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Index: sys/libkern/arc4random.c
===================================================================
--- sys/libkern/arc4random.c
+++ sys/libkern/arc4random.c
@@ -217,3 +217,39 @@
arc4rand(ptr, len, 0);
}
+
+/*
+ * Calculate a uniformly distributed random number less than upper_bound
+ * avoiding "modulo bias".
+ *
+ * Uniformity is achieved by generating new random numbers until the one
+ * returned is outside the range [0, 2**32 % upper_bound). This
+ * guarantees the selected random number will be inside
+ * [2**32 % upper_bound, 2**32) which maps back to [0, upper_bound)
+ * after reduction modulo upper_bound.
+ */
+uint32_t
+arc4random_uniform(uint32_t upper_bound)
+{
+ uint32_t r, min;
+
+ if (upper_bound < 2)
+ return 0;
+
+ /* 2**32 % x == (2**32 - x) % x */
+ min = -upper_bound % upper_bound;
+
+ /*
+ * This could theoretically loop forever but each retry has
+ * p > 0.5 (worst case, usually far better) of selecting a
+ * number inside the range we need, so it should rarely need
+ * to re-roll.
+ */
+ for (;;) {
+ r = arc4random();
+ if (r >= min)
+ break;
+ }
+
+ return r % upper_bound;
+}
Index: sys/sys/libkern.h
===================================================================
--- sys/sys/libkern.h
+++ sys/sys/libkern.h
@@ -127,6 +127,7 @@
struct malloc_type;
uint32_t arc4random(void);
void arc4random_buf(void *, size_t);
+uint32_t arc4random_uniform(uint32_t upper_bound);
void arc4rand(void *, u_int, int);
int timingsafe_bcmp(const void *, const void *, size_t);
void *bsearch(const void *, const void *, size_t,

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