//-------------------------------------------------------------------
//	tempcdev.c
//
//	This module executes some Linux system-calls which perform 
//	operations normally requiring 'root' privileges:
//
//		int sys_mknod( const char *, int, unsigned );
//		int sys_chmod( const char *, int );
//		int sys_unlink( const char * );
//
//	We need to have the '/dev/cmos' character-mode device-file 
//	available for our first in-class exercise on device-driver
//	programming for Linux.  
//
//	NOTE: Written and tested using Linux kernel version 2.6.22.
//
//	programmer: ALLAN CRUSE
//	written on: 30 AUG 2007
//	revised on: 31 AUG 2007 -- to close a security-vulnerability
//	revised on: 03 SEP 2007 -- corrected 'dev_t' parameter-value  
//
//	revised by: DAVID ROBINSON for Linux x86_64
//	tested for: Kernel version 3.0.4
//-------------------------------------------------------------------

#include <linux/module.h>	// for init_module() 
#include <asm/uaccess.h>	// for get_ds(), set_fs()
#include <asm/syscall.h>
#include <linux/syscalls.h>

char modname[] = "tempcdev";
char 	path[] = "/dev/cmos";
mode_t	mode = S_IFCHR | S_IRUSR | S_IRGRP | S_IROTH;
dev_t	dev  = (70 << 8);	// <--- modified on 9/3/07
int	retval;
void **syscall_table;

asmlinkage long (* mknod) ( const char __user *filename, umode_t mode,
		unsigned dev );
asmlinkage long (* unlink) ( const char __user *pathname );

static void **get_sys_call_table( void )
{
	void **sct = (void **) PAGE_OFFSET;

	while ( sct  < (void **) ULLONG_MAX ) {
		if ( sct[__NR_close] == (void *) sys_close ) 
			return sct;

		sct++;
	}

	return NULL;
}

static int __init my_init( void )
{
	mm_segment_t	savedFS = get_fs();	// preserve address-limit

	syscall_table = get_sys_call_table();
	if ( syscall_table == NULL ) {
		printk( "unable to find sys_call_table[]\n" );
		printk( "could not create the \'%s\' device-node \n", path );
		return -1;
	}

	printk( "<1>\nInstalling \'%s\' module\n", modname );

	set_fs( get_ds() );	// lets our module do the system-calls
	mknod = syscall_table[__NR_mknod];
	retval = mknod( path, mode, dev );
	set_fs( savedFS );	// restore the former address-limit value

	if ( retval == 0 ) 
		printk( "successfully created \'%s\' device-node \n", path );
	else	printk( "could not create the \'%s\' device-node \n", path );

	return	retval;
}

static void __exit my_exit( void )
{
	mm_segment_t	savedFS = get_fs();	// preserve address-limit

	printk( "<1>Removing \'%s\' module\n", modname );

	set_fs( get_ds() );	// lets our module do the system-call
	unlink = syscall_table[__NR_unlink];
	retval = unlink( path );
	set_fs( savedFS );	// restore the former address-limit value

	if ( retval == 0 ) 
		printk( "successfully deleted \'%s\' device-node \n", path );
	else	printk( "could not delete the \'%s\' device-node \n", path );
}

module_init( my_init );
module_exit( my_exit );
MODULE_LICENSE("GPL");