layout/svg/AutoReferenceChainGuard.h

   1 /* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
   2 /* vim: set ts=8 sts=2 et sw=2 tw=80: */
   3 /* This Source Code Form is subject to the terms of the Mozilla Public
   4  * License, v. 2.0. If a copy of the MPL was not distributed with this
   5  * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
   6
   7 #ifndef LAYOUT_SVG_AUTOREFERENCECHAINGUARD_H_
   8 #define LAYOUT_SVG_AUTOREFERENCECHAINGUARD_H_
   9
  10 #include "Element.h"
  11 #include "mozilla/Assertions.h"
  12 #include "mozilla/Attributes.h"
  13 #include "mozilla/ReentrancyGuard.h"
  14 #include "mozilla/Likely.h"
  15 #include "nsDebug.h"
  16 #include "mozilla/dom/Document.h"
  17 #include "nsIFrame.h"
  18
  19 namespace mozilla {
  20
  21 /**
  22  * This helper class helps us to protect against two related issues that can
  23  * occur in SVG content: reference loops, and reference chains that we deem to
  24  * be too long.
  25  *
  26  * Some SVG effects can reference another effect of the same type to produce
  27  * a chain of effects to be applied (e.g. clipPath), while in other cases it is
  28  * possible that while processing an effect of a certain type another effect
  29  * of the same type may be encountered indirectly (e.g. pattern).  In order to
  30  * avoid stack overflow crashes and performance issues we need to impose an
  31  * arbitrary limit on the length of the reference chains that SVG content may
  32  * try to create.  (Some SVG authoring tools have been known to create absurdly
  33  * long reference chains.  For example, bug 1253590 details a case where Adobe
  34  * Illustrator was used to created an SVG with a chain of 5000 clip paths which
  35  * could cause us to run out of stack space and crash.)
  36  *
  37  * This class is intended to be used with the nsIFrame's of SVG effects that
  38  * may involve reference chains.  To use it add a boolean member, something
  39  * like this:
  40  *
  41  *   // Flag used to indicate whether a methods that may reenter due to
  42  *   // following a reference to another instance is currently executing.
  43  *   bool mIsBeingProcessed;
  44  *
  45  * Make sure to initialize the member to false in the class' constructons.
  46  *
  47  * Then add the following to the top of any methods that may be reentered due
  48  * to following a reference:
  49  *
  50  *   static int16_t sRefChainLengthCounter = AutoReferenceChainGuard::noChain;
  51  *
  52  *   AutoReferenceChainGuard refChainGuard(this, &mIsBeingProcessed,
  53  *                                         &sRefChainLengthCounter);
  54  *   if (MOZ_UNLIKELY(!refChainGuard.Reference())) {
  55  *     return; // Break reference chain
  56  *   }
  57  *
  58  * Note that mIsBeingProcessed and sRefChainLengthCounter should never be used
  59  * by the frame except when it initialize them as indicated above.
  60  */
  61 class MOZ_RAII AutoReferenceChainGuard {
  62   static const int16_t sDefaultMaxChainLength = 10;  // arbitrary length
  63
  64  public:
  65   static const int16_t noChain = -2;
  66
  67   /**
  68    * @param aFrame The frame for an effect that may involve a reference chain.
  69    * @param aFrameInUse The member variable on aFrame that is used to indicate
  70    *   whether one of aFrame's methods that may involve following a reference
  71    *   to another effect of the same type is currently being executed.
  72    * @param aChainCounter A static variable in the method in which this class
  73    *   is instantiated that is used to keep track of how many times the method
  74    *   is reentered (and thus how long the a reference chain is).
  75    * @param aMaxChainLength The maximum number of links that are allowed in
  76    *   a reference chain.
  77    */
  78   AutoReferenceChainGuard(nsIFrame* aFrame, bool* aFrameInUse,
  79                           int16_t* aChainCounter,
  80                           int16_t aMaxChainLength = sDefaultMaxChainLength)
  81       : mFrame(aFrame),
  82         mFrameInUse(aFrameInUse),
  83         mChainCounter(aChainCounter),
  84         mMaxChainLength(aMaxChainLength),
  85         mBrokeReference(false) {
  86     MOZ_ASSERT(aFrame && aFrameInUse && aChainCounter);
  87     MOZ_ASSERT(aMaxChainLength > 0);
  88     MOZ_ASSERT(*aChainCounter == noChain ||
  89                (*aChainCounter >= 0 && *aChainCounter < aMaxChainLength));
  90   }
  91
  92   ~AutoReferenceChainGuard() {
  93     if (mBrokeReference) {
  94       // We didn't change mFrameInUse or mChainCounter
  95       return;
  96     }
  97
  98     *mFrameInUse = false;
  99
 100     // If we fail this assert then there were more destructor calls than
 101     // Reference() calls (a consumer forgot to to call Reference()), or else
 102     // someone messed with the variable pointed to by mChainCounter.
 103     MOZ_ASSERT(*mChainCounter < mMaxChainLength);
 104
 105     (*mChainCounter)++;
 106
 107     if (*mChainCounter == mMaxChainLength) {
 108       *mChainCounter = noChain;  // reset ready for use next time
 109     }
 110   }
 111
 112   /**
 113    * Returns true on success (no reference loop/reference chain length is
 114    * within the specified limits), else returns false on failure (there is a
 115    * reference loop/the reference chain has exceeded the specified limits).
 116    * If it returns false then an error message will be reported to the DevTools
 117    * console (only once).
 118    */
 119   [[nodiscard]] bool Reference() {
 120     if (MOZ_UNLIKELY(*mFrameInUse)) {
 121       mBrokeReference = true;
 122       ReportErrorToConsole();
 123       return false;
 124     }
 125
 126     if (*mChainCounter == noChain) {
 127       // Initialize - we start at aMaxChainLength and decrement towards zero.
 128       *mChainCounter = mMaxChainLength;
 129     } else {
 130       // If we fail this assertion then either a consumer failed to break a
 131       // reference loop/chain, or else they called Reference() more than once
 132       MOZ_ASSERT(*mChainCounter >= 0);
 133
 134       if (MOZ_UNLIKELY(*mChainCounter < 1)) {
 135         mBrokeReference = true;
 136         ReportErrorToConsole();
 137         return false;
 138       }
 139     }
 140
 141     // We only set these once we know we're returning true.
 142     *mFrameInUse = true;
 143     (*mChainCounter)--;
 144
 145     return true;
 146   }
 147
 148  private:
 149   void ReportErrorToConsole() {
 150     AutoTArray<nsString, 2> params;
 151     dom::Element* element = mFrame->GetContent()->AsElement();
 152     element->GetTagName(*params.AppendElement());
 153     element->GetId(*params.AppendElement());
 154     auto doc = mFrame->GetContent()->OwnerDoc();
 155     auto warning = *mFrameInUse ? dom::Document::eSVGRefLoop
 156                                 : dom::Document::eSVGRefChainLengthExceeded;
 157     doc->WarnOnceAbout(warning, /* asError */ true, params);
 158   }
 159
 160   nsIFrame* mFrame;
 161   bool* mFrameInUse;
 162   int16_t* mChainCounter;
 163   const int16_t mMaxChainLength;
 164   bool mBrokeReference;
 165 };
 166
 167 }  // namespace mozilla
 168
 169 #endif  // LAYOUT_SVG_AUTOREFERENCECHAINGUARD_H_