How do I use a Linux System call from a Linux Kernel Module

Question

I am having some difficulty calling a system call from inside a Linux Kernel Module. The system calls have been tested and work properly from a standard c user space program but I can't seem to get the kernel module to compile and run them.

In my user program I include the following code and the system call works:

#include <linux/unistd.h>   
#define __NR_sys_mycall 343

extern long int _syscall(long int_sysno,...)__THROW;

//and then a simple call is done as such
long value = syscall(__NR_sys_mycall);

printf("The value is %ld\n",value);

But when I try the same thing in my Linux Kernel Module I get a bunch of errors that either say error: implicit declaration of function 'syscall' (if I don't include the _syscall definition) or a long list of errors about syntax if I do...so my assumption is that I need the kernel space version to call the system call. Am I right or wrong?

//My LKM code
#include <linux/module.h>
#include <linux/unistd.h>
#define __NR_sys_mycall 343

static int start_init(void)
{
   long value = syscall(__NR_sys_mycall);
   printk("The value is %ld\n",value);

   return 0;
}

static void finish_exit(void)
{
      printk("Done!\n");
}

module_init(start_init);
module_exit(finish_exit);

Answer 1

You can directly call sys_mycall.

#include <linux/module.h>
#include <linux/unistd.h>


static int start_init(void)
{
   long value = sys_mycall (pass_arguments)
   printk("The value is %ld\n",value);

   return 0;
}

static void finish_exit(void)
{
      printk("Done!\n");
}

module_init(start_init);
module_exit(finish_exit);

Answer 2

Most of system calls uses asmlinkage , which means find the arguments on the stack rather than registers. Make sure that , when you call system call , pass the arguments on stack.

Also lot many system calls just uses copy_from_user. If you pass kernel address into such system call , they do fail.