libkern: Add arc4random_uniform

This variant get a random number up to the limit passed as the argument.
This is simply a copy of the libc version.

Sponsored-by: The FreeBSD Foundation
Reviewed by:	cem, hselasky (previous version)
Differential Revision:	https://reviews.freebsd.org/D24962

sys/conf/files

@@ -3952,6 +3952,7 @@ kgssapi/gsstest.c		optional kgssapi_debug
 # the file should be moved to conf/files.<arch> from here.
 #
 libkern/arc4random.c		standard
+libkern/arc4random_uniform.c	standard
 libkern/asprintf.c		standard
 libkern/bcd.c			standard
 libkern/bsearch.c		standard

sys/libkern/arc4random_uniform.c

@@ -0,0 +1,58 @@
+/*	$OpenBSD: arc4random_uniform.c,v 1.3 2019/01/20 02:59:07 bcook Exp $	*/
+
+/*
+ * Copyright (c) 2008, Damien Miller <djm@openbsd.org>
+ *
+ * Permission to use, copy, modify, and distribute this software for any
+ * purpose with or without fee is hereby granted, provided that the above
+ * copyright notice and this permission notice appear in all copies.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES
+ * WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF
+ * MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR
+ * ANY SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
+ * WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN
+ * ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF
+ * OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
+ *
+ * $FreeBSD$
+ */
+
+#include <sys/types.h>
+#include <sys/libkern.h>
+
+/*
+ * 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;
+}

sys/sys/libkern.h

@@ -127,6 +127,7 @@ extern int arc4rand_iniseed_state;
 struct malloc_type;
 uint32_t arc4random(void);
 void	 arc4random_buf(void *, size_t);
+uint32_t arc4random_uniform(uint32_t);
 void	 arc4rand(void *, u_int, int);
 int	 timingsafe_bcmp(const void *, const void *, size_t);
 void	*bsearch(const void *, const void *, size_t,