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// SPDX-FileCopyrightText: 2022-2024, Alejandro Colomar <alx@kernel.org>
// SPDX-License-Identifier: BSD-3-Clause
#include <config.h>
#ident "$Id$"
#include <fcntl.h>
#include <limits.h>
#include <stdint.h>
#include <stdlib.h>
#include <unistd.h>
#if __has_include(<sys/random.h>)
# include <sys/random.h>
#endif
#include "bit.h"
#include "defines.h"
#include "prototypes.h"
#include "shadowlog.h"
#include "sizeof.h"
static uint32_t csrand32(void);
static uint32_t csrand_uniform32(uint32_t n);
static unsigned long csrand_uniform_slow(unsigned long n);
/*
* Return a uniformly-distributed CS random u_long value.
*/
unsigned long
csrand(void)
{
int fd;
unsigned long r;
#ifdef HAVE_GETENTROPY
/* getentropy may exist but lack kernel support. */
if (getentropy(&r, sizeof(r)) == 0)
return r;
#endif
#ifdef HAVE_GETRANDOM
/* Likewise getrandom. */
if (getrandom(&r, sizeof(r), 0) == sizeof(r))
return r;
#endif
#ifdef HAVE_ARC4RANDOM_BUF
/* arc4random_buf can never fail. */
arc4random_buf(&r, sizeof(r));
return r;
#endif
/* Use /dev/urandom as a last resort. */
fd = open("/dev/urandom", O_RDONLY);
if (fd == -1)
goto fail;
if (read(fd, &r, sizeof(r)) != sizeof(r)) {
close(fd);
goto fail;
}
close(fd);
return r;
fail:
fprintf(log_get_logfd(), _("Unable to obtain random bytes.\n"));
exit(1);
}
/*
* Return a uniformly-distributed CS random value in the interval [0, n-1].
*/
unsigned long
csrand_uniform(unsigned long n)
{
if (n == 0 || n > UINT32_MAX)
return csrand_uniform_slow(n);
return csrand_uniform32(n);
}
/*
* Return a uniformly-distributed CS random value in the interval [min, max].
*/
unsigned long
csrand_interval(unsigned long min, unsigned long max)
{
return csrand_uniform(max - min + 1) + min;
}
static uint32_t
csrand32(void)
{
return csrand();
}
/*
* Fast Random Integer Generation in an Interval
* ACM Transactions on Modeling and Computer Simulation 29 (1), 2019
* <https://arxiv.org/abs/1805.10941>
*/
static uint32_t
csrand_uniform32(uint32_t n)
{
uint32_t bound, rem;
uint64_t r, mult;
if (n == 0)
return csrand32();
bound = -n % n; // analogous to `2^32 % n`, since `x % y == (x-y) % y`
do {
r = csrand32();
mult = r * n;
rem = mult; // analogous to `mult % 2^32`
} while (rem < bound); // p = (2^32 % n) / 2^32; W.C.: n=2^31+1, p=0.5
r = mult >> WIDTHOF(n); // analogous to `mult / 2^32`
return r;
}
static unsigned long
csrand_uniform_slow(unsigned long n)
{
unsigned long r, max, mask;
max = n - 1;
mask = bit_ceil_wrapul(n) - 1;
do {
r = csrand();
r &= mask; // optimization
} while (r > max); // p = ((mask+1) % n) / (mask+1); W.C.: p=0.5
return r;
}
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