Adding upstream version 110.0.1.upstream/110.0.1upstream

Signed-off-by: Daniel Baumann <daniel.baumann@progress-linux.org>

1 files changed, 125 insertions, 0 deletions

diff --git a/mfbt/NonDereferenceable.h b/mfbt/NonDereferenceable.h
new file mode 100644
index 0000000000..30c4cac853
--- /dev/null
+++ b/mfbt/NonDereferenceable.h
@@ -0,0 +1,125 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#ifndef mozilla_NonDereferenceable_h
+#define mozilla_NonDereferenceable_h
+
+/* A pointer wrapper indicating that the pointer should not be dereferenced. */
+
+#include "mozilla/Attributes.h"
+
+#include <cstdint>
+
+// Macro indicating that a function manipulates a pointer that will not be
+// dereferenced, and therefore there is no need to check the object.
+#if defined(__clang__)
+#  define NO_POINTEE_CHECKS __attribute__((no_sanitize("vptr")))
+#else
+#  define NO_POINTEE_CHECKS /* nothing */
+#endif
+
+namespace mozilla {
+
+// NonDereferenceable<T> wraps a raw pointer value of type T*, but prevents
+// dereferencing.
+//
+// The main use case is for pointers that referencing memory that may not
+// contain a valid object, either because the object has already been freed, or
+// is under active construction or destruction (and hence parts of it may be
+// uninitialized or destructed.)
+// Such a pointer may still be useful, e.g., for its numeric value for
+// logging/debugging purposes, which may be accessed with `value()`.
+// Using NonDereferenceable with such pointers will make this intent clearer,
+// and prevent misuses.
+//
+// Note that NonDereferenceable is only a wrapper and is NOT an owning pointer,
+// i.e., it will not release/free the object.
+//
+// NonDereferenceable allows conversions between compatible pointer types, e.g.,
+// to navigate a class hierarchy and identify parent/sub-objects. Note that the
+// converted pointers stay safely NonDereferenceable.
+//
+// Use of NonDereferenceable is required to avoid errors from sanitization tools
+// like `clang++ -fsanitize=vptr`, and should prevent false positives while
+// pointers are manipulated within NonDereferenceable objects.
+//
+template <typename T>
+class NonDereferenceable {
+ public:
+  // Default construction with a null value.
+  NonDereferenceable() : mPtr(nullptr) {}
+
+  // Default copy construction and assignment.
+  NO_POINTEE_CHECKS
+  NonDereferenceable(const NonDereferenceable&) = default;
+  NO_POINTEE_CHECKS
+  NonDereferenceable<T>& operator=(const NonDereferenceable&) = default;
+  // No move operations, as we're only carrying a non-owning pointer, so
+  // copying is most efficient.
+
+  // Construct/assign from a T* raw pointer.
+  // A raw pointer should usually point at a valid object, however we want to
+  // leave the ability to the user to create a NonDereferenceable from any
+  // pointer. Also, strictly speaking, in a constructor or destructor, `this`
+  // points at an object still being constructed or already partially
+  // destructed, which some very sensitive sanitizers could complain about.
+  NO_POINTEE_CHECKS
+  explicit NonDereferenceable(T* aPtr) : mPtr(aPtr) {}
+  NO_POINTEE_CHECKS
+  NonDereferenceable& operator=(T* aPtr) {
+    mPtr = aPtr;
+    return *this;
+  }
+
+  // Construct/assign from a compatible pointer type.
+  template <typename U>
+  NO_POINTEE_CHECKS explicit NonDereferenceable(U* aOther)
+      : mPtr(static_cast<T*>(aOther)) {}
+  template <typename U>
+  NO_POINTEE_CHECKS NonDereferenceable& operator=(U* aOther) {
+    mPtr = static_cast<T*>(aOther);
+    return *this;
+  }
+
+  // Construct/assign from a NonDereferenceable with a compatible pointer type.
+  template <typename U>
+  NO_POINTEE_CHECKS MOZ_IMPLICIT
+  NonDereferenceable(const NonDereferenceable<U>& aOther)
+      : mPtr(static_cast<T*>(aOther.mPtr)) {}
+  template <typename U>
+  NO_POINTEE_CHECKS NonDereferenceable& operator=(
+      const NonDereferenceable<U>& aOther) {
+    mPtr = static_cast<T*>(aOther.mPtr);
+    return *this;
+  }
+
+  // Explicitly disallow dereference operators, so that compiler errors point
+  // at these lines:
+  T& operator*() = delete;   // Cannot dereference NonDereferenceable!
+  T* operator->() = delete;  // Cannot dereference NonDereferenceable!
+
+  // Null check.
+  NO_POINTEE_CHECKS
+  explicit operator bool() const { return !!mPtr; }
+
+  // Extract the pointer value, untyped.
+  NO_POINTEE_CHECKS
+  uintptr_t value() const { return reinterpret_cast<uintptr_t>(mPtr); }
+
+ private:
+  // Let other NonDereferenceable templates access mPtr, to permit construction/
+  // assignment from compatible pointer types.
+  template <typename>
+  friend class NonDereferenceable;
+
+  T* MOZ_NON_OWNING_REF mPtr;
+};
+
+}  // namespace mozilla
+
+#undef NO_POINTEE_CHECKS
+
+#endif /* mozilla_NonDereferenceable_h */