//-------------------------------------------------------------------
//	newcall.c
//
//	This module dynamically installs a new 'method' for a Linux
//	system-call (without the necessity to recompile the kernel)
//	by overwriting an obsolete entry in the 'sys_call_table[]'.
//	NOTE: Our 'myexport.o' module must be previously installed.
//	
//	Our new 'method' merely increments the value of the integer
//	whose address is passed as the system-call's only argument.   
//
//	NOTE: Written and tested using Linux kernel version 2.4.20.
//
//	programmer: ALLAN CRUSE
//	written on: 22 JUN 2004
//	revised on: 28 SEP 2004 -- for Linux kernel version 2.4.26
//-------------------------------------------------------------------

#include <linux/module.h>	// for init_module() 
#include <asm/uaccess.h>	// for copy_to/from_user()
#include <asm/unistd.h>		// for __NR_break

#define	SUCCESS	0
#define OBSOLETE_ID __NR_break	// system-call ID-number

static char modname[] = "newcall";
static unsigned long save_old_syscall;
extern unsigned long *sys_call_table;

static asmlinkage long my_syscall( int * arg )
{
	int	tmp;

	if ( copy_from_user( &tmp, arg, sizeof( int ) ) ) return -EFAULT;
	++tmp;
	if ( copy_to_user( arg, &tmp, sizeof( int ) ) ) return -EFAULT; 
	return	SUCCESS;
}	

int init_module( void )
{
	printk( "<1>\nInstalling \'%s\' \n", modname );

	// modify the designated 'sys_call_table' entry 	
	save_old_syscall = sys_call_table[ OBSOLETE_ID ];
	sys_call_table[ OBSOLETE_ID ] = (unsigned long)my_syscall;

	printk( " Installed a new \'method\' for " );
	printk( "system-call number %d \n", OBSOLETE_ID );
	return	SUCCESS;
}

void cleanup_module( void )
{
	printk( "<1>Removing \'%s\' module\n", modname );
	sys_call_table[ OBSOLETE_ID ] = save_old_syscall;
}

MODULE_LICENSE("GPL");