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 /*
  60  * Handle minimum values for seeds
  61  */
  62 static inline u32 __seed(u32 x, u32 m)
  63 {
  64         return (x < m) ? x + m : x;
  65 }
  66
  67 /**
  68  *      random32 - pseudo random number generator
  69  *
  70  *      A 32 bit pseudo-random number is generated using a fast
  71  *      algorithm suitable for simulation. This algorithm is NOT
  72  *      considered safe for cryptographic use.
  73  */
  74 u32 random32(void)
  75 {
  76         unsigned long r;
  77         struct rnd_state *state = &get_cpu_var(net_rand_state);
  78         r = __random32(state);
  79         put_cpu_var(state);
  80         return r;
  81 }
  82 EXPORT_SYMBOL(random32);
  83
  84 /**
  85  *      srandom32 - add entropy to pseudo random number generator
  86  *      @seed: seed value
  87  *
  88  *      Add some additional seeding to the random32() pool.
  89  */
  90 void srandom32(u32 entropy)
  91 {
  92         int i;
  93         /*
  94          * No locking on the CPUs, but then somewhat random results are, well,
  95          * expected.
  96          */
  97         for_each_possible_cpu (i) {
  98                 struct rnd_state *state = &per_cpu(net_rand_state, i);
  99                 state->s1 = __seed(state->s1 ^ entropy, 1);
 100         }
 101 }
 102 EXPORT_SYMBOL(srandom32);
 103
 104 /*
 105  *      Generate some initially weak seeding values to allow
 106  *      to start the random32() engine.
 107  */
 108 static int __init random32_init(void)
 109 {
 110         int i;
 111
 112         for_each_possible_cpu(i) {
 113                 struct rnd_state *state = &per_cpu(net_rand_state,i);
 114
 115 #define LCG(x)  ((x) * 69069)   /* super-duper LCG */
 116                 state->s1 = __seed(LCG(i + jiffies), 1);
 117                 state->s2 = __seed(LCG(state->s1), 7);
 118                 state->s3 = __seed(LCG(state->s2), 15);
 119
 120                 /* "warm it up" */
 121                 __random32(state);
 122                 __random32(state);
 123                 __random32(state);
 124                 __random32(state);
 125                 __random32(state);
 126                 __random32(state);
 127         }
 128         return 0;
 129 }
 130 core_initcall(random32_init);
 131
 132 /*
 133  *      Generate better values after random number generator
 134  *      is fully initalized.
 135  */
 136 static int __init random32_reseed(void)
 137 {
 138         int i;
 139
 140         for_each_possible_cpu(i) {
 141                 struct rnd_state *state = &per_cpu(net_rand_state,i);
 142                 u32 seeds[3];
 143
 144                 get_random_bytes(&seeds, sizeof(seeds));
 145                 state->s1 = __seed(seeds[0], 1);
 146                 state->s2 = __seed(seeds[1], 7);
 147                 state->s3 = __seed(seeds[2], 15);
 148
 149                 /* mix it in */
 150                 __random32(state);
 151         }
 152         return 0;
 153 }
 154 late_initcall(random32_reseed);