Adding a new System call to Linux 3.2.x with a loadable kernel module [closed]

Question

I want to add a particular new system call in the linux kernel 3.2.x but as a loadable kernel module (as I don't want to recompile the kernel again and again)

I read through many posts on internet and also on SO, and some places claim that implementing system calls as loadable modules is not possible, while others say it is possible.

Which is it? How is it done if it's possible?

Answer 1

It is not possible because system call table (called sys_call_table) is a static size array. And its size is determined at compile time by the number of registered syscalls. This means there is no space for another one.

You can check implementation for example for x86 architecture in arch/x86/kernel/syscall_64.c file, where sys_call_table is defined. Its size is exactly __NR_syscall_max+1. __NR_syscall_max is defined in kernel/asm-offsets_64.c as sizeof(syscalls) - 1 (it's the number of last syscall), wher syscall is a table with all the syscalls.

The only possible thing is to replace some existing syscall with yours as this won't require more space in memory. It's not really easy from kernel module as kernel does not export sys_call_table to modules as of version 2.6 (the last kernel version that had this symbol exported was 2.5.41. You can use this technique if you are developing new syscall and just want to avoid rebooting while experimenting. You will have to define your new system call and then replace it from module. To do this, you will have to change your kernel to export sys_call_table symbol to modules. To do this, you have to add following to lines to kernel/kallsyms.c (don't do this on production machines):

extern void *sys_call_table;
EXPORT_SYMBOL(sys_call_table);

Answer 2

Unfortunately you can't add system calls to kernel as loadable modules. You have to take the pain of compiling the kernel each time you add a new system call.