WRL and the ComPtr

More recently I have been forced out of my comfort zone of C-style systems programming where you manage your own memory and concurrency and in to the world of writing code for WinRT ABI.

In case you don't know, WinRT (universal, modern, tailored, Windows Store?) apps can be written in C#, C++, or even *gasp* JavaScript.  It actually doesn't matter which laguage you use, because they all thunk down to the WinRT ABI.  The ABI is native (read: c++) code, so it is fast and efficient.  Under the covers, with WinRT API is just COM, or more accurately modern COM.  The MIDL syntax has been updated to make way modern runtime APIs.

How does a systems programmer write modern COM for ABI code?  In short, the Windows Runtime Library or WRL. Modern COM still has classic COM under the hood.  One thing they tried to do was to abstract away some of the error prone aspects of COM while making it more developer-friendly.  In some ways it is similar in purpose to ATL but without all of the ATL grossness.  It uses ComPtr instead of CComPtr for smart pointers, and I don't totally hate them like I did CComPtrs.  Modern COM uses a lot of templates types, and doesn't use exceptions.  Most Windows systems programmers avoid exceptions to make the code easier to debug with the KD.  Personally I also hate debugging templated code, but I guess it is needed for the generic programming concepts to work.

ComPtr

ComPtr is the Modern COM smart pointer.  Its behavior is similar to that of the ^ (hat) operator in WinRT CX code.  ComPtr is only for COM objects, and the lifetime automatically managed.  The ^ can be a smart pointer for non-COM things.

Managing ASSERT & NT_ASSERT in WinDBG & KD

What are ASSERTs and how to use them:

In general, Windows developers may use ASSERT and NT_ASSERT to validate excepted assumptions in their code.  In CHK (checked non-optimized) binaries, an exception will be thrown if the expression does not evaluate to true.

For example, you may have an assumption in a function, that a pointer should never be NULL and the buffer size should be less than 1k.  Then you can write an ASSERT like this:

ASSERT(pBuff && (cbBuff < 1024));

This assertion will fire if the expression is false, or in this case: !pBuff || cbBuff >= 1024.

0:003> p
Assertion s:\dllsrv\info.cpp(83): pBuff && (cbBuff < 1024)
...

There are differences between the two, but normally it is better to use NT_ASSERT, but this is not what this article is about.  There is also a NT_VERIFY which will assert even in FRE (free optimized binaries), but I can 't think of any reasons of the top of my head why you would need to use it.

As a rule of thumb, you shouldn't use ASSERTs too heavily.  For instance, in my example, you can validate the same thing using SAL annotations where bugs can be found at compile time with static analysis tools like prefast which is always preferable to runtime debugging.  You may consider adding ASSERTs while developing your code, and removing a bunch of them once it has stabilized.

ASSERTs in WinDBG & KD

Normally when I am testing my code, I run my code under WinDBG or KD.  I normally copy private CHK versions of my binaries with symbols.  It is always a good idea to enable application verifier as well, or use gflags to add standard checks.

In the simplest form, lets say your ASSERT throws an exception because it evaluates false, you do the normal debugging thing and figure out why your assumptions were not correct, fix the bug(s), replace the binary, and restart the test.  That is the point of the asset after all.

But, lets say you want to continue on.  What are the commands to do that?

gh - this is the basic one.  It is like pressing g or F5 except you are telling to not worry about the exception.

ah - you use ah to control a specific assert.  For instance, you can use ahi to ignore an assertion, if it is noisy and you don't want or can't change the binary to remove or fix it.

sx - is used to control exceptions globally.  sx* asrt will disable all asserts for instance.

Postmortem Debugging with WinDBG or NTSD

I have mentioned several times before that using application verifier will help root cause and solve many bugs like for example memory bugs (leaks, heap corruptions, etc.), locking bugs, etc.  If in user mode (UM) you can just attach the debugger to your UM process and run your tests until in breaks in, or if in KM you would use a KD but the same thing applies.  

