing one because the size of the array is a #define.

Keep in mind there are user space changes required as well, at least if you want to be able to actually use the new system call.