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      1 /*	$NetBSD: ct.c,v 1.2 2017/01/28 21:31:50 christos Exp $	*/
      2 
      3 /*
      4  * Copyright (c) 2009 Kungliga Tekniska Hgskolan
      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
     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