Subject: Re: [RFC] enhanced version of net_random()

From: Jean-Luc Cooke <[email-masked]>
Date: Fri, 20 Aug 2004 13:59:52 -0400

Is there a reason why get_random_bytes() is unsuitable?

Keeping the number of PRNGs in the kernel to a minimum should a goal we can
all share.

JLC

On Thu, Aug 12, 2004 at 10:48:35AM -0700, Stephen Hemminger wrote:
> While doing the network emulator, I discovered that the default net_random()
> is too stupid, and get_random_bytes() is more than needed. Rather than put
> another function in just for sch_netem, how about making net_random() smarter?
> The tin-hat crowd already replace net_random() with get_random_bytes anyway.
> 
> Here is a proposed alternative to use a longer period PRNG for net_random().
> The choice of TT800 was because it was freely available, had a long period,
> was fast and relatively small footprint. The existing net_random() was not
> really thread safe, but was immune to thread corruption. 
> 
> Signed-off-by: Stephen Hemminger <[email-masked]>
> 
> diff -Nru a/net/core/utils.c b/net/core/utils.c
> --- a/net/core/utils.c	2004-08-12 10:40:05 -07:00
> +++ b/net/core/utils.c	2004-08-12 10:40:05 -07:00
> @@ -2,6 +2,7 @@
>   *	Generic address resultion entity
>   *
>   *	Authors:
> + *	net_random update to TT800 Stephen Hemminger
>   *	net_random Alan Cox
>   *	net_ratelimit Andy Kleen
>   *
> @@ -23,6 +24,7 @@
>  #include <asm/system.h>
>  #include <asm/uaccess.h>
>  
> +#ifdef CONFIG_EMBEDDED
>  static unsigned long net_rand_seed = 152L;
>  
>  unsigned long net_random(void)
> @@ -34,8 +36,79 @@
>  void net_srandom(unsigned long entropy)
>  {
>  	net_rand_seed ^= entropy;
> + 	net_random();
> +}
> +#else
> +/*
> + * This is the TT800 twisted Global Finite Shift Register random number
> + * generator originally by M. Matsumoto, email: [email-masked].
> + * July 8th 1996 Version 
> + *
> + * See: ACM Transactions on Modelling and Computer Simulation, 
> + * Vol. 4, No. 3, 1994, pages 254-266. 
> + *
> + * It has a large period 2^800 and good distribution properties
> + * up to dimension 25, and passes statistical tests.
> + *
> + * Don't use for cryptographic purposes, see get_random_bytes instead.
> + */
> +#define N 25
> +#define M 7
> +
> +static unsigned long net_rand_seed[N] = {
> +	0x95f24dab, 0x0b685215, 0xe76ccae7, 0xaf3ec239, 0x715fad23,
> +	0x24a590ad, 0x69e4b5ef, 0xbf456141, 0x96bc1b7b, 0xa7bdf825,
> +	0xc1de75b7, 0x8858a9c9, 0x2da87693, 0xb657f9dd, 0xffdc8a9f,
> +	0x8121da71, 0x8b823ecb, 0x885d05f5, 0x4e20cd47, 0x5a9ad5d9,
> +	0x512c0c03, 0xea857ccd, 0x4cc1d30f, 0x8891a8a1, 0xa6b7aadb
> +};
> +
> +static spinlock_t net_random_lock = SPIN_LOCK_UNLOCKED;
> +
> +unsigned long net_random(void)
> +{
> +	unsigned long y;
> +	unsigned long flags;
> +	static int k;
> +	static const unsigned long mag01[2]={ 0x0, 0x8ebfd028 };
> +#define X net_rand_seed
> +
> +	/* generate N words at one time */
> +	spin_lock_irqsave(&net_random_lock, flags);
> +	if (k == N) {
> +		int kk;
> +		for (kk=0; kk< N - M; kk++) {
> +			X[kk] = X[kk+M] ^ (X[kk] >> 1) ^ mag01[X[kk] % 2];
> +		}
> +
> +		for (; kk < N; kk++) {
> +			X[kk] = X[kk+(M-N)] ^ (X[kk] >> 1) ^ mag01[X[kk] % 2];
> +		}
> +		k = 0;
> +	}
> +
> +	y = X[k];
> +	y ^= (y << 7) & 0x2b5b2500; /* s and b, magic vectors */
> +	y ^= (y << 15) & 0xdb8b0000; /* t and c, magic vectors */
> +
> +	/* 
> +	 * the following line was added by Makoto Matsumoto in the 1996 version
> +	 * to improve lower bit's corellation.
> +	 * Delete this line to o use the code published in 1994.
> +	 */
> +	y ^= (y >> 16); /* added to the 1994 version */
> +	k++;
> +	spin_unlock_irqrestore(&net_random_lock, flags);
> +#undef X
> +	return y;
> +}
> +
> +void net_srandom(unsigned long entropy)
> +{
> +	net_rand_seed[0] ^= entropy;
>  	net_random();
>  }
> +#endif
>  
>  int net_msg_cost = 5*HZ;
>  int net_msg_burst = 10;
> -
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