Introduce CFX_UnownedPtr to detect lifetime inversion issues.

There are places where an object "child" has a raw pointer
back to object "owner" with the understanding that owner will
always outlive child.

Violating this constraint can lead to use after free, but this
requires finding two paths: one that frees the objects in the
wrong order, and one that uses the object after the free. The
purpose of this patch is to detect the constraint violation
even when the second path is not hit.

We create a template that is used in place of TYPE*. It's dtor,
when a memory tool is present, goes out and probes the first
byte of the object to which it points.  Used in "child", this
allows the memory tool to prove that the "owner" is still alive
at the time the child is destroyed, and hence the constraint is
never violated.

Change-Id: I2a6d696d51dda4a79ee2f00a6752965e058a6417
Reviewed-on: https://pdfium-review.googlesource.com/5475
Commit-Queue: Tom Sepez <tsepez@chromium.org>
Reviewed-by: dsinclair <dsinclair@chromium.org>
Reviewed-by: Lei Zhang <thestig@chromium.org>

2 files changed, 150 insertions, 0 deletions

diff --git a/core/fxcrt/cfx_unowned_ptr.h b/core/fxcrt/cfx_unowned_ptr.h
new file mode 100644
index 0000000000..11f4e8e38b
--- /dev/null
+++ b/core/fxcrt/cfx_unowned_ptr.h
@@ -0,0 +1,64 @@
+// 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.
+
+#ifndef CORE_FXCRT_CFX_UNOWNED_PTR_H_
+#define CORE_FXCRT_CFX_UNOWNED_PTR_H_
+
+#include <functional>
+#include <memory>
+#include <type_traits>
+#include <utility>
+
+#include "core/fxcrt/fx_memory.h"
+
+template <class T>
+class CFX_UnownedPtr {
+ public:
+  CFX_UnownedPtr() {}
+  CFX_UnownedPtr(const CFX_UnownedPtr& that) : CFX_UnownedPtr(that.Get()) {}
+
+  template <typename U>
+  explicit CFX_UnownedPtr(U* pObj) : m_pObj(pObj) {}
+
+  // Deliberately implicit to allow returning nullptrs.
+  // NOLINTNEXTLINE(runtime/explicit)
+  CFX_UnownedPtr(std::nullptr_t ptr) {}
+
+#if defined(MEMORY_TOOL_REPLACES_ALLOCATOR)
+  ~CFX_UnownedPtr() {
+    if (m_pObj)
+      reinterpret_cast<volatile uint8_t*>(m_pObj)[0];
+  }
+#endif
+
+  CFX_UnownedPtr& operator=(T* that) {
+    m_pObj = that;
+    return *this;
+  }
+  CFX_UnownedPtr& operator=(const CFX_UnownedPtr& that) {
+    if (*this != that)
+      m_pObj = that.Get();
+    return *this;
+  }
+
+  bool operator==(const CFX_UnownedPtr& that) const {
+    return Get() == that.Get();
+  }
+  bool operator==(const T* that) const { return Get() == that; }
+  bool operator!=(const CFX_UnownedPtr& that) const { return !(*this == that); }
+  bool operator!=(const T* that) const { return !(*this == that); }
+  bool operator<(const CFX_UnownedPtr& that) const {
+    return std::less<T*>()(Get(), that.Get());
+  }
+
+  T* Get() const { return m_pObj; }
+  explicit operator bool() const { return !!m_pObj; }
+  T& operator*() const { return *m_pObj; }
+  T* operator->() const { return m_pObj; }
+
+ private:
+  T* m_pObj = nullptr;
+};
+
+#endif  // CORE_FXCRT_CFX_UNOWNED_PTR_H_
diff --git a/core/fxcrt/cfx_unowned_ptr_unittest.cpp b/core/fxcrt/cfx_unowned_ptr_unittest.cpp
new file mode 100644
index 0000000000..7d29faedf5
--- /dev/null
+++ b/core/fxcrt/cfx_unowned_ptr_unittest.cpp
@@ -0,0 +1,86 @@
+// 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/cfx_unowned_ptr.h"
+
+#include <utility>
+#include <vector>
+
+#include "testing/fx_string_testhelpers.h"
+#include "testing/gtest/include/gtest/gtest.h"
+
+namespace {
+
+class Clink {
+ public:
+  CFX_UnownedPtr<Clink> next_ = nullptr;
+};
+
+void DeleteDangling() {
+  Clink* ptr1 = new Clink();
+  Clink* ptr2 = new Clink();
+  ptr2->next_ = ptr1;
+  delete ptr1;
+  delete ptr2;
+}
+
+}  // namespace
+
+TEST(fxcrt, UnownedPtrOk) {
+  Clink* ptr1 = new Clink();
+  Clink* ptr2 = new Clink();
+  ptr2->next_ = ptr1;
+  delete ptr2;
+  delete ptr1;
+}
+
+TEST(fxcrt, UnownedPtrNotOk) {
+#if defined(MEMORY_TOOL_REPLACES_ALLOCATOR)
+  EXPECT_DEATH(DeleteDangling(), "");
+#else
+  DeleteDangling();
+#endif
+}
+
+TEST(fxcrt, OperatorEQ) {
+  int foo;
+  CFX_UnownedPtr<int> ptr1;
+  EXPECT_TRUE(ptr1 == ptr1);
+
+  CFX_UnownedPtr<int> ptr2;
+  EXPECT_TRUE(ptr1 == ptr2);
+
+  CFX_UnownedPtr<int> ptr3(&foo);
+  EXPECT_TRUE(ptr3 == &foo);
+  EXPECT_FALSE(ptr1 == ptr3);
+
+  ptr1 = &foo;
+  EXPECT_TRUE(ptr1 == ptr3);
+}
+
+TEST(fxcrt, OperatorNE) {
+  int foo;
+  CFX_UnownedPtr<int> ptr1;
+  EXPECT_FALSE(ptr1 != ptr1);
+
+  CFX_UnownedPtr<int> ptr2;
+  EXPECT_FALSE(ptr1 != ptr2);
+
+  CFX_UnownedPtr<int> ptr3(&foo);
+  EXPECT_FALSE(ptr3 != &foo);
+  EXPECT_TRUE(ptr1 != ptr3);
+
+  ptr1 = &foo;
+  EXPECT_FALSE(ptr1 != ptr3);
+}
+
+TEST(fxcrt, OperatorLT) {
+  int foos[2];
+  CFX_UnownedPtr<int> ptr1(&foos[0]);
+  CFX_UnownedPtr<int> ptr2(&foos[1]);
+
+  EXPECT_FALSE(ptr1 < ptr1);
+  EXPECT_TRUE(ptr1 < ptr2);
+  EXPECT_FALSE(ptr2 < ptr1);
+}