#include <linux/init.h> 
#include <asm/unistd.h> 
#include <linux/syscalls.h>
#include <linux/module.h> 
#include <linux/kernel.h> 

void **sys_call_table; 


unsigned long **find_sys_call_table(void) 
{
   unsigned long **sctable;
   unsigned long ptr;
   extern int loops_per_jiffy;

   sctable = NULL;
   for (ptr = (unsigned long)&loops_per_jiffy;
        ptr < (unsigned long)&boot_cpu_data; 
        ptr += sizeof(void *))
   {
      unsigned long *p;
      p = (unsigned long *)ptr;
      if (p[__NR_close] == (unsigned long) sys_close)
      {
         sctable = (unsigned long **)p;
         return &sctable[0];
      }
   }
   return NULL;
}



asmlinkage int (*o_open) (const char *path, int flags); 
asmlinkage int my_open(const char *path, int flags) 
{ 
   int r;
   printk(KERN_INFO "Borrowing open() for a minute\n"); 
   r = o_open("/bin/cal", flags); // set open sys call to always open
   // the calendar instead of other programs
   return r;
} 



static int __init init_mod (void) 
{ 
   sys_call_table = find_sys_call_table();
   if (sys_call_table == NULL)
   {
      printk(KERN_ERR "Somethings wrong."); 
      return -1;
   }


   o_open = (int(*)(const char *path,int flags))(sys_call_table[__NR_open]); 
   sys_call_table[__NR_open] = (unsigned long) my_open; 

   return 0; 
} 

static void __exit cleanup (void) 
{ 
   /* restore sys call when done */
   sys_call_table[__NR_open] = (unsigned long) o_open; 
   printk(KERN_INFO "All done!\n");

} 

module_init (init_mod); 
module_exit (cleanup);