Let’s say that you have a heap corruption that is hard to track down in your UM process.  Normally you would do this:

1.       Isolate your code into a single process with a unique image name.

2.       Enable page heap on that image name using gflags.

3.       Start that process and attach a debugger (windbg)

4.       Reproduce the issue that causes the corruption and watch the debugger break in

5.       Then use the handy !heap –triage command to give you some more clues as to what is going on

Alternatively depending on what the memory problem is, in step 2, application verifier could also help.  It can be enable on your image with appverif.exe or gflags.exe.  If in KM, use driver verifier and KD. 

However with some bugs relating to race conditions, or memory corruptions, enabling these tools on your process or attaching the debugger its self could be enough to make it so that the issue doesn’t repro anymore.  That can be frustrating.

If directly debugging your process causes you to lose the repro, there are a few other techniques you can employ:

-          Tracing: adding WPP tracing to your code can be a great way to trouble shoot some issues with your code that are hard to capture with a debugger.  This is an especially good method for race conditions and deadlocks that don’t hit with a debugger.

-          KD: you can set up your machine for kernel debugging.  When you UM process has an unhandled exception, then the KD will break in.  KD is always a solid choice, but kernel debugging may be harder for some than UM debugging.  Also, some machines are hard to setup for KD like Ultrabooks where the debugable USB port may be internal to the chassis and soldered to the web cam, or it is an ARM tablet.

-          WER: Windows Error Reporting can be configured to capture dumps of your process when it crashes.  You can debug the dump after the fact.  Dump files are not as nice as debugging a live machine, but this could be a great option where your code is deployed to a lot of machines, and you want to see what happened on a machine where the bug repros.

-          Postmortem Debugging: you can configure Windows to launch and attach a debugger in response to a un handled exception.  This is called postmortem debugging.

All of these have their uses, but I will focus on postmortem debugging because it was the only way I could chase down a heap corruption that I have been trying to root cause for the past few days.

First off, you can use this link to read about it in detail if you like:

http://msdn.microsoft.com/en-us/library/windows/hardware/ff542967(v=vs.85).aspx

The basics are:

-          Simply type in an elevated command prompt “windbg –I” to make windbg the postmortem debugger.  This works great for processes in your login session. 

-          Use NTSD if you need to used named pipe debugging.  Type “ntsd –iae”.  You can use the –iaec “extra options” to add extra options, but it doesn’t work for –server like you need for name pipe debugging.  You need to manually edit the reg key  at \\HKEY_LOCAL_MACHINE\Software\Microsoft\Windows NT\CurrentVersion\AeDebug. 

GFlags.exe and Pageheap

I was tracking down a heap corruption today, and it occurred to me that I haven't mentioned how to enable pageheap.  For instance, when you use the handy "!heap -traige", if there was a heap corruption, sometimes it will tell to try the repro again with pageheap enabled.  How do you do that?

There are actually several ways to enable pageheap, but I will only talk about one, modifying the global flags using gflags.exe.  It should get installed when you install windbg.

Basically for a user mode (UM) process, you go to the image file tab, and type in the name of your exe.  If you run in a svchost.exe, then you should probably break out your service to a uniquely named servicehost.exe.  I use myhost.exe normally for debugging a service.  After that, check "Enable page heap."

Next, you need to run the code in question under  the debugger, and then you reproduce the heap corruption.  Normally the debugger will break in This time you will be able to get a lot more useful information out of !heap and !analyze.

See the OS Version in WinDBG

I seem to forget this command every so often, so it is ripe for a post.  Often I will be debugging a remote KD from some other team, and I want to know what version of the OS I am debugging.  The command for this is vertarget.  Super obvious.  Obviously the next most obvious command would be version, which is for the version of the debugger, and ver which is nothing.

