170d24a251
Those comments were written when this function used 64 bits (and
temporary variables of 128 bits). Now it uses 32 bits, with temporaries
of 64 bits, so some values have changed.
Fixes: 2a61122b5e ("Unoptimize the higher part of the domain of csrand_uniform()")
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 `2^32 % 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 % 2^32`
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} while (rem < bound); // p = (2^32 % n) / 2^32; W.C.: n=2^31+1, p=0.5
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r = mult >> WIDTHOF(n); // analogous to `mult / 2^32`
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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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