lbmk/util/libreboot-utils/lib/num.c

/* SPDX-License-Identifier: MIT
 * Copyright (c) 2026 Leah Rowe <leah@libreboot.org>
 *
 * Numerical functions.
 */

/*
TODO: properly handle errno in this file
 */

#ifdef __OpenBSD__
#include <sys/param.h>
#endif
#include <sys/types.h>
#if defined(FALLBACK_RAND_1989) && \
    (FALLBACK_RAND_1989) > 0
#include <sys/time.h>
#endif

#include <errno.h>
#if !((defined(__OpenBSD__) && (OpenBSD) >= 201) || \
    defined(__FreeBSD__) || \
    defined(__NetBSD__) || defined(__APPLE__))
#include <fcntl.h> /* if not arc4random: /dev/urandom */
#endif
#include <limits.h>
#include <stddef.h>
#include <string.h>
#if defined(FALLBACK_RAND_1989) && \
    (FALLBACK_RAND_1989) > 0
#include <time.h>
#endif
#include <unistd.h>

#include "../include/common.h"

/* TODO:
 * make this and errno handling more
 * flexible

  in particular:
  hextonum could be modified to
  write into a buffer instead,
  with the converted numbers,
  of an arbitrary length
 */
unsigned short
hextonum(char ch_s)
{
	int saved_errno = errno;

	/* rlong() can return error,
	   but preserves errno if no
	   error. we need to detect
	   this because it handles
	   /dev/urandom sometimes

	   therefore, if it's zero
	   at start, we know if there
	   was an err at the end, by
	   return value zero, if errno
	   was set; this is technically
	   valid, since zero is also
	   a valid random number!

	it's an edge case that i had
	to fix. i'll rewrite the code
	better later. for now, it
	should be ok.
	 */
	errno = 0;

	unsigned char ch;
	size_t rval;

	ch = (unsigned char)ch_s;

	if ((unsigned int)(ch - '0') <= 9) {

		rval = ch - '0';
		goto hextonum_success;
	}

	ch |= 0x20;

	if ((unsigned int)(ch - 'a') <= 5) {

		rval = ch - 'a' + 10;
		goto hextonum_success;
	}

	if (ch == '?' || ch == 'x') {

		rval = rlong();
		if (errno > 0)
			goto err_hextonum;

		goto hextonum_success;
	}

	goto err_hextonum;

hextonum_success:

	errno = saved_errno;
	return (unsigned short)rval & 0xf;

err_hextonum:

	if (errno == saved_errno)
		errno = EINVAL;
	else
		return 17; /* 17 indicates getrandom/urandom fail */

	return 16; /* invalid character */

	/* caller just checks >15. */
}

/* Random numbers
 */

/* when calling this: save errno
 * first, then set errno to zero.
 * on error, this function will
 * set errno and possibly return
 *
 * rlong also preserves errno
 * and leaves it unchanged on
 * success, so if you do it
 * right, you can detect error.
 * this is because it uses
 * /dev/urandom which can err.
 * ditto getrandom (EINTR),
 * theoretically.
 */

size_t
rlong(void)
{
#if !(defined(FALLBACK_RAND_1989) && \
    ((FALLBACK_RAND_1989) > 0))
#if (defined(__OpenBSD__) && (OpenBSD) >= 201) || \
    defined(__FreeBSD__) || \
    defined(__NetBSD__) || defined(__APPLE__)

	int saved_errno = errno;
	size_t rval;

	arc4random_buf(&rval, sizeof(size_t));

	errno = saved_errno;
	return rval;
#else
	static int fd = -1;
	static ssize_t nr = -1;
	static size_t off = 0;
#if defined (BUFSIZ)
	static char rbuf[BUFSIZ];
#else
#ifndef PORTABLE
	static char rbuf[4096];
#elif ((PORTABLE) > 0)
	static char rbuf[256]; /* scarce memory on old systems */
#else
	static char rbuf[4096]; /* typical 32-bit BUFSIZ */
#endif
#endif
	size_t rval;
	ssize_t new_nr;

	int retries = 0;
	int max_retries = 100;

	int saved_errno = errno;

#if defined(__linux__)
#if defined(HAVE_GETRANDOM) || \
    defined(HAVE_GETRANDOM_SYSCALL)

	/* linux getrandom()
	 *
	 * we *can* use arc4random on
	 * modern linux, but not on
	 * every libc. better use the
	 * official linux function
	 */

	if (fallback_rand_getrandom(&rval, sizeof(rval)) == 0) {
		errno = saved_errno;
		return rval;
	}

	/*
	 * now fall back to urandom if getrandom failed:
	 */
#endif
#endif

	/* reading from urandom is inherently
	 * unreliable on old systems, even if
	 * newer systems make it more reliable
	 *
	 * modern linux/bsd make it safe, but
	 * we have to assume that someone is
	 * compiling this on linux from 1999
	 *
	 * this logic therefore applies various
	 * tricks to mitigate possible os bugs
	 */

retry_urandom_read:

	if (++retries > max_retries)
		goto err_rlong;

	if (nr < 0 || nr < (ssize_t)sizeof(size_t)) {

		if (fd < 0) {

			fd = open("/dev/urandom",
			    O_RDONLY | O_BINARY | O_NOFOLLOW |
			    O_CLOEXEC | O_NOCTTY);

