Add timingsafe_memcmp()

This function is used by LibreSSL and OpenSSH and is provided by the
OpenBSD libc.

	* libc/include/string.h (timingsafe_memcmp): Declare.
	* libc/string/timingsafe_memcmp.c: New file.
	* libc/string/Makefile.am: Add new file.
	* libc/string/Makefile.in: Regenerate.

1 files changed, 46 insertions, 0 deletions

diff --git a/newlib/libc/string/timingsafe_memcmp.c b/newlib/libc/string/timingsafe_memcmp.c
new file mode 100644
index 000000000..04e2ac5e2
--- /dev/null
+++ b/newlib/libc/string/timingsafe_memcmp.c
@@ -0,0 +1,46 @@
+/*	$OpenBSD: timingsafe_memcmp.c,v 1.1 2014/06/13 02:12:17 matthew Exp $	*/
+/*
+ * 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 <limits.h>
+#include <string.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);
+}