Fix 32-bit mmap/mmap64 handling of negative offsets.

We don't actually need to worry about sign extension if we reject
negative values ourselves. Previously it was possible to come up
with negative but aligned values that we would pass to the kernel;
in the case of mmap (as opposed to mmap64) we'd incorrectly turn
those into large positive offsets.

Change-Id: I2aa583e0f892d59bb77429aea8730b72db32dcb0

tests/sys_mman_test.cpp

@@ -0,0 +1,30 @@
+/*
+ * Copyright (C) 2014 The Android Open Source Project
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+#include <gtest/gtest.h>
+
+#include <sys/mman.h>
+#include <unistd.h>
+
+TEST(sys_mman, mmap_negative) {
+  off_t off = -sysconf(_SC_PAGESIZE); // Aligned but negative.
+  ASSERT_EQ(MAP_FAILED, mmap(NULL, 4096, PROT_READ|PROT_WRITE, MAP_ANON|MAP_PRIVATE, -1, off));
+}
+
+TEST(sys_mman, mmap64_negative) {
+  off64_t off64 = -sysconf(_SC_PAGESIZE); // Aligned but negative.
+  ASSERT_EQ(MAP_FAILED, mmap64(NULL, 4096, PROT_READ|PROT_WRITE, MAP_ANON|MAP_PRIVATE, -1, off64));
+}