Windows Hotpatching: A Walkthrough

As discussed in the last post, Windows 2003 SP1 introduced a technology known as Hotpatching. An integral part of this technology is Hotpatching, which refers to the process of applying an updated on the fly by using runtime code modification techniques.

Although Hotpatching has caught a bit of attention, suprisingly little information has been published about its inner workings. As the technology is patented, however, there is quite a bit of information that can be obtained by reading the patent description. Moreover, there is this (admittedly very terse) discussion about the actual implementation of hotpatching.

Armed with this information, it is possible to get into more detail by looking what is actually happening under the hood when a hoftix is applied: I did so and chose KB911897 as an example, which fixes some flaw in mrxsmb.sys and rdbss.sys. I have also gone through the hassle of translating key parts of the respective assembly code back to C.

Preparing the machine

First, we need a proper machine image which can be used for the experiment. Unfortunately, KB911897 is an SP1 package, so we have to use an old Win 2003 Server SP1 system to apply this update. Once we have the machine running, we can attach the kernel debugger and see what is happening when the hotfix is installed.

Observing the update

When launched with /hotpatch:enable, after some initialization work, the updater calls NtSetSystemInformation (which delegates to ExApplyCodePatch) to apply the hotpatch. Hotpatching includes a coldpatch, which I do not care about here and the actual hotpatch. The first two calls to NtSetSystemInformation (and thus to ExApplyCodePatch) are coldpatching-related and I will thus ignore them here. The third call, however, is made to apply the actual hotpatch, so let’s observe this one further.

Requiring a kernel mode-patch, ExApplyCodePatch then calls MmHotPatchRoutine, which is where the fun starts. Expressed in C, MmHotPatchRoutine, MmHotPatchRoutine roughly looks like this (reverse engineered from assembly, might be slightly incorrect):

NTSTATUS MmHotPatchRoutine(
  __in PSYSTEM_HOTPATCH_CODE_INFORMATION RemoteInfo
  )
{
  UNICODE_STRING ImageFileName;
  DWORD Flags = RemoteInfo->Flags;
  PVOID ImageBaseAddress;
  PVOID ImageHandle;
  NTSTATUS Status, LoadStatus;
  KTHREAD CurrentThread;

  ImageFileName.Length = RemoteInfo->KernelInfo.NameLength;
  ImageFileName.MaximumLength = RemoteInfo->KernelInfo.NameLength;
  ImageFileName.Buffer = ( PBYTE ) RemoteInfo + NameOffset;

  CurrentThread = KeGetCurrentThread();
  KeEnterCriticalRegion( CurrentThread );

  KeWaitForSingleObject(
    MmSystemLoadLock,
    WrVirtualMemory,
    0,
    0,
    0 );

  LoadStatus = MmLoadSystemImage(
    &ImageFileName,
    0,
    0,
    0,
    &ImageHandle,
    &ImageBaseAddress );
  if ( NT_SUCCESS( Status ) || Status == STATUS_IMAGE_ALREADY_LOADED )
  {

    Status = MiPerformHotPatch(
      ImageHandle,
      ImageBaseAddress,
      Flags );
    
    if ( NT_SUCCESS( Status ) || LoadStatus == STATUS_IMAGE_ALREADY_LOADED )
    {
      NOTHING;
    }
    else
    {
      MmUnloadSystemImage( ImageHandle );
    }
    
    LoadStatus = Status;
  }


  KeReleaseMutant(
    MmSystemLoadLock,
    1,  // increment
    FALSE,
    FALSE );

  KeLeaveCriticalRegion( CurrentThread );

  return LoadStatus;
}

As you see in the code, MmHotPatchRoutine will try load the hotpatch image — we can verify this in the debugger:

kd> bp nt!MmLoadSystemImage

kd> g
Breakpoint 3 hit
nt!MmLoadSystemImage:
808ec4b5 6878010000      push    178h

kd> k
ChildEBP RetAddr  
f6acbb28 80990c9e nt!MmLoadSystemImage
f6acbb68 809b2d67 nt!MmHotPatchRoutine+0x59
f6acbba8 808caeff nt!ExApplyCodePatch+0x191
f6acbd50 8082337b nt!NtSetSystemInformation+0xa1e
f6acbd50 7c82ed54 nt!KiFastCallEntry+0xf8
0006bc50 7c821f24 ntdll!KiFastSystemCallRet
0006bd44 7c8304c9 ntdll!ZwSetSystemInformation+0xc
[...]

