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

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Date:	Thu, 04 Oct 2012 17:12:11 -0400 (EDT)
From:	David Miller <davem@...emloft.net>
To:	stephen.hemminger@...tta.com
Cc:	dada1@...mosbay.com, netdev@...r.kernel.org
Subject: Re: [PATCH] udp: port starting location not random

From: Stephen Hemminger <stephen.hemminger@...tta.com>
Date: Thu, 04 Oct 2012 14:05:26 -0700 (PDT)

>> From: Stephen Hemminger <shemminger@...tta.com>
>> Date: Thu, 4 Oct 2012 13:56:56 -0700
>> 
>> > -		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.
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