Here is some sample output of vertarget:

0:005> vertarget
Windows 8 Version 9200 MP (4 procs) Free x64 <= looks like the output of GetVersionEx which lies after 8, and always says it Windows 8.
Product: WinNt, suite: SingleUserTS
kernel32.dll version: 6.3.9418.0 (winmain.130530-1753) <= This looks the actual version you would get fro m RtlGetVersion
Machine Name: "bob"
Debug session time: Fri May 31 19:12:24.000 2013 (UTC - 7:00)
System Uptime: 0 days 1:03:12.334
Process Uptime: 0 days 0:16:35.000
  Kernel time: 0 days 0:00:00.000
  User time: 0 days 0:00:00.000

This is what it looks like from a KD:
0: kd> vertarget
Windows 8 Kernel Version 9658 MP (4 procs) Free x64
Product: WinNt, suite: TerminalServer SingleUserTS
Built by: 9658.0.amd64fre.winmain.131120-1618
Machine Name: "bobby-brown"
Kernel base = 0xfffff801`d3e15000 PsLoadedModuleList = 0xfffff801`d40dfb90
Debug session time: Fri Nov 22 14:28:29.746 2013 (UTC - 8:00)
System Uptime: 0 days 0:45:58.445

WinDBG: dt vs ??

dt

The dt command stands for display type.

usage:

dt var

dt address

??

The ?? command is for evaluating a C++ expression.  You can use it like dt, but you can use it to do even more.  I personally like to just use it most of the time because C are more accessible to me often than trying to remember which switches I need to use with dt.

usage:

?? sizeof(var)

?? this

?? this->m_member_obj

?? array[25].member_ptr->val

?? g_some_global

Checking an Event's Signaled State in KM w/ WinDBG

I've talked about debugging events in the past in UM in simple terms.  !handle works great, and although is use is slightly different in UM vs KM it provides a lot of good information, but lets say you are in KD and want to know if your event is signaled or not.  !handle will not tell you that.  This is how you do it.  Note: you need to have a KD to look at the kernel's objects.

1. Find the index of handle


1: kd> dt g_hMyEvent
my_bin!g_hMyEvent
0x00001218
Void

2. Use !handle to get the address of the event object.

1: kd> !handle 0x00001218 f -1 Event

Searching for handles of type Event

PROCESS 845f4b40  SessionId: 0  Cid: 040c    Peb: 7fbb4000  ParentCid: 023c
    DirBase: 7ffea200  ObjectTable: a194d280  HandleCount: 1071.
    Image: svchost.exe

Handle table at a194d280 with 1071 entries in use

1218: Object: aa934288  GrantedAccess: 001f0003 Entry: bb591430
Object: aa934288  Type: (845692b8) Event
    ObjectHeader: aa934270 (new version)
        HandleCount: 1  PointerCount: 62

3. Cast that pointer to the dispatch header it is

1: kd> ?? (PDISPATCHER_HEADER)0xaa934288

PDISPATCHER_HEADER 0xaa934288

   +0x000 Type             : 0 ''

   +0x001 TimerControlFlags : 0 ''

   +0x001 Absolute         : 0y0

   +0x001 Wake             : 0y0

   +0x001 EncodedTolerableDelay : 0y000000 (0)

   +0x001 Abandoned        : 0 ''

   +0x001 Signalling       : 0 ''

   +0x002 ThreadControlFlags : 0x4 ''

   +0x002 CycleProfiling   : 0y0

   +0x002 CounterProfiling : 0y0

   +0x002 GroupScheduling  : 0y1

   +0x002 AffinitySet      : 0y0

   +0x002 Reserved         : 0y0000

   +0x002 Hand             : 0x4 ''

   +0x002 Size             : 0x4 ''

   +0x003 TimerMiscFlags   : 0 ''

   +0x003 Index            : 0y0

   +0x003 Processor        : 0y00000 (0)

   +0x003 Inserted         : 0y0

   +0x003 Expired          : 0y0

   +0x003 DebugActive      : 0 ''

   +0x003 DpcActive        : 0 ''

   +0x000 Lock             : 0n262144

   +0x000 LockNV           : 0n262144

   +0x004 SignalState      : 0n1

   +0x008 WaitListHead     : _LIST_ENTRY [ 0xaa934290 - 0xaa934290 ]

There you go, you can see this event is in a signaled state.

By the way, the member Type 0 means event notification.  Other common values:

    EventNotificationObject = 0,
    EventSynchronizationObject = 1,
    MutantObject = 2,
    ProcessObject = 3,
    QueueObject = 4,
    SemaphoreObject = 5,
    ThreadObject = 6,