Debugging walkthrough: Diagnosing a __purecall failure
Prerequisite: Understanding what
I was asked to help diagnose an issue in which a program managed to stumble
XYZ!_purecall: 00a14509 a100000000 mov eax,dword ptr ds:[00000000h] ds:0023:00000000=????????
The stack at the point of failure looked like this:
XYZ!_purecall XYZ!CViewFrame::SetFrame+0x14d XYZ!CViewFrame::SetPresentation+0x355 XYZ!CViewFrame::BeginView+0x1fe
The line at
called the mystic
__purecall was a simple
pSomething->AddRef(); // crashes in __purecall
From what we know of
this means that somebody called into a virtual method on a derived
class after the derived class’s destructor has run.
Okay, well, let’s see if we can find the object in question.
Since the method being called is a COM method,
__stdcall calling convention applies,
which means that the
this pointer is on the stack.
0:023> dd esp+4 l1 0529f76c 06a88d58
Using our knowledge of the layout of a COM object, we can navigate through memory to find the vtable.
0:023> dps 06a88d58 06a88d58 009b2eac XYZ!CRegistrationSink::`vftable' 06a88d5c 06b20058 06a88d60 00000002 06a88d64 00998930 XYZ!CObjectWithBrush::`vftable' 06a88d68 00000000 06a88d6c 009c9c80 XYZ!CBrowseSite::`vftable' 06a88d70 009c9c70 XYZ!CBrowseSite::`vftable' 06a88d74 00000000 .... 0:023> dps 009b2eac 009b2eac 00a14509 XYZ!_purecall // virtual QueryInterface() = 0 009b2eb0 00a14509 XYZ!_purecall // virtual AddRef() = 0 009b2eb4 00a14509 XYZ!_purecall // virtual Release() = 0 009b2eb8 009cb1e4 XYZ!CRegistrationSink::Register 009b2ebc 009b3d2d XYZ!CRegistrationSink::Unregister
We see that the object has been destructed down to the
CRegistrationSink base class,
and the attempt to increment its reference count has led us
into the abyss of
But what was this object before it descended into madness?
Well, we know that the object was something derived from
And the other values in memory tell us that the object most
likely also derived from
Just for fun, here’s the
to confirm that we destructed down to that point:
00998930 00a14509 XYZ!_purecall // virtual QueryInterface() = 0 00998934 00a14509 XYZ!_purecall // virtual AddRef() = 0 00998938 00a14509 XYZ!_purecall // virtual Release() = 0 0099893c 0099880d XYZ!CObjectWithBrush::SetBrush 00998940 00a319ee XYZ!CObjectWithBrush::GetBrush 00998944 00a13fd9 XYZ!CObjectWithBrush::`scalar deleting destructor'
Ooh, it looks like
CObjectWithBrush has a
Probably to destroy the brush.
A check of the source code tells us that nobody derives from
CBrowseSite, so that is almost certainly the
original object type.
As a cross-check, we check whether what we have matches
the memory layout of a
0:023> dt XYZ!CBrowseSite 06a88d58 +0x000 __VFN_table : 0x009b2eac +0x004 m_prgreg : 0x06a88d58 Registration +0x008 m_creg : 2 +0x00c __VFN_table : 0x00998930 +0x010 m_hbr : (null) +0x014 __VFN_table : 0x009c9c80 +0x018 __VFN_table : 0x009c9c70 +0x01c m_cRef : 0
Looks not unreasonable.
(Well, aside from the fact that we have a bug…)
The object has most likely begun its destruction because its
reference count (
_cRef) went to zero.
At this point, there was enough information to ask the developers
CBrowseSite to work out
CViewFrame ended up running around with a pointer
to an object that has already been destructed.