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/random.h>
  40
  41 struct rnd_state {
  42         u32 s1, s2, s3;
  43 };
  44
  45 static DEFINE_PER_CPU(struct rnd_state, net_rand_state);
  46
  47 static u32 __random32(struct rnd_state *state)
  48 {
  49 #define TAUSWORTHE(s,a,b,c,d) ((s&c)<<d) ^ (((s <<a) ^ s)>>b)
  50
  51         state->s1 = TAUSWORTHE(state->s1, 13, 19, 4294967294UL, 12);
  52         state->s2 = TAUSWORTHE(state->s2, 2, 25, 4294967288UL, 4);
  53         state->s3 = TAUSWORTHE(state->s3, 3, 11, 4294967280UL, 17);
  54
  55         return (state->s1 ^ state->s2 ^ state->s3);
  56 }
  57
  58 static void __set_random32(struct rnd_state *state, unsigned long s)
  59 {
  60         if (s == 0)
  61                 s = 1;      /* default seed is 1 */
  62
  63 #define LCG(n) (69069 * n)
  64         state->s1 = LCG(s);
  65         state->s2 = LCG(state->s1);
  66         state->s3 = LCG(state->s2);
  67
  68         /* "warm it up" */
  69         __random32(state);
  70         __random32(state);
  71         __random32(state);
  72         __random32(state);
  73         __random32(state);
  74         __random32(state);
  75 }
  76
  77 /**
  78  *      random32 - pseudo random number generator
  79  *
  80  *      A 32 bit pseudo-random number is generated using a fast
  81  *      algorithm suitable for simulation. This algorithm is NOT
  82  *      considered safe for cryptographic use.
  83  */
  84 u32 random32(void)
  85 {
  86         unsigned long r;
  87         struct rnd_state *state = &get_cpu_var(net_rand_state);
  88         r = __random32(state);
  89         put_cpu_var(state);
  90         return r;
  91 }
  92 EXPORT_SYMBOL(random32);
  93
  94 /**
  95  *      srandom32 - add entropy to pseudo random number generator
  96  *      @seed: seed value
  97  *
  98  *      Add some additional seeding to the random32() pool.
  99  *      Note: this pool is per cpu so it only affects current CPU.
 100  */
 101 void srandom32(u32 entropy)
 102 {
 103         struct rnd_state *state = &get_cpu_var(net_rand_state);
 104         __set_random32(state, state->s1 ^ entropy);
 105         put_cpu_var(state);
 106 }
 107 EXPORT_SYMBOL(srandom32);
 108
 109 /*
 110  *      Generate some initially weak seeding values to allow
 111  *      to start the random32() engine.
 112  */
 113 static int __init random32_init(void)
 114 {
 115         int i;
 116
 117         for_each_possible_cpu(i) {
 118                 struct rnd_state *state = &per_cpu(net_rand_state,i);
 119                 __set_random32(state, i + jiffies);
 120         }
 121         return 0;
 122 }
 123 core_initcall(random32_init);
 124
 125 /*
 126  *      Generate better values after random number generator
 127  *      is fully initalized.
 128  */
 129 static int __init random32_reseed(void)
 130 {
 131         int i;
 132         unsigned long seed;
 133
 134         for_each_possible_cpu(i) {
 135                 struct rnd_state *state = &per_cpu(net_rand_state,i);
 136
 137                 get_random_bytes(&seed, sizeof(seed));
 138                 __set_random32(state, seed);
 139         }
 140         return 0;
 141 }
 142 late_initcall(random32_reseed);