kd> dt _UNICODE_STRING poi(@esp+4)
ntdll!_UNICODE_STRING
 "\??\c:\windows\system32\drivers\hpf3.tmp"
   +0x000 Length           : 0x50
   +0x002 MaximumLength    : 0x50
   +0x004 Buffer           : 0x81623fa8  "\??\c:\windows\system32\drivers\hpf3.tmp"
   
kd> gu

kd> lm
start    end        module name
[...]           
f6ba4000 f6bad000   hpf3       (deferred)  
[...]
f95cb000 f9641000   mrxsmb     (deferred)  
f9641000 f9671000   rdbss      (deferred)      
[...]

Having loaded the hotpatch image, MmHotPatchRoutine proceeds be calling MiPerformHotPatch, which looks about like this:

NTSTATUS
MiPerformHotPatch(
  IN PLDR_DATA_TABLE_ENTRY ImageHandle,
  IN PVOID ImageBaseAddress,
  IN DWORD Flags
  )
{
  PHOTPATCH_HEADER SectionData ;
  PRTL_PATCH_HEADER Header;    
  NTSTATUS Status;
  PVOID LockVariable;
  PVOID LockedBuffer;
  BOOLEAN f;
  PLDR_DATA_TABLE_ENTRY LdrEntry;

  SectionData = RtlGetHotpatchHeader( ImageBaseAddress );
  if ( ! SectionData  )
  {
    return STATUS_INVALID_PARAMETER;
  }
  
  //
  // Try to get header from MiHotPatchList
  //
  Header = RtlFindRtlPatchHeader(
    MiHotPatchList,
    ImageHandle );

  if ( ! Header )
  {
    PLIST_ENTRY Entry;

    if ( Flags & FLG_HOTPATCH_ACTIVE )
    {
      return STATUS_NOT_SUPPORTED;
    }

    Status = RtlCreateHotPatch(
      &Header,
      SectionData,
      ImageHandle,
      Flags
      );
    if ( ! NT_SUCCESS( Status ) )
    {
      return Status;
    }

    ExAcquireResourceExclusiveLite(
      PsLoadedModuleResource,
      TRUE
      );

    Entry =  PsLoadedModuleList;
    while ( Entry != PsLoadedModuleList )
    {
      LdrEntry = DataTableEntry = CONTAINING_RECORD( Entry,
                                            KLDR_DATA_TABLE_ENTRY,
                                            InLoadOrderLinks )
      if ( LdrEntry->DllBase DllBase >= MiSessionImageEnd )
      {
        if ( RtlpIsSameImage( Header, LdrEntry ) )
        {
          break;
        }
      }
    }

    ExReleaseResourceLite( PsLoadedModuleResource );

    if ( ! PatchHeader->TargetDllBase )
    {
      Status = STATUS_DLL_NOT_FOUND ;
    }

    Status = ExLockUserBuffer(
      ImageHandle->DllBase,
      ImageHandle->SizeOfImage,
      KernelMode,
      IoWriteAccess,
      LockedBuffer,
      LockVariable
      );
    if ( ! NT_SUCCESS( Status ) )
    {
      FreeHotPatchData( Header );
      return Status;
    }


    Status = RtlInitializeHotPatch(
      ( PRTL_PATCH_HEADER ) Header,
      ( PBYTE ) LockedBuffer - ImageHandle->DllBase
      );

    ExUnlockUserBuffer( LockVariable );

    if ( ! NT_SUCCESS( Status ) )
    {
      FreeHotPatchData( ImageHandle );
      return Status;
    }

    f = 1;
  }
  else
  {
    if ( ( Flags ^ ImageHandle->CodeInfo->Flags ) & FLG_HOTPATCH_ACTIVE )
    {
      return STATUS_NOT_SUPPORTED;
    }

    if ( ! ( ImageHandle->CodeInfo->Flags & FLG_HOTPATCH_ACTIVE ) )
    {
      Status = RtlReadHookInformation( Header );
      if ( ! NT_SUCCESS( Status ) )
      {
        return Status;
      }
    }

    f = 0;
  }
  
  Status = MmLockAndCopyMemory(
    ImageHandle->CodeInfo,
    KernelMode
    );
  if ( NT_SUCCESS( Status ) )
  {
    if ( ! f  )
    {
      return Status;
    }

