shadow/libmisc/csrand.c
Alejandro Colomar 2a61122b5e Unoptimize the higher part of the domain of csrand_uniform()
__int128, which is needed for optimizing that part of the range, is not
always available.  We need the unoptimized version for portability
reasons.

Closes: <https://github.com/shadow-maint/shadow/issues/634>
Fixes: 1a0e13f94e ("Optimize csrand_uniform()")
Reported-by: Adam Sampson <ats@offog.org>
Cc: Iker Pedrosa <ipedrosa@redhat.com>
Signed-off-by: Alejandro Colomar <alx@kernel.org>
2023-01-30 18:24:15 +01:00

142 lines
2.5 KiB
C

/*
* SPDX-FileCopyrightText: Alejandro Colomar <alx@kernel.org>
*
* SPDX-License-Identifier: BSD-3-Clause
*/
#include <config.h>
#ident "$Id$"
#include <limits.h>
#include <stdint.h>
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#if HAVE_SYS_RANDOM_H
#include <sys/random.h>
#endif
#include "bit.h"
#include "defines.h"
#include "prototypes.h"
#include "shadowlog.h"
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)
{
FILE *fp;
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. */
fp = fopen("/dev/urandom", "r");
if (NULL == fp) {
goto fail;
}
if (fread(&r, sizeof(r), 1, fp) != 1) {
fclose(fp);
goto fail;
}
fclose(fp);
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;
}
/*
* 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 csrand();
bound = -n % n; // analogous to `2^64 % n`, since `x % y == (x-y) % y`
do {
r = csrand();
mult = r * n;
rem = mult; // analogous to `mult % 2^64`
} while (rem < bound); // p = (2^64 % n) / 2^64; W.C.: n=2^63+1, p=0.5
r = mult >> WIDTHOF(n); // analogous to `mult / 2^64`
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;
}