tsmemcmp.c revision 1.3 
1/*	$NetBSD: tsmemcmp.c,v 1.3 2020/05/25 20:47:20 christos Exp $	*/
2
3/*
4 * Copyright (C) 2004-2007, 2011, 2012  Internet Systems Consortium, Inc. ("ISC")
5 * Copyright (C) 1999-2001, 2003  Internet Software Consortium.
6 *
7 * Permission to use, copy, modify, and/or distribute this software for any
8 * purpose with or without fee is hereby granted, provided that the above
9 * copyright notice and this permission notice appear in all copies.
10 *
11 * THE SOFTWARE IS PROVIDED "AS IS" AND ISC DISCLAIMS ALL WARRANTIES WITH
12 * REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF MERCHANTABILITY
13 * AND FITNESS.  IN NO EVENT SHALL ISC BE LIABLE FOR ANY SPECIAL, DIRECT,
14 * INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES WHATSOEVER RESULTING FROM
15 * LOSS OF USE, DATA OR PROFITS, WHETHER IN AN ACTION OF CONTRACT, NEGLIGENCE
16 * OR OTHER TORTIOUS ACTION, ARISING OUT OF OR IN CONNECTION WITH THE USE OR
17 * PERFORMANCE OF THIS SOFTWARE.
18 */
19
20/* Id */
21
22/*! \file */
23
24#include <config.h>
25#include <limits.h>
26#include <isc/string.h>
27
28/* Making a portable memcmp that has no internal branches and loops always
29 * once for every byte without early-out shortcut has a few challenges.
30 *
31 * Inspired by 'timingsafe_memcmp()' from the BSD system and
32 * https://github.com/libressl-portable/openbsd/blob/master/src/lib/libc/string/timingsafe_memcmp.c
33 *
34 * Sadly, that one is not portable C: It makes assumptions on the representation
35 * of negative integers and assumes sign-preserving right-shift of negative
36 * signed values. This is a rewrite from scratch that should not suffer from
37 * such issues.
38 *
39 * 2015-12-12, J. Perlinger (perlinger-at-ntp-dot-org)
40 */
41int
42isc_tsmemcmp(const void *p1, const void *p2, size_t nb) {
43	const unsigned char *ucp1 = p1;
44	const unsigned char *ucp2 = p2;
45	unsigned int isLT = 0u;
46	unsigned int isGT = 0u;
47	volatile unsigned int mask = (1u << CHAR_BIT);
48
49	for (/*NOP*/; 0 != nb; --nb, ++ucp1, ++ucp2) {
50		isLT |= mask &
51		    ((unsigned int)*ucp1 - (unsigned int)*ucp2);
52		isGT |= mask &
53		    ((unsigned int)*ucp2 - (unsigned int)*ucp1);
54		mask &= ~(isLT | isGT);
55	}
56	return (int)(isGT >> CHAR_BIT) - (int)(isLT >> CHAR_BIT);
57}
58