    LdrEntry->EntryPointActivationContext = Header;  // ???
    InsertTailList( MiHotPatchList, LdrEntry->PatchList );
  }
  else
  {
    if ( f ) 
    {
      RtlFreeHotPatchData( Header );
    }
  }

  return Status;
}

So MiPerformHotPatch inspects the hotpatch information stored in the hotpatch image. This data includes information about which code regions need to be updated. After the neccessary information has been gathered, it applies the code changes.

Two basic problems have to be overcome now: On the one hand, all code sections of drivers are mapped read/execute only. Overwring the instructions thus does not work. On the other hand, the system has to properly synchronize the patching process, i.e. it has to make sure no CPU is currently executing the code that is about to be patched.

To overcome the memory protection problems, Windows facilitates a trick I previously only knew from malware: It creates a memory descriptor list (MDL) for the affected code region, maps the MDL, and updates the code through this mapped region. The memory protection is thus circumvented. As it turns, out, there is even a handy, undocumented helper routine for this purpose: ExLockUserBuffer, which is used by MiPerformHotPatch.

To proceed, MiPerformHotPatch calls MmLockAndCopyMemory to do the actual patching. So how does Windows synchronize the update process? Again, it uses a technique I assumed was a malware trick: It schedules CPU-specific DPCs on all CPUs but the current and keeps those DPCs busy while the current thread is uddating the code. Again, Windows provides a neat routine for that: KeGenericCallDpc. In addition to this, Windows raises the IRQL to clock level in order to mask all interrupts.

Here is the pseudo-code for MmLockAndCopyMemory and its helper, MiDoCopyMemory:

NTSTATUS
MmLockAndCopyMemory (
    IN PSYSTEM_HOTPATCH_CODE_INFORMATION PatchInfo,
    IN KPROCESSOR_MODE ProbeMode
    )
{
  PVOID Buffer;
  NTSTATUS Status;
  UINT Index;

  if ( 0 == PatchInfo->CodeInfo.DescriptorsCount )
  {
    return STATUS_SUCCESS;
  }

  Buffer = ExAllocatePoolWithQuotaTag( 
    9,
    PatchInfo->CodeInfo.DescriptorsCount * 2,
    'PtoH' );
  if ( ! Buffer )
  {
    return STATUS_INSUFFICIENT_RESOURCES;
  }
  RtlZeroMemory( Buffer, PatchInfo->CodeInfo.DescriptorsCount * 2 );

  if ( 0 == PatchInfo->CodeInfo.DescriptorsCount )
  {
    Status = STATUS_INVALID_PARAMETER;
    goto Cleanup;
  }

  for ( Index = 0; Index CodeInfo.DescriptorsCount; Index++ )
  {
    if ( PatchInfo->CodeInfo.CodeDescriptors[ Index ].CodeOffset > PatchInfo->InfoSize ||
       PatchInfo->CodeInfo.CodeDescriptors[ Index ].CodeSize > PatchInfo->InfoSize ||
       PatchInfo->CodeInfo.CodeDescriptors[ Index ].CodeOffset +
       PatchInfo->CodeInfo.CodeDescriptors[ Index ].CodeSize > PatchInfo->InfoSize || 
       /* other checks... */ )
    {
      Status = STATUS_INVALID_PARAMETER;
      goto Cleanup;
    }

    Status = ExLockUserBuffer(
      TargetAddress,
      PatchInfo->CodeInfo.CodeDescriptors[ Index ].CodeSize
      ProbeMode,
      IoWriteAccess,
      &PatchInfo->CodeInfo.CodeDescriptors[ Index ].MappedAddress,
      Buffer[ Index ]
      );
    if ( ! NT_SUCCESS( Status ) )
    {
      goto Cleanup;
    }
  }

