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File: [cvs.NetBSD.org] / src / common / lib / libc / string / consttime_memequal.c (download)

Revision 1.4, Wed Aug 28 19:31:14 2013 UTC (3 years, 11 months ago) by riastradh
Branch: MAIN
CVS Tags: yamt-pagecache-base9, rmind-smpnet-nbase, rmind-smpnet-base, riastradh-xf86-video-intel-2-7-1-pre-2-21-15, riastradh-drm2-base3
Branch point for: yamt-pagecache, tls-earlyentropy
Changes since 1.3: +11 -1 lines

Comment on possible data-dependent branch in `!res'.

/* $NetBSD: consttime_memequal.c,v 1.4 2013/08/28 19:31:14 riastradh Exp $ */

#if !defined(_KERNEL) && !defined(_STANDALONE)
#include "namespace.h"
#include <string.h>
#ifdef __weak_alias
__weak_alias(consttime_memequal,_consttime_memequal)
#endif
#else
#include <lib/libkern/libkern.h>
#endif

int
consttime_memequal(const void *b1, const void *b2, size_t len)
{
	const char *c1 = b1, *c2 = b2;
	int res = 0;

	while (len --)
		res |= *c1++ ^ *c2++;

	/*
	 * If the compiler for your favourite architecture generates a
	 * conditional branch for `!res', it will be a data-dependent
	 * branch, in which case this should be replaced by
	 *
	 *	return (1 - (1 & ((res - 1) >> 8)));
	 *
	 * or rewritten in assembly.
	 */
	return !res;
}