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