  PatchInfo->Flags |= FLG_HOTPATCH_ACTIVE;

  KeGenericCallDpc(
    MiDoCopyMemory,
    PatchInfo );

  if ( PatchInfo->Flags & FLG_HOTPATCH_VERIFICATION_ERROR )
  {
    PatchInfo->Flags &= ~FLG_HOTPATCH_ACTIVE;
    PatchInfo->Flags &= ~FLG_HOTPATCH_VERIFICATION_ERROR;
    Status = STATUS_DATA_ERROR;
  }

Cleanup:
  if ( PatchInfo->CodeInfo.DescriptorsCount > 0 )
  {
    for ( Index = 0; Index CodeInfo.DescriptorsCount; Index++ )
    {
      ExUnlockUserBuffer( Buffer[ Index ] );
    }
  }

  ExFreePoolWithTag( Buffer, 0 );
  return Status;
}

VOID MiDoCopyMemory(
  IN PKDPC Dpc,
  IN PSYSTEM_HOTPATCH_CODE_INFORMATION PatchInfo,
  IN ULONG NumberCpus,
  IN DEFERRED_REVERSE_BARRIER ReverseBarrier
  )
{
  KIRQL OldIrql;
  UNREFERENCED_PARAMETER( Dpc );
  NTSTATUS Status;
  ULONG Index;

  OldIrql = KfRaiseIrql( CLOCK1_LEVEL );

  //
  // Decrement reverse barrier count.
  //
  Status = KeSignalCallDpcSynchronize( ReverseBarrier );
  if ( ! NT_SUCCESS( Status ) )
  {
    goto Cleanup;
  }

  PatchInfo->Flags &= ~FLG_HOTPATCH_VERIFICATION_ERROR;
    
  for ( Index = 0; Index CodeInfo.DescriptorsCount; Index++ )
  {
    if ( PatchInfo->Flags & FLG_HOTPATCH_ACTIVE )
    {
      if ( PatchInfo->CodeInfo.CodeDescriptors[ Index ].ValidationSize != 
        RtlCompareMemory(
          PatchInfo->CodeInfo.CodeDescriptors[ Index ].MappedAddress,
          ( PBYTE ) PatchInfo + PatchInfo->CodeInfo.CodeDescriptors[ Index ].ValidationOffset,
          PatchInfo->CodeInfo.CodeDescriptors[ Index ].ValidationSize ) )
      {

        if ( PatchInfo->CodeInfo.CodeDescriptors[ Index ].CodeSize != 
          RtlCompareMemory(
            PatchInfo->CodeInfo.CodeDescriptors[ Index ].MappedAddress,
            ( PBYTE ) PatchInfo + PatchInfo->CodeInfo.CodeDescriptors[ Index ].OrigCodeOffset,
            PatchInfo->CodeInfo.CodeDescriptors[ Index ].CodeSize ) )
        {
          PatchInfo->Flags &= FLG_HOTPATCH_VERIFICATION_ERROR;
          break;
        }
      }
    }
    else
    {
      if ( PatchInfo->CodeInfo.CodeDescriptors[ Index ].CodeSize !=
        RtlComparememory(
          PatchInfo->CodeInfo.CodeDescriptors[ Index ].MappedAddress,
          ( PBYTE ) PatchInfo + PatchInfo->CodeInfo.CodeDescriptors[ Index ].CodeOffset,
          PatchInfo->CodeInfo.CodeDescriptors[ Index ].CodeSize ) )
      {
        PatchInfo->Flags &= FLG_HOTPATCH_VERIFICATION_ERROR;
        break;
      }
    }
  }

  //loc_479533
  if ( PatchInfo->Flags & FLG_HOTPATCH_VERIFICATION_ERROR ||
     PatchInfo->CodeInfo.DescriptorsCount <= 0 )
  {
    goto Cleanup;
  }

  for ( Index = 0; Index CodeInfo.DescriptorsCount; Index++ )
  {
    PVOID Source;
    if ( PatchInfo->Flags & FLG_HOTPATCH_ACTIVE )
    {
      Source = ( PBYTE ) PatchInfo + PatchInfo->CodeInfo.CodeDescriptors[ Index ].CodeOffset;
    }
    else
    {
      Source = ( PBYTE ) PatchInfo + PatchInfo->CodeInfo.CodeDescriptors[ Index ].OrigCodeOffset;
    }

