random32: seeding improvement
Stephen Hemminger [Wed, 30 Jul 2008 23:29:19 +0000 (16:29 -0700)]
The rationale is:
   * use u32 consistently
   * no need to do LCG on values from (better) get_random_bytes
   * use more data from get_random_bytes for secondary seeding
   * don't reduce state space on srandom32()
   * enforce state variable initialization restrictions

Note: the second paper has a version of random32() with even longer period
and a version of random64() if needed.

Signed-off-by: Stephen Hemminger <shemminger@vyatta.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

lib/random32.c

index ca87d86..217d5c4 100644 (file)
@@ -56,23 +56,12 @@ static u32 __random32(struct rnd_state *state)
        return (state->s1 ^ state->s2 ^ state->s3);
 }
 
-static void __set_random32(struct rnd_state *state, unsigned long s)
+/*
+ * Handle minimum values for seeds
+ */
+static inline u32 __seed(u32 x, u32 m)
 {
-       if (s == 0)
-               s = 1;      /* default seed is 1 */
-
-#define LCG(n) (69069 * n)
-       state->s1 = LCG(s);
-       state->s2 = LCG(state->s1);
-       state->s3 = LCG(state->s2);
-
-       /* "warm it up" */
-       __random32(state);
-       __random32(state);
-       __random32(state);
-       __random32(state);
-       __random32(state);
-       __random32(state);
+       return (x < m) ? x + m : x;
 }
 
 /**
@@ -107,7 +96,7 @@ void srandom32(u32 entropy)
         */
        for_each_possible_cpu (i) {
                struct rnd_state *state = &per_cpu(net_rand_state, i);
-               __set_random32(state, state->s1 ^ entropy);
+               state->s1 = __seed(state->s1 ^ entropy, 1);
        }
 }
 EXPORT_SYMBOL(srandom32);
@@ -122,7 +111,19 @@ static int __init random32_init(void)
 
        for_each_possible_cpu(i) {
                struct rnd_state *state = &per_cpu(net_rand_state,i);
-               __set_random32(state, i + jiffies);
+
+#define LCG(x) ((x) * 69069)   /* super-duper LCG */
+               state->s1 = __seed(LCG(i + jiffies), 1);
+               state->s2 = __seed(LCG(state->s1), 7);
+               state->s3 = __seed(LCG(state->s2), 15);
+
+               /* "warm it up" */
+               __random32(state);
+               __random32(state);
+               __random32(state);
+               __random32(state);
+               __random32(state);
+               __random32(state);
        }
        return 0;
 }
@@ -135,13 +136,18 @@ core_initcall(random32_init);
 static int __init random32_reseed(void)
 {
        int i;
-       unsigned long seed;
 
        for_each_possible_cpu(i) {
                struct rnd_state *state = &per_cpu(net_rand_state,i);
+               u32 seeds[3];
+
+               get_random_bytes(&seeds, sizeof(seeds));
+               state->s1 = __seed(seeds[0], 1);
+               state->s2 = __seed(seeds[1], 7);
+               state->s3 = __seed(seeds[2], 15);
 
-               get_random_bytes(&seed, sizeof(seed));
-               __set_random32(state, seed);
+               /* mix it in */
+               __random32(state);
        }
        return 0;
 }