Re: [PATCH] udp: port starting location not random

From: Stephen Hemminger <redacted>
Date: Thu, 04 Oct 2012 14:05:26 -0700 (PDT)

quoted

From: Stephen Hemminger <redacted>
Date: Thu, 4 Oct 2012 13:56:56 -0700

quoted

-		first = (((u64)rand * remaining) >> 32) + low;
+		first = rand % remaining + low;

I really don't get it, either a modulus via multiplcation
works or it doesn't.  We have this construct all over the
place.

Try out of kernel:

#include <stdio.h>
#include <stdlib.h>
#include <stdint.h>

int main(int ac, char **av) {
	int low, high, remaining;
	unsigned int rand;
	unsigned short first;

	low = atoi(av[1]);
	high = atoi(av[2]);
	remaining = (high - low) + 1;
	rand = atoi(av[3]);

	first = (((uint64_t)rand * remaining) >> 32) + low;

	printf("%d %d %u => %u\n", low, high, rand, first);
	return 0;
}

$ port-test 32768 61000 $RANDOM
32768 61000 7027 => 32768

It just shows that we need to shift "remaining" up into the top N
bits instead of the bottom N bits.