    RtlCopyMemory(
      PatchInfo->CodeInfo.CodeDescriptors[ Index ].MappedAddress,
      Source,
      PatchInfo->CodeInfo.CodeDescriptors[ Index ].CodeSize
      );
  }


Cleanup:
   KeSignalCallDpcSynchronize( ReverseBarrier );
   KfLowerIrql( OldIrql );
   KeSignalCallDpcDone( NumberCpus );
}

To see the code, in action, we set a breakpoint on nt!MiDoCopyMemory:

kd> k
ChildEBP RetAddr  
f6acbac0 8087622f nt!MiDoCopyMemory
f6acbae8 80990a10 nt!KeGenericCallDpc+0x3d
f6acbb0c 80990bea nt!MmLockAndCopyMemory+0xf1
f6acbb34 80990cba nt!MiPerformHotPatch+0x143
f6acbb68 809b2d67 nt!MmHotPatchRoutine+0x75
f6acbba8 808caeff nt!ExApplyCodePatch+0x191
f6acbd50 8082337b nt!NtSetSystemInformation+0xa1e

Before letting MiDoCopyMemory do its work, let’s see what it is about to do. No modifications have yet been done to mrxsmb:

kd> !chkimg mrxsmb
0 errors : mrxsmb 

kd> !chkimg rdbss
0 errors : rdbss

The second argument is a structure holding the information garthered previously, peeking into it reveals:

kd> dd /c 1 poi(esp+8) l 4
81583008  00000001
8158300c  00000149
81583010  00000008   <-- # of code patches
81583014  f9648b1f   <-- hmm...

As it turns out, address 81583014 refers to a variable length array of size 8. Poking aroud with dd, the following listing suggests that the structure is of size 28 bytes:

kd> dd /c 7 81583014
81583014  f9648b1f fa2afb1f 000000ec 00000005 000000f1 000000f6 00000005
81583030  f9648b24 fa2b2b24 000000fb 00000002 000000fd 000000ff 00000002
8158304c  f96585ef fa2b15ef 00000101 00000005 00000106 0000010b 00000005
81583068  f96585f4 fa2b45f4 00000110 00000002 00000112 00000114 00000002
81583084  f9658569 fa2b3569 00000116 00000005 0000011b 00000120 00000005
815830a0  f965856e fa2b656e 00000125 00000002 00000127 00000129 00000002
815830bc  f9653378 fa2b5378 0000012b 00000005 00000130 00000135 00000005
815830d8  f965337d fa2b837d 0000013a 00000005 0000013f 00000144 00000005

Given that rdbss was loaded to address range f9641000-f9671000, it is obvious that the first 2 columns refer to code addresses. The third, fifth and sixth column looks like an offset, the fourth and seventh like the length of the code change. First, let’s see where the first column points to:

kd> u f9648b1f
rdbss!RxInitiateOrContinueThrottling+0x6b:
f9648b1f 90              nop
f9648b20 90              nop
f9648b21 90              nop
f9648b22 90              nop
f9648b23 90              nop
rdbss!RxpCancelRoutine:
f9648b24 8bff            mov     edi,edi
f9648b26 55              push    ebp
f9648b27 8bec            mov     ebp,esp

Now that looks promising, especially since the fourth column holds the value 5. Let’s look at the second row:

kd> u f9648b24
rdbss!RxpCancelRoutine:
f9648b24 8bff            mov     edi,edi

No doubt, the first and second row define the two patches necessary to redirect RxpCancelRoutine. But what to replace this code with? As it turns out, the offsets in column three are relative to the structure and point to the code that is to be written:

kd> u poi(esp+8)+000000ec
815830f4 e9dcc455fd      jmp     7eadf5d5          mov     edi,edi

kd> u poi(esp+8)+000000fb
81583103 ebf9            jmp     815830fe

That makes perfectly sense — the five nops are to be overwritten by a near jump, the mov edi, edi will be replaced by a short jump.

