Fix one-second spin in fx_random.cpp

Take seed generation logic from base's address_space_randomization.cc.
One small tweak is to avoid the bottom three bits of a stack address and
invert, to make leaking ASLR more difficult along the lines of the
freelist masking in base's partition allocator. Another tweak is to
mix in some more time-based information. Another tweak is to add in
the times called so that rapid successive calls return different results.

Bug: pdfium:891
Change-Id: I14238da15cee9c8d4ca72d79e4f7fbb26997c619
Reviewed-on: https://pdfium-review.googlesource.com/13490
Reviewed-by: Henrique Nakashima <hnakashima@chromium.org>
Commit-Queue: Tom Sepez <tsepez@chromium.org>

1 files changed, 24 insertions, 0 deletions

diff --git a/core/fxcrt/fx_random_unittest.cpp b/core/fxcrt/fx_random_unittest.cpp
new file mode 100644
index 0000000000..607f1409b7
--- /dev/null
+++ b/core/fxcrt/fx_random_unittest.cpp
@@ -0,0 +1,24 @@
+// Copyright 2017 PDFium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#include "core/fxcrt/fx_random.h"
+
+#include <array>
+#include <set>
+
+#include "testing/gtest/include/gtest/gtest.h"
+
+TEST(FX_Random, GenerateMT3600times) {
+  // Prove this doesn't spin wait for a second each time.
+  // Since our global seeds are sequential, they wont't collide once
+  // seeded until 2^32 calls, and if the PNRG is any good, we won't
+  // get the same sequence from different seeds, esp. with this few
+  // iterations.
+  std::set<std::array<uint32_t, 16>> seen;
+  std::array<uint32_t, 16> current;
+  for (int i = 0; i < 3600; ++i) {
+    FX_Random_GenerateMT(current.data(), 16);
+    EXPECT_TRUE(seen.insert(current).second);
+  }
+}