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author | Conrad Meyer <cem@FreeBSD.org> | 2019-04-19 20:05:47 +0000 |
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committer | Conrad Meyer <cem@FreeBSD.org> | 2019-04-19 20:05:47 +0000 |
commit | 7deb4b1964834c479406cab511de339a6cc0088e (patch) | |
tree | 36e0f3d5153a3824cc22071cb7a14c67ae8db8c9 /sys/libkern | |
parent | 16b07b25c4782aff9cbebed107ced80344cee187 (diff) | |
download | src-7deb4b1964834c479406cab511de339a6cc0088e.tar.gz src-7deb4b1964834c479406cab511de339a6cc0088e.zip |
libkern: Bring in arc4random_uniform(9) from libc
It is a useful arc4random wrapper in the kernel for much the same reasons as
in userspace. Move the source to libkern (because kernel build is
restricted to sys/, but userspace can include any file it likes) and build
kernel and libc versions from the same source file.
Copy the documentation from arc4random_uniform(3) to the section 9 page.
While here, add missing arc4random_buf(9) symlink.
Sponsored by: Dell EMC Isilon
Notes
Notes:
svn path=/head/; revision=346410
Diffstat (limited to 'sys/libkern')
-rw-r--r-- | sys/libkern/arc4random_uniform.c | 62 |
1 files changed, 62 insertions, 0 deletions
diff --git a/sys/libkern/arc4random_uniform.c b/sys/libkern/arc4random_uniform.c new file mode 100644 index 000000000000..26f91e6532f9 --- /dev/null +++ b/sys/libkern/arc4random_uniform.c @@ -0,0 +1,62 @@ +/* $OpenBSD: arc4random_uniform.c,v 1.2 2015/09/13 08:31:47 guenther 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> +#ifdef _KERNEL +#include <sys/libkern.h> +#else +#include <stdlib.h> +#endif + +/* + * 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; +} |