So let’s run MiDoCopyMemory and have a look at the results. Back in MmLockAndCopyMemory, the code referred to by the first to rows look like this:

kd> u f9648b1f
rdbss!RxInitiateOrContinueThrottling+0x6b:
f9648b1f e9dcc455fd      jmp     hpf3!RxpCancelRoutine (f6ba5000)

kd> u f9648b24
rdbss!RxpCancelRoutine:
f9648b24 ebf9            jmp     rdbss!RxInitiateOrContinueThrottling+0x6b (f9648b1f)
f9648b26 55              push    ebp
f9648b27 8bec            mov     ebp,esp

Voilà, RxpCancelRoutine has been patched and calls are redirected to hpf3!RxpCancelRoutine, the new routine located in the auxiliarry ‘hpf3′ driver. All that remains to be done is cleanup (unlocking the memory etc).

That’s it — that’s how Windows applies patches on the fly using hotpatching. Too bad that the technology is so rarely used in practice.

#ifdef _WIN32

When writing processor-specific code, the _M_IX86, _M_AMD64 and _M_IA64 can be used for conditional compilation — so far, so good. But sometimes code is not exactly processor-specific but rather specific to the natural machine word length (i.e. 32 bit or 64 bit). Fur such situations, there are defines, too — however there is a little catch: For ancient 16 bit code, there is _WIN16. For 64 bit, the WDK build environment defines _WIN64 by default. Given these two macros, it is tempting to conclude that _WIN32 should only be defined for 32 bit builds — however this is not the case. As it turns out, _WIN32 is always defined, both for 32 and 64 bit builds.

And yes, this behaviour is documented on MSDN, but it is stupid anyway.

However, where _WIN32 can be of use is when writing code targeting multiple platforms — as _WIN32 is always defined, it can be used as an indicator that you compile for Windows, regardless of the compiler used (another option is to use _MSC_VER, but that is compiler-specific).

Windows Hotpatching

Several years ago, with Windows Server 2003 SP1, Microsoft introduced a technology and infrastructure called Hotpatching. The basic intent of this infrastructure is to provide a means to apply hotfixes on the fly, i.e. without having to reboot the system — even if the hotfix contains changes on critical system components such as the kernel iteself, important drivers, or user mode libraries such as shell32.dll.

Trying to applying hotfixes on the fly introduces a variety of problems — the most important being:

  • Patching code that is currently in use
  • Atomically replacing files on disk that are currently in use and therefore locked
  • Making sure that all changes take effect for both, processes currently running and processes which are yet to be started (i.e. before the next reboot)
  • Allowing further hotfixes to be applied on system that has not been rebooted since the last hotfix has been applied in an on-the-fly fashion

The Windows Hotpatching infrastructure is capable of handling all these cases — it is, however, not applicable to all kinds of code fixes. Generally speaking, it can only be used for fixes that merely comprise smallish code changes but do not affect layout or semantics of data structures. A fix for a buffer overflow caused by an off-by-one error, however, is a perfect example for a fix that could certainly be applied using the Hotpatching infrastructure.

That all sounds good and nice, but reality is that we still reboot our machines for just about every update Microsoft provides us, right?

Right. The answer for this is threefold. First, as indicated, some hotfixes can be expected to make changes that cannot be safely applied using the Hotpatching system. Secondly, Hotpatching is used on an opt-in basis, so you will not benefit from it automatically: When a hotpatch-enabled hotfix is applied through Windows Update or by launching the corresponding exe file, it is not used and a reboot will be required. The user has to explicitly specify the /hotpatch:enable switch in order to have the hotfix to be applied on the fly.

In the months after the release of SP1, a certain fraction of the hotfixes issued by Microsoft were indeed hotpatch-enabled and could be applied without a reboot. Interestingly, however, I am not aware of a single hotfix issued since Server 2003 SP2 that supported hotpatching!

And thirdly: Whether Microsoft has lost faith in their hotpatching facility, whether the effort to test such hotfixes turned out to be too high or whether there were other reasons speaking against issueing hotpatch-enabled hotfixes — I do not know.

Notwithstanding this observation, Hotpatching is an interesting technology that deserves to be looked at in more detail. Although I will not cover the entire infrastructure, I will spend at least one more blog post on the mechanisms implemented in Windows that allow code modifications to be performed on the fly. That is, I will focus on the hotpatching part of the infrastructure and will ignore coldpatching and other, smaller aspects of the infrastructre.

