ct.c revision 1.2
1/* $NetBSD: ct.c,v 1.2 2017/01/28 21:31:50 christos Exp $ */ 2 3/* 4 * Copyright (c) 2009 Kungliga Tekniska H��gskolan 5 * (Royal Institute of Technology, Stockholm, Sweden). 6 * All rights reserved. 7 * 8 * Redistribution and use in source and binary forms, with or without 9 * modification, are permitted provided that the following conditions 10 * are met: 11 * 12 * 1. Redistributions of source code must retain the above copyright 13 * notice, this list of conditions and the following disclaimer. 14 * 15 * 2. Redistributions in binary form must reproduce the above copyright 16 * notice, this list of conditions and the following disclaimer in the 17 * documentation and/or other materials provided with the distribution. 18 * 19 * 3. Neither the name of the Institute nor the names of its contributors 20 * may be used to endorse or promote products derived from this software 21 * without specific prior written permission. 22 * 23 * THIS SOFTWARE IS PROVIDED BY THE INSTITUTE AND CONTRIBUTORS ``AS IS'' AND 24 * ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE 25 * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE 26 * ARE DISCLAIMED. IN NO EVENT SHALL THE INSTITUTE OR CONTRIBUTORS BE LIABLE 27 * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL 28 * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS 29 * OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) 30 * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT 31 * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY 32 * OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF 33 * SUCH DAMAGE. 34 */ 35 36#include <config.h> 37#include <krb5/roken.h> 38 39/** 40 * Constant time compare to memory regions. The reason for making it 41 * constant time is to make sure that timeing information leak from 42 * where in the function the diffrence is. 43 * 44 * ct_memcmp() can't be used to order memory regions like memcmp(), 45 * for example, use ct_memcmp() with qsort(). 46 * 47 * We use volatile to avoid optimizations where the compiler and/or 48 * linker turn this ct_memcmp() into a plain memcmp(). The pointers 49 * themselves are also marked volatile (not just the memory pointed at) 50 * because in some GCC versions there is a bug which can be worked 51 * around by doing this. 52 * 53 * @param p1 memory region 1 to compare 54 * @param p2 memory region 2 to compare 55 * @param len length of memory 56 * 57 * @return 0 when the memory regions are equal, non zero if not 58 * 59 * @ingroup roken 60 */ 61 62int 63ct_memcmp(const volatile void * volatile p1, 64 const volatile void * volatile p2, 65 size_t len) 66{ 67 /* 68 * There's no need for s1 and s2 to be volatile; only p1 and p2 have 69 * to be in order to work around GCC bugs. 70 * 71 * However, s1 and s2 do have to point to volatile, as we don't know 72 * if the object was originally defined as volatile, and if it was 73 * then we'd get undefined behavior here if s1/s2 were declared to 74 * point to non-volatile memory. 75 */ 76 const volatile unsigned char *s1 = p1; 77 const volatile unsigned char *s2 = p2; 78 size_t i; 79 int r = 0; 80 81 for (i = 0; i < len; i++) 82 r |= (s1[i] ^ s2[i]); 83 return !!r; 84} 85