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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 */
#endif
#include <fcntl.h>
#include <limits.h>
#include <stddef.h>
#include <string.h>
#include <unistd.h>
#include <stdlib.h>
#include <string.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.
*/
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 length 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:
err_no_cleanup(0, ECANCELED,
"Randomisation failure, possibly unsupported in your kernel");
exit(EXIT_FAILURE);
}
#endif
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