How to test MFC applications using Visual Assert or cfix

Automated testing of GUI applications is tricky. It is not only tricky because testing the GUI itself is hard (despite there being good tools around), it is also tricky because GUI applications often tend to be a bit hostile towards unit testing.

One class of GUI applications for which this kind of hostility often applies is MFC applications. Although MFC allows the use of DLLs, components, etc, the framework still encourages the use of relatively monolithic architectures.

Regardless of whether this is a good or bad thing, the question is how to make unit testing MFC applications feasible and less painful.

In one of the last releases, both cfix and Visual Assert introduced the abilify to embed unit tests into executable modules. That is, the framework allows unit tests to be compiled and linked into the main application .exe file. Such an executable can then be run in two modes: On the one hand, it can be launched regularly and the existance of unit tests will largely go unnnoticed. On the other hand, the executable can be launched indirectly via cfix of Visual Assert — and in this case, the application will not have its WinMain routine run, but rather have its unit tests be executed.

Although this feature enables us to do unit testing without having to rethink or tweak the application’s architecture more than necessary, the question still in how far MFC likes this kind of testing.

At this point, a distinction has to be made on whether the unit tests make use of MFC APIs or not. Tests that only exercise internal methods will run smoothless, without further setup or precautions needed. Of course, this also holds for tests that make of of basic MFC functionality such as using classes like CString or CArray.

For test that make use of more “interesting” MFC functions, however, it is quite possible that you will observe slightly strange behavior: API calls failing, fields not having the proper value, or maybe even crashes.

An example for this is the field AFX_MODULE_STATE::m_hCurrentInstanceHandle. During a normal run, this field will hold the address of the current module. In a unit tests, however, the following assertion will fail:

CFIX_ASSERT( AfxGetModuleState()->m_hCurrentInstanceHandle != 0 );

If you think about this for a minute, this should not really come as a surprise. As described previously, Visual Assert/cfix, when attempting to run tests embedded into an executable module, will not call the applocation’s WinMain/main routine. They will ensure that all initializers (in particular: constructors of global C++ objects) are run, but calling the actual main routine is effectively skipped. Many of the MFC APIs, however, rely on quite a bit of initialization work that is performed during startup. Some of this work is conducted as part of constructors of global objects executed, a significant part of this work, however, is done in AfxWinInit.

Quoting MSDN:

This function is called by the MFC-supplied WinMain function, as part of the CWinApp initialization of a GUI-based application, to initialize MFC.

And indeed, if you look at the default MFC WinMain routine, AfxWinMain, you will see that it calls AfxWinInit.

The key to using “interesting” MFC APIs in your unit tests therefore is to make sure that AfxWinMain has been called before.

Unfortunately, there is no counterpart to AfxWinInit, so initializing MFC in a fixture’s Before or Setup routine and shutting MFC down in a After or Teardown routine is not feasible. Rather, you have to call it once per process, something that is a bit unusual for unit testing and requires a tiny bit of manual work. The easiest way to accomplish this is to a lazy initialization helper routine:

static void InitalizeMfc()
{
  static BOOL Initialized = FALSE;
  if ( ! Initialized )
  {
    CFIX_ASSERT( AfxWinInit(
      ::GetModuleHandle(NULL),
      NULL,
      ::GetCommandLine(),
      0 ) );
    Initialized = TRUE;
  }
}

…and call it from each fixture’s Setup routine:

static void SetUp()
{
  InitalizeMfc();
}

With this in place, MFC APIs will now work properly and AfxGetModuleState()->m_hCurrentInstanceHandle will contain a proper address.

With this bit of MFC background in mind, writing unit tests for MFC applications should be not much different than writing tests for any other kind of application.

One final tip: Adding unit tests to your project introduces a dependeny to cfix.dll. In your final build, you’ll want to remove (#ifdef out, for example) all your tests so this dependency will disappear. During development, however, this dependency might be a bit of a drag because there might be machines on which cfix is not available. To alleviate this situation, consider making cfix.dll a delay-load: Unless you want to run unit tests, it is then ok to miss cfix.dll on other machines.

