xenocara/xserver/os/timingsafe_memcmp.c

/*
 * Copyright (c) 2014 Google Inc.
 *
 * Permission to use, copy, modify, and 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 THE AUTHOR DISCLAIMS ALL WARRANTIES
 * WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF
 * MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHOR 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 <dix-config.h>
#include <limits.h>
#include <string.h>
#include <X11/Xfuncproto.h>
#include "os.h"

int
timingsafe_memcmp(const void *b1, const void *b2, size_t len)
{
        const unsigned char *p1 = b1, *p2 = b2;
        size_t i;
        int res = 0, done = 0;

        for (i = 0; i < len; i++) {
                /* lt is -1 if p1[i] < p2[i]; else 0. */
                int lt = (p1[i] - p2[i]) >> CHAR_BIT;

                /* gt is -1 if p1[i] > p2[i]; else 0. */
                int gt = (p2[i] - p1[i]) >> CHAR_BIT;

                /* cmp is 1 if p1[i] > p2[i]; -1 if p1[i] < p2[i]; else 0. */
                int cmp = lt - gt;

                /* set res = cmp if !done. */
                res |= cmp & ~done;

                /* set done if p1[i] != p2[i]. */
                done |= lt | gt;
        }

        return (res);
}