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 *      prandom_u32_state - 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 prandom_u32().
  50 */
  51u32 prandom_u32_state(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(prandom_u32_state);
  62
  63/**
  64 *      prandom_u32 - 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 prandom_u32(void)
  71{
  72        unsigned long r;
  73        struct rnd_state *state = &get_cpu_var(net_rand_state);
  74        r = prandom_u32_state(state);
  75        put_cpu_var(state);
  76        return r;
  77}
  78EXPORT_SYMBOL(prandom_u32);
  79
  80/*
  81 *      prandom_bytes_state - get the requested number of pseudo-random bytes
  82 *
  83 *      @state: pointer to state structure holding seeded state.
  84 *      @buf: where to copy the pseudo-random bytes to
  85 *      @bytes: the requested number of bytes
  86 *
  87 *      This is used for pseudo-randomness with no outside seeding.
  88 *      For more random results, use prandom_bytes().
  89 */
  90void prandom_bytes_state(struct rnd_state *state, void *buf, int bytes)
  91{
  92        unsigned char *p = buf;
  93        int i;
  94
  95        for (i = 0; i < round_down(bytes, sizeof(u32)); i += sizeof(u32)) {
  96                u32 random = prandom_u32_state(state);
  97                int j;
  98
  99                for (j = 0; j < sizeof(u32); j++) {
 100                        p[i + j] = random;
 101                        random >>= BITS_PER_BYTE;
 102                }
 103        }
 104        if (i < bytes) {
 105                u32 random = prandom_u32_state(state);
 106
 107                for (; i < bytes; i++) {
 108                        p[i] = random;
 109                        random >>= BITS_PER_BYTE;
 110                }
 111        }
 112}
 113EXPORT_SYMBOL(prandom_bytes_state);
 114
 115/**
 116 *      prandom_bytes - get the requested number of pseudo-random bytes
 117 *      @buf: where to copy the pseudo-random bytes to
 118 *      @bytes: the requested number of bytes
 119 */
 120void prandom_bytes(void *buf, int bytes)
 121{
 122        struct rnd_state *state = &get_cpu_var(net_rand_state);
 123
 124        prandom_bytes_state(state, buf, bytes);
 125        put_cpu_var(state);
 126}
 127EXPORT_SYMBOL(prandom_bytes);
 128
 129/**
 130 *      prandom_seed - add entropy to pseudo random number generator
 131 *      @seed: seed value
 132 *
 133 *      Add some additional seeding to the prandom pool.
 134 */
 135void prandom_seed(u32 entropy)
 136{
 137        int i;
 138        /*
 139         * No locking on the CPUs, but then somewhat random results are, well,
 140         * expected.
 141         */
 142        for_each_possible_cpu (i) {
 143                struct rnd_state *state = &per_cpu(net_rand_state, i);
 144                state->s1 = __seed(state->s1 ^ entropy, 1);
 145        }
 146}
 147EXPORT_SYMBOL(prandom_seed);
 148
 149/*
 150 *      Generate some initially weak seeding values to allow
 151 *      to start the prandom_u32() engine.
 152 */
 153static int __init prandom_init(void)
 154{
 155        int i;
 156
 157        for_each_possible_cpu(i) {
 158                struct rnd_state *state = &per_cpu(net_rand_state,i);
 159
 160#define LCG(x)  ((x) * 69069)   /* super-duper LCG */
 161                state->s1 = __seed(LCG(i + jiffies), 1);
 162                state->s2 = __seed(LCG(state->s1), 7);
 163                state->s3 = __seed(LCG(state->s2), 15);
 164
 165                /* "warm it up" */
 166                prandom_u32_state(state);
 167                prandom_u32_state(state);
 168                prandom_u32_state(state);
 169                prandom_u32_state(state);
 170                prandom_u32_state(state);
 171                prandom_u32_state(state);
 172        }
 173        return 0;
 174}
 175core_initcall(prandom_init);
 176
 177/*
 178 *      Generate better values after random number generator
 179 *      is fully initialized.
 180 */
 181static int __init prandom_reseed(void)
 182{
 183        int i;
 184
 185        for_each_possible_cpu(i) {
 186                struct rnd_state *state = &per_cpu(net_rand_state,i);
 187                u32 seeds[3];
 188
 189                get_random_bytes(&seeds, sizeof(seeds));
 190                state->s1 = __seed(seeds[0], 1);
 191                state->s2 = __seed(seeds[1], 7);
 192                state->s3 = __seed(seeds[2], 15);
 193
 194                /* mix it in */
 195                prandom_u32_state(state);
 196        }
 197        return 0;
 198}
 199late_initcall(prandom_reseed);
 200
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