LXR linux/lib/random32.c

   1/*
   2  This is a maximally equidistributed combined Tausworthe generator
   3  based on code from GNU Scientific Library 1.5 (30 Jun 2004)
   4
   5   x_n = (s1_n ^ s2_n ^ s3_n)
   6
   7   s1_{n+1} = (((s1_n & 4294967294) <<12) ^ (((s1_n <<13) ^ s1_n) >>19))
   8   s2_{n+1} = (((s2_n & 4294967288) << 4) ^ (((s2_n << 2) ^ s2_n) >>25))
   9   s3_{n+1} = (((s3_n & 4294967280) <<17) ^ (((s3_n << 3) ^ s3_n) >>11))
  10
  11   The period of this generator is about 2^88.
  12
  13   From: P. L'Ecuyer, "Maximally Equidistributed Combined Tausworthe
  14   Generators", Mathematics of Computation, 65, 213 (1996), 203--213.
  15
  16   This is available on the net from L'Ecuyer's home page,
  17
  18   http://www.iro.umontreal.ca/~lecuyer/myftp/papers/tausme.ps
  19   ftp://ftp.iro.umontreal.ca/pub/simulation/lecuyer/papers/tausme.ps
  20
  21   There is an erratum in the paper "Tables of Maximally
  22   Equidistributed Combined LFSR Generators", Mathematics of
  23   Computation, 68, 225 (1999), 261--269:
  24   http://www.iro.umontreal.ca/~lecuyer/myftp/papers/tausme2.ps
  25
  26        ... the k_j most significant bits of z_j must be non-
  27        zero, for each j. (Note: this restriction also applies to the
  28        computer code given in [4], but was mistakenly not mentioned in
  29        that paper.)
  30
  31   This affects the seeding procedure by imposing the requirement
  32   s1 > 1, s2 > 7, s3 > 15.
  33
  34*/
  35
  36#include <linux/types.h>
  37#include <linux/percpu.h>
  38#include <linux/module.h>
  39#include <linux/jiffies.h>
  40#include <linux/random.h>
  41
  42struct rnd_state {
  43        u32 s1, s2, s3;
  44};
  45
  46static DEFINE_PER_CPU(struct rnd_state, net_rand_state);
  47
  48static u32 __random32(struct rnd_state *state)
  49{
  50#define TAUSWORTHE(s,a,b,c,d) ((s&c)<<d) ^ (((s <<a) ^ s)>>b)
  51
  52        state->s1 = TAUSWORTHE(state->s1, 13, 19, 4294967294UL, 12);
  53        state->s2 = TAUSWORTHE(state->s2, 2, 25, 4294967288UL, 4);
  54        state->s3 = TAUSWORTHE(state->s3, 3, 11, 4294967280UL, 17);
  55
  56        return (state->s1 ^ state->s2 ^ state->s3);
  57}
  58
  59/*
  60 * Handle minimum values for seeds
  61 */
  62static inline u32 __seed(u32 x, u32 m)
  63{
  64        return (x < m) ? x + m : x;
  65}
  66
  67/**
  68 *      random32 - pseudo random number generator
  69 *
  70 *      A 32 bit pseudo-random number is generated using a fast
  71 *      algorithm suitable for simulation. This algorithm is NOT
  72 *      considered safe for cryptographic use.
  73 */
  74u32 random32(void)
  75{
  76        unsigned long r;
  77        struct rnd_state *state = &get_cpu_var(net_rand_state);
  78        r = __random32(state);
  79        put_cpu_var(state);
  80        return r;
  81}
  82EXPORT_SYMBOL(random32);
  83
  84/**
  85 *      srandom32 - add entropy to pseudo random number generator
  86 *      @seed: seed value
  87 *
  88 *      Add some additional seeding to the random32() pool.
  89 */
  90void srandom32(u32 entropy)
  91{
  92        int i;
  93        /*
  94         * No locking on the CPUs, but then somewhat random results are, well,
  95         * expected.
  96         */
  97        for_each_possible_cpu (i) {
  98                struct rnd_state *state = &per_cpu(net_rand_state, i);
  99                state->s1 = __seed(state->s1 ^ entropy, 1);
 100        }
 101}
 102EXPORT_SYMBOL(srandom32);
 103
 104/*
 105 *      Generate some initially weak seeding values to allow
 106 *      to start the random32() engine.
 107 */
 108static int __init random32_init(void)
 109{
 110        int i;
 111
 112        for_each_possible_cpu(i) {
 113                struct rnd_state *state = &per_cpu(net_rand_state,i);
 114
 115#define LCG(x)  ((x) * 69069)   /* super-duper LCG */
 116                state->s1 = __seed(LCG(i + jiffies), 1);
 117                state->s2 = __seed(LCG(state->s1), 7);
 118                state->s3 = __seed(LCG(state->s2), 15);
 119
 120                /* "warm it up" */
 121                __random32(state);
 122                __random32(state);
 123                __random32(state);
 124                __random32(state);
 125                __random32(state);
 126                __random32(state);
 127        }
 128        return 0;
 129}
 130core_initcall(random32_init);
 131
 132/*
 133 *      Generate better values after random number generator
 134 *      is fully initalized.
 135 */
 136static int __init random32_reseed(void)
 137{
 138        int i;
 139
 140        for_each_possible_cpu(i) {
 141                struct rnd_state *state = &per_cpu(net_rand_state,i);
 142                u32 seeds[3];
 143
 144                get_random_bytes(&seeds, sizeof(seeds));
 145                state->s1 = __seed(seeds[0], 1);
 146                state->s2 = __seed(seeds[1], 7);
 147                state->s3 = __seed(seeds[2], 15);
 148
 149                /* mix it in */
 150                __random32(state);
 151        }
 152        return 0;
 153}
 154late_initcall(random32_reseed);
 155