Question

I have downloaded kernel 2.6.38-5 and want to add a system call.
I did the following:

1. I have added my system call to system call table;

   <src folder>/arc/x86/kernel/syscall_table_32.S
   
   .long sys_mycall
   

2. I have added the system call number;

   <src folder>/include/asm-generic/unistd.h
   
   #define __NR_mycall 244
   __SYSCALL(__NR_mycall, sys_mycall)
   

3. I have added the prototype to syscalls.h;

   <src follder>/include/linux/syscalls.h
   
   asmlinkage long sys_mycall(long input);
   

4. And, here is my system call;

   asmlinkage long sys_mycall(long input) {
       return (input * 2);
   }
   

5. I have edited the Makefiles too.

Now after compilation, when I use it via syscall() it gives me BAD ADDRESS with errno set to 14.

What should I do?

Answer 1

On x86, system call number 244 is already taken by get_thread_area(), which takes as first argument a pointer to a struct user_desc:

int get_thread_area(struct user_desc *u_info);

You are passing a number instead of a pointer, the kernel is trying to interpret it as a pointer, it’s determining that it points outside of your process, and returning -EFAULT.