From 4ee15987fac1a48460d47ddb7db5a9c65c3ded8f Mon Sep 17 00:00:00 2001
From: Jonas 'Sortie' Termansen <sortie@maxsi.org>
Date: Fri, 20 Jun 2014 18:20:34 +0200
Subject: [PATCH] Add timingsafe_memcmp(3).

---
 libc/Makefile                     |  1 +
 libc/include/string.h             |  1 +
 libc/string/timingsafe_memcmp.cpp | 54 +++++++++++++++++++++++++++++++
 3 files changed, 56 insertions(+)
 create mode 100644 libc/string/timingsafe_memcmp.cpp

diff --git a/libc/Makefile b/libc/Makefile
index ae6f5b97..daa68ab3 100644
--- a/libc/Makefile
+++ b/libc/Makefile
@@ -228,6 +228,7 @@ string/strtok_r.o \
 string/strverscmp.o \
 string/strxfrm_l.o \
 string/strxfrm.o \
+string/timingsafe_memcmp.o \
 time/asctime.o \
 time/asctime_r.o \
 time/gmtime.o \
diff --git a/libc/include/string.h b/libc/include/string.h
index c3baac4b..3bd814cb 100644
--- a/libc/include/string.h
+++ b/libc/include/string.h
@@ -132,6 +132,7 @@ size_t strlcat(char* __restrict, const char* __restrict, size_t);
 size_t strlcpy(char* __restrict, const char* __restrict, size_t);
 /* TODO: strncasecmp_l */
 int strverscmp(const char*, const char*);
+int timingsafe_memcmp(const void*, const void*, size_t);
 #endif
 
 /* Functions that are Sortix extensions. */
diff --git a/libc/string/timingsafe_memcmp.cpp b/libc/string/timingsafe_memcmp.cpp
new file mode 100644
index 00000000..d4ecb2d8
--- /dev/null
+++ b/libc/string/timingsafe_memcmp.cpp
@@ -0,0 +1,54 @@
+/*	$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>
+
+// TODO: I don't fully trust that this code is _always_ timing safe in the
+//       presence of very smart compilers that can prove that this computes the
+//       same value as memcmp and could be short-circuited. We should invent and
+//       rely on compiler extensions that inform the compiler that sensitive
+//       information is involved, which forbids the compiler code generation
+//       from branching on the information (and such).
+// TODO: We should add testcases that verify this is actually timing safe.
+
+extern "C"
+int timingsafe_memcmp(const void* a_ptr, const void* b_ptr, size_t size)
+{
+	const unsigned char* a = (const unsigned char*) a_ptr;
+	const unsigned char* b = (const unsigned char*) b_ptr;
+	int result = 0;
+	int done = 0;
+	for ( size_t i = 0; i < size; i++ )
+	{
+		/* lt is -1 if a[i] < b[i]; else 0. */
+		int lt = (a[i] - b[i]) >> CHAR_BIT;
+
+		/* gt is -1 if a[i] > b[i]; else 0. */
+		int gt = (b[i] - a[i]) >> CHAR_BIT;
+
+		/* cmp is 1 if a[i] > b[i]; -1 if a[i] < b[i]; else 0. */
+		int cmp = lt - gt;
+
+		/* set result = cmp if !done. */
+		result |= cmp & ~done;
+
+		/* set done if a[i] != b[i]. */
+		done |= lt | gt;
+	}
+	return result;
+}