author Boris Zbarsky <>
Sat, 02 Feb 2019 03:25:07 +0000
changeset 456513 f39008382451ae45616ff3263421c2affc716cbc
parent 452446 f0a91d36587266d7454a450c6044d573664fbed5
child 478255 89b5e0496b75de48a77f44ab25953bb54b2e0a4b
permissions -rw-r--r--
Bug 1521907 part 6. Start using CheckedUnwrapStatic/Dynamic in toolkit. r=peterv Differential Revision:

/* -*- 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 */


#include "Element.h"
#include "mozilla/Assertions.h"
#include "mozilla/Attributes.h"
#include "mozilla/ReentrancyGuard.h"
#include "mozilla/Likely.h"
#include "nsDebug.h"
#include "mozilla/dom/Document.h"
#include "nsIFrame.h"

namespace mozilla {

 * This helper class helps us to protect against two related issues that can
 * occur in SVG content: reference loops, and reference chains that we deem to
 * be too long.
 * Some SVG effects can reference another effect of the same type to produce
 * a chain of effects to be applied (e.g. clipPath), while in other cases it is
 * possible that while processing an effect of a certain type another effect
 * of the same type may be encountered indirectly (e.g. pattern).  In order to
 * avoid stack overflow crashes and performance issues we need to impose an
 * arbitrary limit on the length of the reference chains that SVG content may
 * try to create.  (Some SVG authoring tools have been known to create absurdly
 * long reference chains.  For example, bug 1253590 details a case where Adobe
 * Illustrator was used to created an SVG with a chain of 5000 clip paths which
 * could cause us to run out of stack space and crash.)
 * This class is intended to be used with the nsIFrame's of SVG effects that
 * may involve reference chains.  To use it add a boolean member, something
 * like this:
 *   // Flag used to indicate whether a methods that may reenter due to
 *   // following a reference to another instance is currently executing.
 *   bool mIsBeingProcessed;
 * Make sure to initialize the member to false in the class' constructons.
 * Then add the following to the top of any methods that may be reentered due
 * to following a reference:
 *   static int16_t sRefChainLengthCounter = AutoReferenceChainGuard::noChain;
 *   AutoReferenceChainGuard refChainGuard(this, &mIsBeingProcessed,
 *                                         &sRefChainLengthCounter);
 *   if (MOZ_UNLIKELY(!refChainGuard.Reference())) {
 *     return; // Break reference chain
 *   }
 * Note that mIsBeingProcessed and sRefChainLengthCounter should never be used
 * by the frame except when it initialize them as indicated above.
class MOZ_RAII AutoReferenceChainGuard {
  static const int16_t sDefaultMaxChainLength = 10;  // arbitrary length

  static const int16_t noChain = -2;

   * @param aFrame The frame for an effect that may involve a reference chain.
   * @param aFrameInUse The member variable on aFrame that is used to indicate
   *   whether one of aFrame's methods that may involve following a reference
   *   to another effect of the same type is currently being executed.
   * @param aChainCounter A static variable in the method in which this class
   *   is instantiated that is used to keep track of how many times the method
   *   is reentered (and thus how long the a reference chain is).
   * @param aMaxChainLength The maximum number of links that are allowed in
   *   a reference chain.
  AutoReferenceChainGuard(nsIFrame* aFrame, bool* aFrameInUse,
                          int16_t* aChainCounter,
                          int16_t aMaxChainLength = sDefaultMaxChainLength
      : mFrame(aFrame),
        mBrokeReference(false) {
    MOZ_ASSERT(aFrame && aFrameInUse && aChainCounter);
    MOZ_ASSERT(aMaxChainLength > 0);
    MOZ_ASSERT(*aChainCounter == noChain ||
               (*aChainCounter >= 0 && *aChainCounter < aMaxChainLength));

  ~AutoReferenceChainGuard() {
    if (mBrokeReference) {
      // We didn't change mFrameInUse or mChainCounter

    *mFrameInUse = false;

    // If we fail this assert then there were more destructor calls than
    // Reference() calls (a consumer forgot to to call Reference()), or else
    // someone messed with the variable pointed to by mChainCounter.
    MOZ_ASSERT(*mChainCounter < mMaxChainLength);


    if (*mChainCounter == mMaxChainLength) {
      *mChainCounter = noChain;  // reset ready for use next time

   * Returns true on success (no reference loop/reference chain length is
   * within the specified limits), else returns false on failure (there is a
   * reference loop/the reference chain has exceeded the specified limits).
   * If it returns false then an error message will be reported to the DevTools
   * console (only once).
  MOZ_MUST_USE bool Reference() {
    if (MOZ_UNLIKELY(*mFrameInUse)) {
      mBrokeReference = true;
      return false;

    if (*mChainCounter == noChain) {
      // Initialize - we start at aMaxChainLength and decrement towards zero.
      *mChainCounter = mMaxChainLength;
    } else {
      // If we fail this assertion then either a consumer failed to break a
      // reference loop/chain, or else they called Reference() more than once
      MOZ_ASSERT(*mChainCounter >= 0);

      if (MOZ_UNLIKELY(*mChainCounter < 1)) {
        mBrokeReference = true;
        return false;

    // We only set these once we know we're returning true.
    *mFrameInUse = true;

    return true;

  void ReportErrorToConsole() {
    nsAutoString tag, id;
    dom::Element* element = mFrame->GetContent()->AsElement();
    const char16_t* params[] = {tag.get(), id.get()};
    auto doc = mFrame->GetContent()->OwnerDoc();
    auto warning = *mFrameInUse ? dom::Document::eSVGRefLoop
                                : dom::Document::eSVGRefChainLengthExceeded;
    doc->WarnOnceAbout(warning, /* asError */ true, params,

  nsIFrame* mFrame;
  bool* mFrameInUse;
  int16_t* mChainCounter;
  const int16_t mMaxChainLength;
  bool mBrokeReference;

}  // namespace mozilla