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  *      prandom_u32_state - 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 prandom_u32().
  50  */
  51 u32 prandom_u32_state(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(prandom_u32_state);
  62
  63 /**
  64  *      prandom_u32 - 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 prandom_u32(void)
  71 {
  72         unsigned long r;
  73         struct rnd_state *state = &get_cpu_var(net_rand_state);
  74         r = prandom_u32_state(state);
  75         put_cpu_var(state);
  76         return r;
  77 }
  78 EXPORT_SYMBOL(prandom_u32);
  79
  80 /*
  81  *      prandom_bytes_state - get the requested number of pseudo-random bytes
  82  *
  83  *      @state: pointer to state structure holding seeded state.
  84  *      @buf: where to copy the pseudo-random bytes to
  85  *      @bytes: the requested number of bytes
  86  *
  87  *      This is used for pseudo-randomness with no outside seeding.
  88  *      For more random results, use prandom_bytes().
  89  */
  90 void prandom_bytes_state(struct rnd_state *state, void *buf, int bytes)
  91 {
  92         unsigned char *p = buf;
  93         int i;
  94
  95         for (i = 0; i < round_down(bytes, sizeof(u32)); i += sizeof(u32)) {
  96                 u32 random = prandom_u32_state(state);
  97                 int j;
  98
  99                 for (j = 0; j < sizeof(u32); j++) {
 100                         p[i + j] = random;
 101                         random >>= BITS_PER_BYTE;
 102                 }
 103         }
 104         if (i < bytes) {
 105                 u32 random = prandom_u32_state(state);
 106
 107                 for (; i < bytes; i++) {
 108                         p[i] = random;
 109                         random >>= BITS_PER_BYTE;
 110                 }
 111         }
 112 }
 113 EXPORT_SYMBOL(prandom_bytes_state);
 114
 115 /**
 116  *      prandom_bytes - get the requested number of pseudo-random bytes
 117  *      @buf: where to copy the pseudo-random bytes to
 118  *      @bytes: the requested number of bytes
 119  */
 120 void prandom_bytes(void *buf, int bytes)
 121 {
 122         struct rnd_state *state = &get_cpu_var(net_rand_state);
 123
 124         prandom_bytes_state(state, buf, bytes);
 125         put_cpu_var(state);
 126 }
 127 EXPORT_SYMBOL(prandom_bytes);
 128
 129 /**
 130  *      prandom_seed - add entropy to pseudo random number generator
 131  *      @seed: seed value
 132  *
 133  *      Add some additional seeding to the prandom pool.
 134  */
 135 void prandom_seed(u32 entropy)
 136 {
 137         int i;
 138         /*
 139          * No locking on the CPUs, but then somewhat random results are, well,
 140          * expected.
 141          */
 142         for_each_possible_cpu (i) {
 143                 struct rnd_state *state = &per_cpu(net_rand_state, i);
 144                 state->s1 = __seed(state->s1 ^ entropy, 2);
 145                 prandom_u32_state(state);
 146         }
 147 }
 148 EXPORT_SYMBOL(prandom_seed);
 149
 150 /*
 151  *      Generate some initially weak seeding values to allow
 152  *      to start the prandom_u32() engine.
 153  */
 154 static int __init prandom_init(void)
 155 {
 156         int i;
 157
 158         for_each_possible_cpu(i) {
 159                 struct rnd_state *state = &per_cpu(net_rand_state,i);
 160
 161 #define LCG(x)  ((x) * 69069)   /* super-duper LCG */
 162                 state->s1 = __seed(LCG(i + jiffies), 2);
 163                 state->s2 = __seed(LCG(state->s1), 8);
 164                 state->s3 = __seed(LCG(state->s2), 16);
 165
 166                 /* "warm it up" */
 167                 prandom_u32_state(state);
 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         }
 174         return 0;
 175 }
 176 core_initcall(prandom_init);
 177
 178 static void __prandom_timer(unsigned long dontcare);
 179 static DEFINE_TIMER(seed_timer, __prandom_timer, 0, 0);
 180
 181 static void __prandom_timer(unsigned long dontcare)
 182 {
 183         u32 entropy;
 184
 185         get_random_bytes(&entropy, sizeof(entropy));
 186         prandom_seed(entropy);
 187         /* reseed every ~60 seconds, in [40 .. 80) interval with slack */
 188         seed_timer.expires = jiffies + (40 * HZ + (prandom_u32() % (40 * HZ)));
 189         add_timer(&seed_timer);
 190 }
 191
 192 static void prandom_start_seed_timer(void)
 193 {
 194         set_timer_slack(&seed_timer, HZ);
 195         seed_timer.expires = jiffies + 40 * HZ;
 196         add_timer(&seed_timer);
 197 }
 198
 199 /*
 200  *      Generate better values after random number generator
 201  *      is fully initialized.
 202  */
 203 static void __prandom_reseed(bool late)
 204 {
 205         int i;
 206         unsigned long flags;
 207         static bool latch = false;
 208         static DEFINE_SPINLOCK(lock);
 209
 210         /* only allow initial seeding (late == false) once */
 211         spin_lock_irqsave(&lock, flags);
 212         if (latch && !late)
 213                 goto out;
 214         latch = true;
 215
 216         for_each_possible_cpu(i) {
 217                 struct rnd_state *state = &per_cpu(net_rand_state,i);
 218                 u32 seeds[3];
 219
 220                 get_random_bytes(&seeds, sizeof(seeds));
 221                 state->s1 = __seed(seeds[0], 2);
 222                 state->s2 = __seed(seeds[1], 8);
 223                 state->s3 = __seed(seeds[2], 16);
 224
 225                 /* mix it in */
 226                 prandom_u32_state(state);
 227         }
 228 out:
 229         spin_unlock_irqrestore(&lock, flags);
 230 }
 231
 232 void prandom_reseed_late(void)
 233 {
 234         __prandom_reseed(true);
 235 }
 236
 237 static int __init prandom_reseed(void)
 238 {
 239         __prandom_reseed(false);
 240         prandom_start_seed_timer();
 241         return 0;
 242 }
 243 late_initcall(prandom_reseed);