//-------------------------------------------------------------------
//	myexport.c
//
//	This module creates a pseudo-file (named '/proc/myexport') 	
//	which allows other modules to be dynamically linked to the
//	kernel's 'sys_call_table[]' data-structure, simulating the
//	effect of having recompiled the kernel with this symbol as
//	an 'exported' (i.e., publicly visible) object.  The actual
//	address at which that array resides within the kernel will 
//	depend upon specific configuration options in effect while
//	the kernel was being compiled, but can be determined by an
//	inspection of the (uncompressed) kernel binary ('vmlinux')
//	or from its associated 'System.map' file; in any case, the
//	address must be supplied by the user as a module-parameter 
//	at the time this module is being installed, for example:
//
//	     root# /sbin/insmod myexport.o svctable=C032F974
//
//	For convenience, we recommend that a shell-script be used
//	so as to automate these address-lookup and module-install 
//	steps (and avoid hazards caused by parameter mis-typing). 
//
//	Warning: This technique is definitely a kernel 'hack' and 
//	justified only for strictly limited educational purposes! 
//
//	NOTE: Written and tested with Linux kernel version 2.4.26.
//
//	programmer: ALLAN CRUSE
//	written on: 28 SEP 2004
//-------------------------------------------------------------------

#include <linux/module.h>	// for init_module(), printk() 
#include <linux/proc_fs.h>	// for create_proc_info_entry() 

#define PROC_DIR  NULL		// use the default directory
#define FILE_MODE  0		// use the default file mode

static char modname[] = "myexport";	// name for the file 
unsigned long *sys_call_table;		// exported variable


char *svctable = "";			// parameter pointer
MODULE_PARM( svctable, "s" );		// setup by 'insmod' 


static int my_get_info( char *system_buffer, char **my_buffer, 
			off_t file_position, int system_buffer_length )
{
	int	len = 0;
	len += sprintf( system_buffer+len, "sys_call_table[]=" );
	len += sprintf( system_buffer+len, "%08X \n", sys_call_table );
	return	len;
}



int init_module( void )
{
	unsigned long	myval;
	
	printk( "<1>\nInstalling \'%s\' module ", modname );
	printk( "with svctable=%s \n", svctable );

	// converts the character-string argument to binary
	myval = simple_strtoul( svctable, NULL, 16 );

	// assigns the resulting value to a public variable
	sys_call_table = (unsigned long *)myval;

	// a 'sanity check' (in case parameter was omitted)
	if ( sys_call_table == NULL ) return -EINVAL;
	
	create_proc_info_entry( modname, 0, NULL, my_get_info );
	return	0;  //SUCCESS
}

void cleanup_module( void )
{
	remove_proc_entry( modname, NULL );
	printk( "<1>Removing \'%s\' module\n", modname );
}

MODULE_LICENSE("GPL");