//-------------------------------------------------------------------
//	gdt.c				(Global Descriptor Table)
//
//	This is a Linux character-mode device-driver which allows 
//	user-level programs to read the processor's GDT as though
//	it were a 'read-only' file, once the required device-node 
//	has been created by a System Administrator:     
//
//		root# /sbin/mknod /dev/gdt c 98 0
//
//	NOTE: Written and tested with Linux kernel version 2.4.20
//
//	programmer: ALLAN CRUSE
//	written on: 14 APR 2003
//-------------------------------------------------------------------

#define	__KERNEL__
#define	  MODULE  

#include <linux/module.h>	// for init_module() 
#include <asm/uaccess.h>	// for copy_to_user()

static char modname[] = "gdt";
static int my_major = 98;
static long gdtsize; 
static char *gdtbase;

static ssize_t
my_read( struct file *file, char *buf, size_t count, loff_t *pos )
{
	char	*from = gdtbase + *pos;
	
	if ( *pos >= gdtsize ) return 0;
	if ( *pos + count > gdtsize ) count = gdtsize - *pos;
	if ( copy_to_user( buf, from, count ) ) return -EFAULT;
	*pos += count;
	return	count;
}

static loff_t my_seek( struct file *file, loff_t offset, int whence )
{
	loff_t	newpos;

	switch( whence )
		{
		case 0:	newpos = offset; break; 		// SEEK_SET
		case 1: newpos = file->f_pos + offset; break; 	// SEEK_CUR
		case 2:	newpos = gdtsize + offset; break; 	// SEEK_END
		default: return	-EINVAL;	// this should never occur
		}
	if ( newpos < 0 ) return -EINVAL;
	file->f_pos = newpos;
	return	newpos;	
}	


static struct file_operations 
my_fops =	{
		owner:		THIS_MODULE,
		read:		my_read,
		llseek:		my_seek,
		};


int init_module( void )
{
	unsigned short	gdtr[3];	

	// initialize device-driver's global variables
	asm(" sgdt %0 " : "=m" (gdtr) );
	gdtsize = 1 + gdtr[0];
	gdtbase = (char *)(*(unsigned long *)(gdtr+1));

	// register this device-driver with the kernel
	return	register_chrdev( my_major, modname, &my_fops );
}


void cleanup_module( void )
{
	unregister_chrdev( my_major, modname );
}

MODULE_LICENSE("GPL");