svn commit: r361418 - in head/sys: conf libkern sys
Emmanuel Vadot
manu at FreeBSD.org
Sat May 23 17:51:07 UTC 2020
Author: manu
Date: Sat May 23 17:51:06 2020
New Revision: 361418
URL: https://svnweb.freebsd.org/changeset/base/361418
Log:
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
Added:
head/sys/libkern/arc4random_uniform.c (contents, props changed)
Modified:
head/sys/conf/files
head/sys/sys/libkern.h
Modified: head/sys/conf/files
==============================================================================
--- head/sys/conf/files Sat May 23 16:42:27 2020 (r361417)
+++ head/sys/conf/files Sat May 23 17:51:06 2020 (r361418)
@@ -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
Added: head/sys/libkern/arc4random_uniform.c
==============================================================================
--- /dev/null 00:00:00 1970 (empty, because file is newly added)
+++ head/sys/libkern/arc4random_uniform.c Sat May 23 17:51:06 2020 (r361418)
@@ -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 at 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;
+}
Modified: head/sys/sys/libkern.h
==============================================================================
--- head/sys/sys/libkern.h Sat May 23 16:42:27 2020 (r361417)
+++ head/sys/sys/libkern.h Sat May 23 17:51:06 2020 (r361418)
@@ -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,
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