[src/trunk]: src/common/lib/libc/string Switch to the suggested constant-time...

[riastradh <riastradh%NetBSD.org@localhost>]

details:   https://anonhg.NetBSD.org/src/rev/4af4a4274ffd
branches:  trunk
changeset: 336775:4af4a4274ffd
user:      riastradh <riastradh%NetBSD.org@localhost>
date:      Wed Mar 18 20:11:35 2015 +0000

description:
Switch to the suggested constant-time result conversion.

Not hard to find CPU/compiler combinations with branches for `!res'.

While here, make everything unsigned for good measure.

diffstat:

 common/lib/libc/string/consttime_memequal.c |  22 +++++++++++-----------
 1 files changed, 11 insertions(+), 11 deletions(-)

diffs (40 lines):

diff -r 2393c02bfc7c -r 4af4a4274ffd common/lib/libc/string/consttime_memequal.c
--- a/common/lib/libc/string/consttime_memequal.c       Wed Mar 18 17:43:20 2015 +0000
+++ b/common/lib/libc/string/consttime_memequal.c       Wed Mar 18 20:11:35 2015 +0000
@@ -1,4 +1,4 @@
-/* $NetBSD: consttime_memequal.c,v 1.5 2014/06/24 16:39:39 drochner Exp $ */
+/* $NetBSD: consttime_memequal.c,v 1.6 2015/03/18 20:11:35 riastradh Exp $ */
 
 /*
  * Written by Matthias Drochner <drochner%NetBSD.org@localhost>.
@@ -18,20 +18,20 @@
 int
 consttime_memequal(const void *b1, const void *b2, size_t len)
 {
-       const char *c1 = b1, *c2 = b2;
-       int res = 0;
+       const unsigned char *c1 = b1, *c2 = b2;
+       unsigned int res = 0;
 
-       while (len --)
+       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
+        * Map 0 to 1 and [1, 256) to 0 using only constant-time
+        * arithmetic.
         *
-        *      return (1 - (1 & ((res - 1) >> 8)));
-        *
-        * or rewritten in assembly.
+        * This is not simply `!res' because although many CPUs support
+        * branchless conditional moves and many compilers will take
+        * advantage of them, certain compilers generate branches on
+        * certain CPUs for `!res'.
         */
-       return !res;
+       return (1 & ((res - 1) >> 8));
 }