/*

 * Copyright (c) 2009 Kungliga Tekniska Högskolan

 * (Royal Institute of Technology, Stockholm, Sweden).

 * All rights reserved.

 *

 * Redistribution and use in source and binary forms, with or without

 * modification, are permitted provided that the following conditions

 * are met:

 *

 * 1. Redistributions of source code must retain the above copyright

 *    notice, this list of conditions and the following disclaimer.

 *

 * 2. Redistributions in binary form must reproduce the above copyright

 *    notice, this list of conditions and the following disclaimer in the

 *    documentation and/or other materials provided with the distribution.

 *

 * 3. Neither the name of the Institute nor the names of its contributors

 *    may be used to endorse or promote products derived from this software

 *    without specific prior written permission.

 *

 * THIS SOFTWARE IS PROVIDED BY THE INSTITUTE AND CONTRIBUTORS ``AS IS'' AND

 * ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE

 * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE

 * ARE DISCLAIMED.  IN NO EVENT SHALL THE INSTITUTE OR CONTRIBUTORS BE LIABLE

 * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL

 * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS

 * OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)

 * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT

 * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY

 * OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF

 * SUCH DAMAGE.

 */

#include <config.h>

#include "roken.h"

/**

 * Constant time compare to memory regions. The reason for making it

 * constant time is to make sure that timeing information leak from

 * where in the function the diffrence is.

 *

 * ct_memcmp() can't be used to order memory regions like memcmp(),

 * for example, use ct_memcmp() with qsort().

 *

 * We use volatile to avoid optimizations where the compiler and/or

 * linker turn this ct_memcmp() into a plain memcmp().  The pointers

 * themselves are also marked volatile (not just the memory pointed at)

 * because in some GCC versions there is a bug which can be worked

 * around by doing this.

 *

 * @param p1 memory region 1 to compare

 * @param p2 memory region 2 to compare

 * @param len length of memory

 *

 * @return 0 when the memory regions are equal, non zero if not

 *

 * @ingroup roken

 */

int

ct_memcmp(const volatile void * volatile p1,

          const volatile void * volatile p2,

          size_t len)

{

    /*

     * There's no need for s1 and s2 to be volatile; only p1 and p2 have

     * to be in order to work around GCC bugs.

     *

     * However, s1 and s2 do have to point to volatile, as we don't know

     * if the object was originally defined as volatile, and if it was

     * then we'd get undefined behavior here if s1/s2 were declared to

     * point to non-volatile memory.

     */

    const volatile unsigned char *s1 = p1;

    const volatile unsigned char *s2 = p2;

    size_t i;

    int r = 0;

    for (i = 0; i < len; i++)

  r |= (s1[i] ^ s2[i]);

    return !!r;

}