/* packet_dropper.c
 *
 * This program provides an example of how to install a module into a
 *   slightly modified kernel that will randomly drop packets for a specific
 *   (hard-coded) host.
 *
 * See linux/drivers/char/random.c for details of get_random_bytes().
 *
 * Usage (must be root to use):
 *   /sbin/insmod packet_dropper
 *   /sbin/rmmod packet_dropper
 */

#define MODULE
#define MAX_UNSIGNED_SHORT 65535


#include <linux/module.h>
#include <linux/skbuff.h>  /* for struct sk_buff */
#include <linux/ip.h>      /* for struct iphdr */


extern int (*xmit_test_function)(struct sk_buff *);  /* calling function */
unsigned short cutoff;                               /* drop cutoff */
float rate   = 0.14;                                /* drop percentage */

__u32 target = 0x0202A8C0;	/* dst address 192.168.2.2 */

/* ************** added by lin begin ( ***********/
inline unsigned short getUnsignedShortRandom()
{
	unsigned l, h;
	unsigned short low, high;

	 __asm__ volatile("rdtsc": "=a" (l), "=d" (h));
	 /* get the lower 16 bits */
	 low = (unsigned short) l & 0xFFFF; 
	 /* cyclicly right shift 6 bits  */
	 high = low << 6;
	 low = low >> 10;
	 low = low | high;

	 return low;
}
/* ************** added by lin end   ) ***********/


/************************************************************ packet_dropper
 * this is what dev_queue_xmit will call while this module is installed */
int packet_dropper(struct sk_buff *skb) 
{
  unsigned short t;
  if (skb->nh.iph->daddr == target) {

  /* the following code is modified by lin : begin (*/
	t = getUnsignedShortRandom();
  	if (t < cutoff) return 1;    /* drop this packet */
  /* modified by lin : end ) */

  }
  return 0;                       /* continue with normal routine */
}  /* packet_dropper */



/*************************************************************** init_module
 * this function replaces the null pointer with a real one */
int init_module() {
  /*****************  modified by lin begin **************** */
  //EXPORT_NO_SYMBOLS;
  //cutoff =  rate * MAX_UNSIGNED_SHORT;
  /* experimnts on UDP test shows that the number of packets lost is always
   * greater than the number we expect. So here we low down the cutoff a little
   * bit. 
   */
  cutoff = rate*(1-rate)*MAX_UNSIGNED_SHORT;
  /*****************  modified by lin end ) *****************/

  xmit_test_function = packet_dropper;
  printk("<1> packet_dropper: now dropping packets below %u\n",cutoff);
  return 0;
}  /* init_module */



/************************************************************ cleanup_module
 * this function resets the function pointer back to null */
void cleanup_module() {
  xmit_test_function = 0;
  printk("<1> packet_dropper: uninstalled\n");
}  /* cleanup_module */