2a61122b5e
__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>
142 lines
2.5 KiB
C
142 lines
2.5 KiB
C
/*
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* SPDX-FileCopyrightText: Alejandro Colomar <alx@kernel.org>
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*
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* SPDX-License-Identifier: BSD-3-Clause
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*/
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#include <config.h>
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#ident "$Id$"
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#include <limits.h>
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#include <stdint.h>
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#include <stdio.h>
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#include <stdlib.h>
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#include <unistd.h>
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#if HAVE_SYS_RANDOM_H
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#include <sys/random.h>
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#endif
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#include "bit.h"
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#include "defines.h"
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#include "prototypes.h"
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#include "shadowlog.h"
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static uint32_t csrand_uniform32(uint32_t n);
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static unsigned long csrand_uniform_slow(unsigned long n);
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/*
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* Return a uniformly-distributed CS random u_long value.
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*/
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unsigned long
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csrand(void)
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{
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FILE *fp;
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unsigned long r;
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#ifdef HAVE_GETENTROPY
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/* getentropy may exist but lack kernel support. */
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if (getentropy(&r, sizeof(r)) == 0)
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return r;
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#endif
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#ifdef HAVE_GETRANDOM
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/* Likewise getrandom. */
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if (getrandom(&r, sizeof(r), 0) == sizeof(r))
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return r;
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#endif
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#ifdef HAVE_ARC4RANDOM_BUF
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/* arc4random_buf can never fail. */
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arc4random_buf(&r, sizeof(r));
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return r;
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#endif
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/* Use /dev/urandom as a last resort. */
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fp = fopen("/dev/urandom", "r");
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if (NULL == fp) {
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goto fail;
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}
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if (fread(&r, sizeof(r), 1, fp) != 1) {
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fclose(fp);
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goto fail;
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}
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fclose(fp);
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return r;
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fail:
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fprintf(log_get_logfd(), _("Unable to obtain random bytes.\n"));
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exit(1);
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}
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/*
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* Return a uniformly-distributed CS random value in the interval [0, n-1].
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*/
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unsigned long
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csrand_uniform(unsigned long n)
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{
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if (n == 0 || n > UINT32_MAX)
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return csrand_uniform_slow(n);
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return csrand_uniform32(n);
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}
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/*
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* Return a uniformly-distributed CS random value in the interval [min, max].
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*/
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unsigned long
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csrand_interval(unsigned long min, unsigned long max)
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{
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return csrand_uniform(max - min + 1) + min;
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}
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/*
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* Fast Random Integer Generation in an Interval
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* ACM Transactions on Modeling and Computer Simulation 29 (1), 2019
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* <https://arxiv.org/abs/1805.10941>
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*/
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static uint32_t
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csrand_uniform32(uint32_t n)
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{
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uint32_t bound, rem;
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uint64_t r, mult;
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if (n == 0)
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return csrand();
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bound = -n % n; // analogous to `2^64 % n`, since `x % y == (x-y) % y`
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do {
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r = csrand();
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mult = r * n;
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rem = mult; // analogous to `mult % 2^64`
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} while (rem < bound); // p = (2^64 % n) / 2^64; W.C.: n=2^63+1, p=0.5
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r = mult >> WIDTHOF(n); // analogous to `mult / 2^64`
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return r;
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}
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static unsigned long
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csrand_uniform_slow(unsigned long n)
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{
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unsigned long r, max, mask;
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max = n - 1;
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mask = bit_ceil_wrapul(n) - 1;
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do {
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r = csrand();
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r &= mask; // optimization
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} while (r > max); // p = ((mask+1) % n) / (mask+1); W.C.: p=0.5
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return r;
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}
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