Missing null pointer check in MouseRelatedEvent::receivedTarget()

[Andreas Kling]

Created attachment 48802 [details]
Backtrace

Original bugreport: http://bugreports.qt.nokia.com/browse/QTBUG-5020

Compiler: Microsoft (R) 32-bit C/C++ Optimizing Compiler Version 15.00.30729.01 for 80x86 

I can't provide you with exact steps to reproduce because bug is very rare and hard to reproduce, but I found a root cause of the bug. 
We use Qt for developing proprietary application with UI partially written with the JavaScript-rich HTML that intensively uses Qt plugins and webkit HTML5 extensions. In some unforeseen circumstances application unexpectedly crashes in $(Qt)/src/3rdparty/webkit/webcore/dom/mouserelatedevent.cpp, in the MouseRelatedEvent::receivedTarget method.

There is a following code in MouseRelatedEvent::receivedTarget: 
======================================================== 
Node* n = targ; 
while (n && !n->renderer()) 
n = n->parent(); 
if (n) { 
RenderLayer* layer = n->renderer()->enclosingLayer(); 
layer->updateLayerPosition(); 
for (; layer; layer = layer->parent()) { 
m_layerX -= layer->xPos(); 
m_layerY -= layer->yPos(); 
} 
} 
======================================================== 

in some circumstances layer is NULL in call to layer->updateLayerPosition() in the code above. 
Null pointer check introduced in for loop, I believe that it shall be introduced above the call to updateLayerPosition, in other words code shall be rewritten as follows: 

Node* n = targ; 
while (n && !n->renderer()) 
n = n->parent(); 
if (n) { 
RenderLayer* layer = n->renderer()->enclosingLayer(); 
if (layer) { 
layer->updateLayerPosition(); 

// this is to avoid redundant null pointer check in the first iteration 
do 
{ 
m_layerX -= layer->xPos(); 
m_layerY -= layer->yPos(); 
} 
while (layer = layer->parent()); 
} 
}

[Emil A Eklund]

Fixed in r82225: <http://trac.webkit.org/changeset/82225>