#ifdef USE_OLD_DEV_RANDOM
#if (USE_OLD_DEV_RANDOM) > 0
			/* WARNING:
			 *   /dev/random may block
			 *   forever and does **NOT**
			 *   guarantee better entropy
			 *   on old systems
			 *
			 *   only use it if needed
			 */

			if (fd < 0)
				fd = open("/dev/random",
				    O_RDONLY | O_BINARY | O_NOFOLLOW |
				    O_CLOEXEC | O_NOCTTY);
#endif
#endif

			if (fd < 0)
				goto retry_urandom_read;

			retries = 0;
		}

		new_nr = rw_file_exact(fd, (unsigned char *)rbuf,
		    sizeof(rbuf), 0, IO_READ, LOOP_EAGAIN,
		    LOOP_EINTR, MAX_ZERO_RW_RETRY, OFF_ERR);

		if (new_nr < 0 || new_nr < (ssize_t)sizeof(rbuf))
			goto retry_urandom_read;

		/* only reset buffer after successful refill */
		nr = new_nr;
		off = 0;

		/* to mitigate file descriptor
		 * injection, we do not re-use
		 * the same descriptor each time
		 */
		(void) close_on_eintr(fd);
		fd = -1;
	}

	fd = -1;
	retries = 0;

	memcpy(&rval, rbuf + off, sizeof(size_t));

	nr -= (ssize_t)sizeof(size_t);
	off += sizeof(size_t);

	errno = saved_errno;

	return rval;

err_rlong:

	if (errno == saved_errno)
		errno = EIO;

	return 0;

#endif
#else /* FALLBACK_RAND_1989 */
	/* your computer is from a museum
	 */
	size_t mix = 0;
	int nr = 0;
	int saved_errno = errno;

	/* 100 times, for entropy
	 */
	for (nr = 0; nr < 100; nr++)
		mix ^= fallback_rand_1989();

	errno = saved_errno;
	return mix;
#endif
}

#if !(defined(FALLBACK_RAND_1989) && \
    ((FALLBACK_RAND_1989) > 0))
#if defined(__linux__)
#if defined(HAVE_GETRANDOM) || \
    defined(HAVE_GETRANDOM_SYSCALL)
int /* yes, linux is a fallback */
fallback_rand_getrandom(void *buf, size_t len)
{
	size_t off = 0;
	ssize_t rval = -1;
	int saved_errno = errno;

	if (!len) {
		errno = EINVAL;
		return -1;
	}

	if (buf == NULL) {
		errno = EFAULT;
		return -1;
	}

#if defined(HAVE_GETRANDOM) || \
    defined(HAVE_GETRANDOM_SYSCALL)

	while (off < len) {

#if defined(HAVE_GETRANDOM)
		rval = (ssize_t)getrandom((char *)buf + off, len - off, 0);
#elif defined(HAVE_GETRANDOM_SYSCALL)
		rval = (ssize_t)syscall(SYS_getrandom,
		    (char *)buf + off, len - off, 0);
#endif

		if (rval < 0) {
			if (errno == EINTR || errno == EAGAIN)
				continue;

			errno = EIO;
			return -1; /* unsupported by kernel */
		}

		if (rval == 0) {
			errno = EIO;
			return -1;
		}

		off += (size_t)rval;
	}

	errno = saved_errno;
	return 0;

#else
	(void)buf;
	(void)len;

	errno = EIO;
	return -1;
#endif
}
#endif
#endif
#else
size_t
fallback_rand_1989(void)
{
	static size_t mix = 0;
	static size_t counter = 0;

	struct timeval tv;

	/* nobody should use this
	 * (not crypto-safe)
	 */

	gettimeofday(&tv, NULL);

	mix ^= (size_t)tv.tv_sec
	    ^ (size_t)tv.tv_usec
	    ^ (size_t)getpid()
	    ^ (size_t)&mix
	    ^ counter++
	    ^ entropy_jitter();

	/*
	 * Stack addresses can vary between
	 * calls, thus increasing entropy.
	 */
	mix ^= (size_t)&mix;
	mix ^= (size_t)&tv;
	mix ^= (size_t)&counter;

	return mix;
}

size_t
entropy_jitter(void)
{
	size_t mix;

	struct timeval a, b;
	ssize_t mix_diff;

	int c;

	mix = 0;

	gettimeofday(&a, NULL);

	for (c = 0; c < 32; c++) {

		getpid();
		gettimeofday(&b, NULL);

		/*
		 * prevent negative numbers to prevent overflow,
		 * which would bias rand to large numbers
		 */
		mix_diff = (ssize_t)(b.tv_usec - a.tv_usec);
		if (mix_diff < 0)
			mix_diff = -mix_diff;

		mix ^= (size_t)(mix_diff);

		mix ^= (size_t)&mix;

	}

	return mix;
}
#endif

void
check_bin(size_t a, const char *a_name)
{
	if (a > 1)
		err(EINVAL, "%s must be 0 or 1, but is %lu",
		    a_name, (size_t)a);
}