1 /* $NetBSD: ct.c,v 1.2 2017/01/28 21:31:50 christos Exp $ */
2
3 /*
4 * Copyright (c) 2009 Kungliga Tekniska Högskolan
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
62 int
ct_memcmp(const volatile void * volatile p1,const volatile void * volatile p2,size_t len)63 ct_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