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Unoptimize the higher part of the domain of csrand_uniform()

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__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>
This commit is contained in:
Alejandro Colomar 2023-01-30 13:13:36 +01:00 committed by Iker Pedrosa
parent 0712b236c3
commit 2a61122b5e
1 changed files with 46 additions and 12 deletions
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58
libmisc/csrand.c
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@ -9,17 +9,23 @@
#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.
*/
@ -69,16 +75,37 @@ fail:
/*
* Return a uniformly-distributed CS random value in the interval [0, n-1].
*
* Fast Random Integer Generation in an Interval
* ACM Transactions on Modeling and Computer Simulation 29 (1), 2019
* <https://arxiv.org/abs/1805.10941>
*/
unsigned long
csrand_uniform(unsigned long n)
{
unsigned long bound, rem;
unsigned __int128 r, mult;
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();
@ -97,11 +124,18 @@ csrand_uniform(unsigned long n)
}
/*
* Return a uniformly-distributed CS random value in the interval [min, max].
*/
unsigned long
csrand_interval(unsigned long min, unsigned long max)
static unsigned long
csrand_uniform_slow(unsigned long n)
{
return csrand_uniform(max - min + 1) + min;
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;
}