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/* SPDX-License-Identifier: MIT
* Copyright (c) 2026 Leah Rowe <leah@libreboot.org>
*
* Random number generation
*/
#if defined(USE_ARC4) && \
((USE_ARC4) > 0)
#define _DEFAULT_SOURCE 1 /* for arc4random on *linux* */
/* (not needed on bsd - on bsd,
it is used automatically unless
overridden with USE_URANDOM */
#elif defined(USE_URANDOM) && \
((USE_URANDOM) > 0)
#include <fcntl.h> /* if not arc4random: /dev/urandom */
#elif defined(__linux__) && \
!(defined(USE_ARC4) && ((USE_ARC4) > 0))
#ifndef _GNU_SOURCE
#define _GNU_SOURCE 1
#endif
#include <sys/syscall.h>
#include <sys/random.h>
#endif
#ifdef __OpenBSD__
#include <sys/param.h>
#endif
#include <sys/types.h>
#include <errno.h>
#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.
*/
/* random string generator, with
* rejection sampling. NOTE: only
* uses ASCII-safe characters, for
* printing on a unix terminal
*
* you still shouldn't use this for
* password generation; open diceware
* passphrases are better for that
*
* NOTE: the generated strings must
* ALSO be safe for file/directory names
* on unix-like os e.g. linux/bsd
*/
char *
rchars(size_t n) /* emulates spkmodem-decode */
{
static char ch[] =
"abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789";
char *s = NULL;
size_t i;
smalloc(&s, n + 1);
for (i = 0; i < n; i++)
s[i] = ch[rsize(sizeof(ch) - 1)];
*(s + n) = '\0';
return s;
}
size_t
rsize(size_t n)
{
size_t rval = SIZE_MAX;
if (!n)
exitf("rsize: division by zero");
/* rejection sampling (clamp rand to eliminate modulo bias) */
for (; rval >= SIZE_MAX - (SIZE_MAX % n); rset(&rval, sizeof(rval)));
return rval % n;
}
void *
rmalloc(size_t n)
{
void *buf = NULL;
rset(vmalloc(&buf, n), n);
return buf; /* basically malloc() but with rand */
}
void
rset(void *buf, size_t n)
{
int saved_errno = errno;
errno = 0;
if (if_err(buf == NULL, EFAULT))
goto err;
if (n == 0)
exitf("rset: zero-byte request");
/* on linux, getrandom is recommended,
but you can pass -DUSE_ARC4=1 to use arc4random.
useful for portability testing from linux.
*/
#if (defined(USE_ARC4) && ((USE_ARC4) > 0)) || \
((defined(__OpenBSD__) || defined(__FreeBSD__) || \
defined(__NetBSD__) || defined(__APPLE__) || \
defined(__DragonFly__)) && !(defined(USE_URANDOM) && \
((USE_URANDOM) > 0)))
arc4random_buf(buf, n);
#else
size_t off = 0;
retry_rand: {
#if defined(USE_URANDOM) && \
((USE_URANDOM) > 0)
ssize_t rval;
int fd = -1;
open_file_on_eintr("/dev/urandom", &fd, O_RDONLY, 0400, NULL);
while (rw_retry(saved_errno,
rval = rw(fd, (unsigned char *)buf + off, n - off, 0, IO_READ)));
#elif defined(__linux__)
long rval;
while (sys_retry(saved_errno,
rval = syscall(SYS_getrandom,
(unsigned char *)buf + off, n - off, 0)));
#else
#error Unsupported operating system (possibly unsecure randomisation)
#endif
if (rval < 0 || /* syscall fehler */
rval == 0) { /* prevent infinite loop on fatal err */
#if defined(USE_URANDOM) && \
((USE_URANDOM) > 0)
xclose(&fd);
#endif
goto err;
}
if ((off += (size_t)rval) < n)
goto retry_rand;
#if defined(USE_URANDOM) && \
((USE_URANDOM) > 0)
xclose(&fd);
#endif
}
#endif
reset_caller_errno(0);
return;
err:
(void) with_fallback_errno(ECANCELED);
exitf("Randomisierungsfehler");
exit(EXIT_FAILURE);
}
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