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/export.h>
  39 #include <linux/jiffies.h>
  40 #include <linux/random.h>
  41
  42 static 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  */
  51 u32 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 }
  61 EXPORT_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  */
  70 u32 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 }
  78 EXPORT_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  */
  86 void 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 }
  98 EXPORT_SYMBOL(srandom32);
  99
 100 /*
 101  *      Generate some initially weak seeding values to allow
 102  *      to start the random32() engine.
 103  */
 104 static 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 }
 126 core_initcall(random32_init);
 127
 128 /*
 129  *      Generate better values after random number generator
 130  *      is fully initialized.
 131  */
 132 static 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 }
 150 late_initcall(random32_reseed);