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/* SPDX-License-Identifier: MIT
* Copyright (c) 2026 Leah Rowe <leah@libreboot.org>
*
* Random number generation
*/
#ifndef RAND_H
#define RAND_H
#ifdef __OpenBSD__
#include <sys/param.h>
#endif
#include <sys/types.h>
#ifndef USE_URANDOM
#define USE_URANDOM 0
#endif
#include <errno.h>
#if defined(USE_URANDOM) && \
((USE_URANDOM) > 0)
#include <fcntl.h> /* if not arc4random: /dev/urandom */
#elif defined(__linux__)
#include <sys/random.h>
#include <sys/syscall.h>
#endif
#include <fcntl.h>
#include <limits.h>
#include <stddef.h>
#include <string.h>
#include <unistd.h>
#include <stdlib.h>
#include <string.h>
#include <stdint.h>
#include <stdio.h>
#include "../include/common.h"
/* Regarding Linux getrandom/urandom:
*
* For maximum security guarantee, we *only*
* use getrandom via syscall, or /dev/urandom;
* use of urandom is ill advised. This is why
* we use the syscall, in case the libc version
* of getrandom() might defer to /dev/urandom
*
* We *abort* on error, for both /dev/urandom
* and getrandom(), because the BSD arc4random
* never returns with error; therefore, for the
* most parity in terms of behaviour, we abort,
* because otherwise the function would have two
* return modes: always successful (BSD), or only
* sometimes (Linux). The BSD arc4random could
* theoretically abort; it is extremely unlikely
* there, and just so on Linux, hence this design.
*
* This is important, because cryptographic code
* for example must not rely on weak randomness.
* We must therefore treat broken randomness as
* though the world is broken, and burn accordingly.
*
* Similarly, any invalid input (NULL, zero bytes
* requested) are treated as fatal errors; again,
* cryptographic code must be reliable. If your
* code erroneously requested zero bytes, you might
* then end up with a non-randomised buffer, where
* you likely intended otherwise.
*
* In other words: call rset() correctly, or your
* program dies, and rset will behave correctly,
* or your program dies.
*/
#ifndef BUFSIZ
#define BUFSIZ 8192 /* reasonably on modern 64-bit systems */
#elif (BUFSIZ <= 0)
#error defined buffer size BUFSIZ below or equal to zero
#endif
int
win_lottery(void) /* are u lucky? */
{
size_t size = rsize();
size_t size2 = rsize();
int rval = 0;
char *s1 = NULL;
char *s2 = NULL;
if (size != size2)
return 0;
s1 = mkrbuf(size);
s2 = mkrbuf(size);
if (!memcmp(s1, s2, size))
rval = 1; /* winner! */
else
rval = 0;
free_if_null(&s1);
free_if_null(&s2);
return rval;
}
size_t
rsize(void)
{
size_t rval = 0;
size_t size = 67108864;
/* clamp rand to prevent modulo bias */
size_t limit = SIZE_MAX - (SIZE_MAX % size);
do {
rset(&rval, sizeof(rval));
} while (rval >= limit);
return rval % size;
}
void *
rmalloc(size_t *rval)
{
size_t size = 16777216;
/* clamp rand to prevent modulo bias */
size_t limit = SIZE_MAX - (SIZE_MAX % size);
if (if_err(rval == NULL, EFAULT))
return NULL;
do {
rset(rval, sizeof(*rval));
} while (*rval >= limit || *rval == 0);
return mkrstr(*rval %= size);
}
char *
mkrstr(size_t n) /* emulates spkmodem-decode */
{
char *s;
size_t i;
if (n == 0)
err_no_cleanup(0, EPERM, "mkrbuf: zero-byte request");
if (n >= SIZE_MAX - 1)
err_no_cleanup(0, EOVERFLOW, "mkrbuf: overflow");
if (if_err((s = mkrbuf(n + 1)) == NULL, EFAULT))
err_no_cleanup(0, EFAULT, "mkrstr: null");
for (i = 0; i < n; i++)
while(*(s + i) == '\0')
rset(s + i, 1);
*(s + n) = '\0';
return s;
}
void *
mkrbuf(size_t n)
{
void *buf;
if (n == 0)
err_no_cleanup(0, EPERM, "mkrbuf: zero-byte request");
if (n >= SIZE_MAX - 1)
err_no_cleanup(0, EOVERFLOW, "integer overflow in mkrbuf");
if ((buf = malloc(n)) == NULL)
err_no_cleanup(0, ENOMEM, "mkrbuf: malloc");
rset(buf, n);
return buf; /* basically malloc() but with rand */
}
void
rset(void *buf, size_t n)
{
int saved_errno = errno;
if (if_err(buf == NULL, EFAULT))
goto err;
if (n == 0)
err_no_cleanup(0, EPERM, "rset: zero-byte request");
#if (defined(__OpenBSD__) || defined(__FreeBSD__) || \
defined(__NetBSD__) || defined(__APPLE__) || \
defined(__DragonFly__)) && !(defined(USE_URANDOM) && \
((USE_URANDOM) > 0))
arc4random_buf(buf, n);
goto out;
#else
size_t off = 0;
ssize_t rc = 0;
#if defined(USE_URANDOM) && \
((USE_URANDOM) > 0)
int fd = -1;
if ((fd = open("/dev/urandom", O_RDONLY)) < 0)
goto err;
retry_rand:
if ((rc = read(fd, (unsigned char *)buf + off, n - off)) < 0) {
#elif defined(__linux__)
retry_rand:
if ((rc = (ssize_t)syscall(SYS_getrandom,
(unsigned char *)buf + off, n - off, 0)) < 0) {
#else
#error Unsupported operating system (possibly unsecure randomisation)
#endif
if (errno == EINTR ||
errno == EAGAIN)
goto retry_rand;
goto err; /* possibly unsupported by kernel */
}
if (rc == 0)
goto err; /* prevent infinite loop on fatal err */
if ((off += (size_t)rc) < n)
goto retry_rand;
#if defined(USE_URANDOM) && \
((USE_URANDOM) > 0)
close_no_err(&fd);
#endif
goto out;
#endif
out:
errno = saved_errno;
return;
err:
#if defined(USE_URANDOM) && \
((USE_URANDOM) > 0)
close_no_err(&fd);
#endif
err_no_cleanup(0, ECANCELED,
"Randomisation failure, possibly unsupported in your kernel");
exit(EXIT_FAILURE);
}
#endif
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