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/* -*- 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).
[[nodiscard]] 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() {
AutoTArray<nsString, 2> params;
dom::Element* element = mFrame->GetContent()->AsElement();
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