| // SPDX-License-Identifier: ISC |
| /* |
| * Copyright (C) 2019 Linaro Limited |
| * Copyright (C) 2004-2007, 2011, 2012 Internet Systems Consortium, Inc. ("ISC") |
| * Copyright (C) 1999-2001, 2003 Internet Software Consortium. |
| * |
| * Permission to use, copy, modify, and/or distribute this software for any |
| * purpose with or without fee is hereby granted, provided that the above |
| * copyright notice and this permission notice appear in all copies. |
| * |
| * THE SOFTWARE IS PROVIDED "AS IS" AND ISC DISCLAIMS ALL WARRANTIES WITH |
| * REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF MERCHANTABILITY |
| * AND FITNESS. IN NO EVENT SHALL ISC BE LIABLE FOR ANY SPECIAL, DIRECT, |
| * INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES WHATSOEVER RESULTING FROM |
| * LOSS OF USE, DATA OR PROFITS, WHETHER IN AN ACTION OF CONTRACT, NEGLIGENCE |
| * OR OTHER TORTIOUS ACTION, ARISING OUT OF OR IN CONNECTION WITH THE USE OR |
| * PERFORMANCE OF THIS SOFTWARE. |
| */ |
| |
| #include <limits.h> |
| #include <string_ext.h> |
| |
| /* Making a portable memcmp that has no internal branches and loops always |
| * once for every byte without early-out shortcut has a few challenges. |
| * |
| * Inspired by 'timingsafe_memcmp()' from the BSD system and |
| * https://github.com/libressl-portable/openbsd/blob/master/src/lib/libc/string/timingsafe_memcmp.c |
| * |
| * Sadly, that one is not portable C: It makes assumptions on the representation |
| * of negative integers and assumes sign-preserving right-shift of negative |
| * signed values. This is a rewrite from scratch that should not suffer from |
| * such issues. |
| * |
| * 2015-12-12, J. Perlinger (perlinger-at-ntp-dot-org) |
| */ |
| int consttime_memcmp(const void *p1, const void *p2, size_t nb) { |
| const unsigned char *ucp1 = p1; |
| const unsigned char *ucp2 = p2; |
| unsigned int isLT = 0u; |
| unsigned int isGT = 0u; |
| volatile unsigned int mask = (1u << CHAR_BIT); |
| |
| for (/*NOP*/; 0 != nb; --nb, ++ucp1, ++ucp2) { |
| isLT |= mask & |
| ((unsigned int)*ucp1 - (unsigned int)*ucp2); |
| isGT |= mask & |
| ((unsigned int)*ucp2 - (unsigned int)*ucp1); |
| mask &= ~(isLT | isGT); |
| } |
| return (int)(isGT >> CHAR_BIT) - (int)(isLT >> CHAR_BIT); |
| } |