Coverage Report

Created: 2023-09-23 17:42

/libfido2/fuzz/uniform_random.c
Line
Count
Source (jump to first uncovered line)
1
/*
2
 * Copyright (c) 2008, Damien Miller <djm@openbsd.org>
3
 *
4
 * Permission to use, copy, modify, and distribute this software for any
5
 * purpose with or without fee is hereby granted, provided that the above
6
 * copyright notice and this permission notice appear in all copies.
7
 *
8
 * THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES
9
 * WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF
10
 * MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR
11
 * ANY SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
12
 * WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN
13
 * ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF
14
 * OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
15
 */
16
17
#include <stdint.h>
18
#include <stdlib.h>
19
20
uint32_t uniform_random(uint32_t);
21
unsigned long prng_uint32(void);
22
23
/*
24
 * Calculate a uniformly distributed random number less than upper_bound
25
 * avoiding "modulo bias".
26
 *
27
 * Uniformity is achieved by generating new random numbers until the one
28
 * returned is outside the range [0, 2**32 % upper_bound).  This
29
 * guarantees the selected random number will be inside
30
 * [2**32 % upper_bound, 2**32) which maps back to [0, upper_bound)
31
 * after reduction modulo upper_bound.
32
 */
33
uint32_t
34
uniform_random(uint32_t upper_bound)
35
18.0M
{
36
18.0M
        uint32_t r, min;
37
38
18.0M
        if (upper_bound < 2)
39
0
                return 0;
40
41
        /* 2**32 % x == (2**32 - x) % x */
42
18.0M
        min = -upper_bound % upper_bound;
43
44
        /*
45
         * This could theoretically loop forever but each retry has
46
         * p > 0.5 (worst case, usually far better) of selecting a
47
         * number inside the range we need, so it should rarely need
48
         * to re-roll.
49
         */
50
18.0M
        for (;;) {
51
18.0M
                r = (uint32_t)prng_uint32();
52
18.0M
                if (r >= min)
53
18.0M
                        break;
54
18.0M
        }
55
56
18.0M
        return r % upper_bound;
57
18.0M
}