linux/lib/random32.c
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   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/export.h>
  39#include <linux/jiffies.h>
  40#include <linux/random.h>
  41
  42static DEFINE_PER_CPU(struct rnd_state, net_rand_state);
  43
  44/**
  45 *      prandom32 - seeded pseudo-random number generator.
  46 *      @state: pointer to state structure holding seeded state.
  47 *
  48 *      This is used for pseudo-randomness with no outside seeding.
  49 *      For more random results, use random32().
  50 */
  51u32 prandom32(struct rnd_state *state)
  52{
  53#define TAUSWORTHE(s,a,b,c,d) ((s&c)<<d) ^ (((s <<a) ^ s)>>b)
  54
  55        state->s1 = TAUSWORTHE(state->s1, 13, 19, 4294967294UL, 12);
  56        state->s2 = TAUSWORTHE(state->s2, 2, 25, 4294967288UL, 4);
  57        state->s3 = TAUSWORTHE(state->s3, 3, 11, 4294967280UL, 17);
  58
  59        return (state->s1 ^ state->s2 ^ state->s3);
  60}
  61EXPORT_SYMBOL(prandom32);
  62
  63/**
  64 *      random32 - pseudo random number generator
  65 *
  66 *      A 32 bit pseudo-random number is generated using a fast
  67 *      algorithm suitable for simulation. This algorithm is NOT
  68 *      considered safe for cryptographic use.
  69 */
  70u32 random32(void)
  71{
  72        unsigned long r;
  73        struct rnd_state *state = &get_cpu_var(net_rand_state);
  74        r = prandom32(state);
  75        put_cpu_var(state);
  76        return r;
  77}
  78EXPORT_SYMBOL(random32);
  79
  80/**
  81 *      srandom32 - add entropy to pseudo random number generator
  82 *      @seed: seed value
  83 *
  84 *      Add some additional seeding to the random32() pool.
  85 */
  86void srandom32(u32 entropy)
  87{
  88        int i;
  89        /*
  90         * No locking on the CPUs, but then somewhat random results are, well,
  91         * expected.
  92         */
  93        for_each_possible_cpu (i) {
  94                struct rnd_state *state = &per_cpu(net_rand_state, i);
  95                state->s1 = __seed(state->s1 ^ entropy, 1);
  96        }
  97}
  98EXPORT_SYMBOL(srandom32);
  99
 100/*
 101 *      Generate some initially weak seeding values to allow
 102 *      to start the random32() engine.
 103 */
 104static int __init random32_init(void)
 105{
 106        int i;
 107
 108        for_each_possible_cpu(i) {
 109                struct rnd_state *state = &per_cpu(net_rand_state,i);
 110
 111#define LCG(x)  ((x) * 69069)   /* super-duper LCG */
 112                state->s1 = __seed(LCG(i + jiffies), 1);
 113                state->s2 = __seed(LCG(state->s1), 7);
 114                state->s3 = __seed(LCG(state->s2), 15);
 115
 116                /* "warm it up" */
 117                prandom32(state);
 118                prandom32(state);
 119                prandom32(state);
 120                prandom32(state);
 121                prandom32(state);
 122                prandom32(state);
 123        }
 124        return 0;
 125}
 126core_initcall(random32_init);
 127
 128/*
 129 *      Generate better values after random number generator
 130 *      is fully initialized.
 131 */
 132static int __init random32_reseed(void)
 133{
 134        int i;
 135
 136        for_each_possible_cpu(i) {
 137                struct rnd_state *state = &per_cpu(net_rand_state,i);
 138                u32 seeds[3];
 139
 140                get_random_bytes(&seeds, sizeof(seeds));
 141                state->s1 = __seed(seeds[0], 1);
 142                state->s2 = __seed(seeds[1], 7);
 143                state->s3 = __seed(seeds[2], 15);
 144
 145                /* mix it in */
 146                prandom32(state);
 147        }
 148        return 0;
 149}
 150late_initcall(random32_reseed);
 151
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