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 #include <linux/timer.h>
  42
  43 static DEFINE_PER_CPU(struct rnd_state, net_rand_state);
  44
  45 /**
  46  *      prandom_u32_state - seeded pseudo-random number generator.
  47  *      @state: pointer to state structure holding seeded state.
  48  *
  49  *      This is used for pseudo-randomness with no outside seeding.
  50  *      For more random results, use prandom_u32().
  51  */
  52 u32 prandom_u32_state(struct rnd_state *state)
  53 {
  54 #define TAUSWORTHE(s,a,b,c,d) ((s&c)<<d) ^ (((s <<a) ^ s)>>b)
  55
  56         state->s1 = TAUSWORTHE(state->s1, 13, 19, 4294967294UL, 12);
  57         state->s2 = TAUSWORTHE(state->s2, 2, 25, 4294967288UL, 4);
  58         state->s3 = TAUSWORTHE(state->s3, 3, 11, 4294967280UL, 17);
  59
  60         return (state->s1 ^ state->s2 ^ state->s3);
  61 }
  62 EXPORT_SYMBOL(prandom_u32_state);
  63
  64 /**
  65  *      prandom_u32 - pseudo random number generator
  66  *
  67  *      A 32 bit pseudo-random number is generated using a fast
  68  *      algorithm suitable for simulation. This algorithm is NOT
  69  *      considered safe for cryptographic use.
  70  */
  71 u32 prandom_u32(void)
  72 {
  73         unsigned long r;
  74         struct rnd_state *state = &get_cpu_var(net_rand_state);
  75         r = prandom_u32_state(state);
  76         put_cpu_var(state);
  77         return r;
  78 }
  79 EXPORT_SYMBOL(prandom_u32);
  80
  81 /*
  82  *      prandom_bytes_state - get the requested number of pseudo-random bytes
  83  *
  84  *      @state: pointer to state structure holding seeded state.
  85  *      @buf: where to copy the pseudo-random bytes to
  86  *      @bytes: the requested number of bytes
  87  *
  88  *      This is used for pseudo-randomness with no outside seeding.
  89  *      For more random results, use prandom_bytes().
  90  */
  91 void prandom_bytes_state(struct rnd_state *state, void *buf, int bytes)
  92 {
  93         unsigned char *p = buf;
  94         int i;
  95
  96         for (i = 0; i < round_down(bytes, sizeof(u32)); i += sizeof(u32)) {
  97                 u32 random = prandom_u32_state(state);
  98                 int j;
  99
 100                 for (j = 0; j < sizeof(u32); j++) {
 101                         p[i + j] = random;
 102                         random >>= BITS_PER_BYTE;
 103                 }
 104         }
 105         if (i < bytes) {
 106                 u32 random = prandom_u32_state(state);
 107
 108                 for (; i < bytes; i++) {
 109                         p[i] = random;
 110                         random >>= BITS_PER_BYTE;
 111                 }
 112         }
 113 }
 114 EXPORT_SYMBOL(prandom_bytes_state);
 115
 116 /**
 117  *      prandom_bytes - get the requested number of pseudo-random bytes
 118  *      @buf: where to copy the pseudo-random bytes to
 119  *      @bytes: the requested number of bytes
 120  */
 121 void prandom_bytes(void *buf, int bytes)
 122 {
 123         struct rnd_state *state = &get_cpu_var(net_rand_state);
 124
 125         prandom_bytes_state(state, buf, bytes);
 126         put_cpu_var(state);
 127 }
 128 EXPORT_SYMBOL(prandom_bytes);
 129
 130 /**
 131  *      prandom_seed - add entropy to pseudo random number generator
 132  *      @seed: seed value
 133  *
 134  *      Add some additional seeding to the prandom pool.
 135  */
 136 void prandom_seed(u32 entropy)
 137 {
 138         int i;
 139         /*
 140          * No locking on the CPUs, but then somewhat random results are, well,
 141          * expected.
 142          */
 143         for_each_possible_cpu (i) {
 144                 struct rnd_state *state = &per_cpu(net_rand_state, i);
 145                 state->s1 = __seed(state->s1 ^ entropy, 2);
 146                 prandom_u32_state(state);
 147         }
 148 }
 149 EXPORT_SYMBOL(prandom_seed);
 150
 151 /*
 152  *      Generate some initially weak seeding values to allow
 153  *      to start the prandom_u32() engine.
 154  */
 155 static int __init prandom_init(void)
 156 {
 157         int i;
 158
 159         for_each_possible_cpu(i) {
 160                 struct rnd_state *state = &per_cpu(net_rand_state,i);
 161
 162 #define LCG(x)  ((x) * 69069)   /* super-duper LCG */
 163                 state->s1 = __seed(LCG(i + jiffies), 2);
 164                 state->s2 = __seed(LCG(state->s1), 8);
 165                 state->s3 = __seed(LCG(state->s2), 16);
 166
 167                 /* "warm it up" */
 168                 prandom_u32_state(state);
 169                 prandom_u32_state(state);
 170                 prandom_u32_state(state);
 171                 prandom_u32_state(state);
 172                 prandom_u32_state(state);
 173                 prandom_u32_state(state);
 174         }
 175         return 0;
 176 }
 177 core_initcall(prandom_init);
 178
 179 static void __prandom_timer(unsigned long dontcare);
 180 static DEFINE_TIMER(seed_timer, __prandom_timer, 0, 0);
 181  static void __prandom_timer(unsigned long dontcare)
 182 {
 183         u32 entropy;
 184         get_random_bytes(&entropy, sizeof(entropy));
 185         prandom_seed(entropy);
 186         /* reseed every ~60 seconds, in [40 .. 80) interval with slack */
 187         seed_timer.expires = jiffies + (40 * HZ + (prandom_u32() % (40 * HZ)));
 188         add_timer(&seed_timer);
 189 }
 190  static void prandom_start_seed_timer(void)
 191 {
 192         set_timer_slack(&seed_timer, HZ);
 193         seed_timer.expires = jiffies + 40 * HZ;
 194         add_timer(&seed_timer);
 195 }
 196
 197 /*
 198  *      Generate better values after random number generator
 199  *      is fully initialized.
 200  */
 201 static void __prandom_reseed(bool late)
 202 {
 203         int i;
 204         unsigned long flags;
 205         static bool latch = false;
 206         static DEFINE_SPINLOCK(lock);
 207         /* only allow initial seeding (late == false) once */
 208         spin_lock_irqsave(&lock, flags);
 209         if (latch && !late)
 210                 goto out;
 211         latch = true;
 212
 213         for_each_possible_cpu(i) {
 214                 struct rnd_state *state = &per_cpu(net_rand_state,i);
 215                 u32 seeds[3];
 216
 217                 get_random_bytes(&seeds, sizeof(seeds));
 218                 state->s1 = __seed(seeds[0], 2);
 219                 state->s2 = __seed(seeds[1], 8);
 220                 state->s3 = __seed(seeds[2], 16);
 221
 222                 /* mix it in */
 223                 prandom_u32_state(state);
 224         }
 225 out:
 226         spin_unlock_irqrestore(&lock, flags);
 227 }
 228
 229 void prandom_reseed_late(void)
 230 {
 231         __prandom_reseed(true);
 232 }
 233
 234 static int __init prandom_reseed(void)
 235 {
 236         __prandom_reseed(false);
 237         prandom_start_seed_timer();
 238         return 0;
 239 }
 240 late_initcall(prandom_reseed);