#include <stdio.h>
#include <net/if.h>
#define _GNU_SOURCE
#include <sched.h>
#include <unistd.h>
#include <sys/types.h>

/* 
 * We import the hash function from the kernel, so we can compute a conflicting
 * name directly in this program and reproduce the bug easily
 */

#define NETDEV_HASHBITS  8

/*************** from include/linux/dcache.h *****************************/

#define init_name_hash()  0
static inline unsigned long
partial_name_hash(unsigned long c, unsigned long prevhash)
{
        return (prevhash + (c << 4) + (c >> 4)) * 11;
}

static inline unsigned long end_name_hash(unsigned long hash)
{
        return (unsigned int) hash;
}

static inline unsigned int
full_name_hash(const unsigned char *name, unsigned int len)
{
        unsigned long hash = init_name_hash();
        while (len--)
                hash = partial_name_hash(*name++, hash);
        return end_name_hash(hash);
}

/***************** from include/linux/hash.h *****************************/

/* 2^31 + 2^29 - 2^25 + 2^22 - 2^19 - 2^16 + 1 */
#define GOLDEN_RATIO_PRIME_32 0x9e370001UL

static inline unsigned int hash_32(unsigned int val, unsigned int bits)
{
        /* On some cpus multiply is faster, on others gcc will do shifts */
        unsigned int hash = val * GOLDEN_RATIO_PRIME_32;

        /* High bits are more random, so use them. */
        return hash >> (32 - bits);
}

/*************************************************************************/


unsigned int dev_name_hash(const char *name)
{
	return hash_32(full_name_hash(name, strnlen(name, IFNAMSIZ)), 
                        NETDEV_HASHBITS);
}

int main(int argc, char *argv[])
{
	char devname[IFNAMSIZ];
	char cmd[4096];
	unsigned int val, val2;
	
	if (getuid()) {
		fprintf(stderr, "you have to be root !\n");
		return -1;
	}

	/*
	 * Unshare the network namespace, we don't mess the network
         * and we can assume the eth%d rename will be eth0
	 */
	if (unshare(CLONE_NEWNET)) {
		perror("unshare");
		return -1;
	}		

	val = dev_name_hash("eth0");

	while (1) {

		snprintf(devname, IFNAMSIZ, "ethXXXXXX");
		mktemp(devname);

		val2 = dev_name_hash(devname);

		if (val == val2) {
			printf("'%s' has same hash entry\n", devname);
			break;
		}
	}

	/*
	 * We create a dummy interface with the conflicting name
	 * and then we rename it with an kernel allocated name
	 * The kernel will fail to rename with -ENFILE.
	 */

	sprintf(cmd, "ip link add %s type dummy", devname);

	if (system(cmd)) {
		perror("system");
		return -1;
	}

	sprintf(cmd, "ip link set name eth%%d dev %s", devname);
	printf("%s\n", cmd);
	system(cmd); 

	return 0;
}