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SubjectRe: [RFC] enhanced version of net_random()
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 <shemminger@osdl.org>
>
> 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: matumoto@math.keio.ac.jp.
> + * 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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