Module Name: src
Committed By: riastradh
Date: Wed Aug 28 19:31:14 UTC 2013
Modified Files:
src/common/lib/libc/string: consttime_memequal.c
Log Message:
Comment on possible data-dependent branch in `!res'.
To generate a diff of this commit:
cvs rdiff -u -r1.3 -r1.4 src/common/lib/libc/string/consttime_memequal.c
Please note that diffs are not public domain; they are subject to the
copyright notices on the relevant files.
Modified files:
Index: src/common/lib/libc/string/consttime_memequal.c
diff -u src/common/lib/libc/string/consttime_memequal.c:1.3 src/common/lib/libc/string/consttime_memequal.c:1.4
--- src/common/lib/libc/string/consttime_memequal.c:1.3 Wed Aug 28 17:47:07 2013
+++ src/common/lib/libc/string/consttime_memequal.c Wed Aug 28 19:31:14 2013
@@ -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)
#include "namespace.h"
@@ -18,5 +18,15 @@ consttime_memequal(const void *b1, const
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;
}
