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
  42 struct rnd_state {
  43         u32 s1, s2, s3;
  44 };
  45
  46 static DEFINE_PER_CPU(struct rnd_state, net_rand_state);
  47
  48 static 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 static void __set_random32(struct rnd_state *state, unsigned long s)
  60 {
  61         if (s == 0)
  62                 s = 1;      /* default seed is 1 */
  63
  64 #define LCG(n) (69069 * n)
  65         state->s1 = LCG(s);
  66         state->s2 = LCG(state->s1);
  67         state->s3 = LCG(state->s2);
  68
  69         /* "warm it up" */
  70         __random32(state);
  71         __random32(state);
  72         __random32(state);
  73         __random32(state);
  74         __random32(state);
  75         __random32(state);
  76 }
  77
  78 /**
  79  *      random32 - pseudo random number generator
  80  *
  81  *      A 32 bit pseudo-random number is generated using a fast
  82  *      algorithm suitable for simulation. This algorithm is NOT
  83  *      considered safe for cryptographic use.
  84  */
  85 u32 random32(void)
  86 {
  87         unsigned long r;
  88         struct rnd_state *state = &get_cpu_var(net_rand_state);
  89         r = __random32(state);
  90         put_cpu_var(state);
  91         return r;
  92 }
  93 EXPORT_SYMBOL(random32);
  94
  95 /**
  96  *      srandom32 - add entropy to pseudo random number generator
  97  *      @seed: seed value
  98  *
  99  *      Add some additional seeding to the random32() pool.
 100  */
 101 void srandom32(u32 entropy)
 102 {
 103         int i;
 104         /*
 105          * No locking on the CPUs, but then somewhat random results are, well,
 106          * expected.
 107          */
 108         for_each_possible_cpu (i) {
 109                 struct rnd_state *state = &per_cpu(net_rand_state, i);
 110                 __set_random32(state, state->s1 ^ entropy);
 111         }
 112 }
 113 EXPORT_SYMBOL(srandom32);
 114
 115 /*
 116  *      Generate some initially weak seeding values to allow
 117  *      to start the random32() engine.
 118  */
 119 static int __init random32_init(void)
 120 {
 121         int i;
 122
 123         for_each_possible_cpu(i) {
 124                 struct rnd_state *state = &per_cpu(net_rand_state,i);
 125                 __set_random32(state, i + jiffies);
 126         }
 127         return 0;
 128 }
 129 core_initcall(random32_init);
 130
 131 /*
 132  *      Generate better values after random number generator
 133  *      is fully initalized.
 134  */
 135 static int __init random32_reseed(void)
 136 {
 137         int i;
 138         unsigned long seed;
 139
 140         for_each_possible_cpu(i) {
 141                 struct rnd_state *state = &per_cpu(net_rand_state,i);
 142
 143                 get_random_bytes(&seed, sizeof(seed));
 144                 __set_random32(state, seed);
 145         }
 146         return 0;
 147 }
 148 late_initcall(random32_reseed);