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

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This commit is contained in:
riastradh 2013-08-28 19:31:14 +00:00
parent c462ebccdd
commit 3db7caeeb3
1 changed files with 11 additions and 1 deletions
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12
common/lib/libc/string/consttime_memequal.c
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@ -1,4 +1,4 @@
/* $NetBSD: consttime_memequal.c,v 1.3 2013/08/28 17:47:07 riastradh Exp $ */ /* $NetBSD: consttime_memequal.c,v 1.4 2013/08/28 19:31:14 riastradh Exp $ */
#if !defined(_KERNEL) && !defined(_STANDALONE) #if !defined(_KERNEL) && !defined(_STANDALONE)
#include "namespace.h" #include "namespace.h"
@ -18,5 +18,15 @@ consttime_memequal(const void *b1, const void *b2, size_t len)
while (len --) while (len --)
res |= *c1++ ^ *c2++; 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; return !res;
} }