1 /* $NetBSD: tsmemcmp.c,v 1.2 2024/08/18 20:47:14 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 */ 41 int 42 isc_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