Date: Sat, 23 May 2020 17:51:06 +0000 (UTC) From: Emmanuel Vadot <manu@FreeBSD.org> To: src-committers@freebsd.org, svn-src-all@freebsd.org, svn-src-head@freebsd.org Subject: svn commit: r361418 - in head/sys: conf libkern sys Message-ID: <202005231751.04NHp6xY043056@repo.freebsd.org>
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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@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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