Visual Assert 1.1 beta and cfix 1.7 released

Slightly delayed, Visual Assert 1.1 beta is now available for download. As announced in a previous post, the most important change in the new version is added suport for the latest version of Visual Studio, Visual Studio 2010.

However, the new version also brings a couple of new features that apply to all versions of Visual Studio. Most importantly, cfix and Visual Assert now expose an API that allows developers to plug in custom event sinks. A custom event sink is implemented as a DLL and receives all events the runtime generates during the execution of a test suite. As such, the API is perfectly suited for implementing custom loggers.

To make this new feature easily usable, the Options dialog (Menu: Tools > Options) has been enhanced appropriately. Moreover, the dialog now includes some further options concerning stack size, current directory adjustment, and VC++ directory registration that have not been exposed previously.

Coming along with the new Visual Assert release, a new cfix release, version 1.7, is now available on Sourceforge.

Coming soon: Visual Assert for Visual Studio 2010

Now that Visual Studio 2010 has oficially been released, I keep getting questions about a Visual Studio 2010-enabled version of Visual Assert.

The fact that Visual Studio 2010 is already out, yet there is no Visual Assert version for it is unfortunate. It would have been nice to have Visual Studio 2010 support ready on Visual Studio’s release date, however, that was not possible due to lack of time. Having solved most compatibilty issues though (of which there were many, Visual Studio 2010 is a truly painful release for AddIn developers), I am now confident to be able to offer a first beta by begin of May.

This version will not only add support for Visual Studio 2010, but will also contain a set of other improvements and new features.

What a weirdo: How the /analyze switch changes its behavior depending on its environment

In Visual Studio 2005 Team System (VSTS), the “ultimate” SKU of Visual Studio 2005, Microsoft introduced the /analyze compiler switch. When the /analyze switch is used, the cl compiler not only does its regular checks, but performs a much more thorough static code analysis.

While /analyze is very useful indeed, it was only available in the top SKU — the Standard and Professional versions of Visual Studio lacked support for this compiler switch (this has changed by now, Professional now also supports this feature). As some smart people quickly figured out though, the compilers shipped as part of the Windows SDK did support /analyze, too.

So given that some compilers do support /analyze while other do not, you may well expect that there are two slightly different types of binaries, one that the SDK and VSTS uses, and one that is shipped with other Visual Studio SKUs.

At least this was what I expected. As it turns out though, this is not quite the case.

Where’s /analyze?

For the past two years, I have been developing using Visual Studio 2005 Team System along with Windows SDK 6.0 and WDK 6000 on a Vista x64 machine. Using this setup, I was able to use the /analyze switch in both, “regular” Visual Studio projects and WDK (build.exe-driven) projects. That led me to the conclusion that the WDK 6000 compilers, like the SDK compilers were in fact /analyze-enabled binaries as well.

Switching to a Windows 7 machine with VSTS 2005 and 2008, SDK 7.0, and WDK 6000 did not change this — /analyze kept working fine in all environments.

Then I set up a build server, installed WDK 6000 and Windows SDK 7.0 and attempted to perform a build — to my surprise, though, I got plenty of complaints about the /anayze switch not being supported.

I verified that the right compilers (WDK 6000) were used and compared cl versions between the build machine and my development machine — both were 14.00.50727.220, so everything seemed right. Running cl.exe /? on both machines, however, I noticed that despite versions being the same, this Code Analsis section was missing in the output on the build machine:

                         -CODE ANALYSIS-

/analyze[:WX-] enable code analysis
    WX- - code analysis warnings should not be treated as errors even if /WX is invoked

So obviously, Code Analysis support is enabled or disabled depending on external factors — not the binary itself, but the environment somehow determines whether the /analyze switch is supported or not.

Observing cl.exe /? with Process Monitor on my development machine resulted in the following output:

Process Monitor tracing the search for c1xxast

This trace leaves little room for interpretation: The code analysis features must (mainly) be implemented in c1xxast.dll. c1xxast.dll, however, is not shipped with the WDK itself, nor is it shipped with the non-VSTS SKUs of Visual Studio. So by default, the WDK’s cl will fail to locate the DLL and will revert to “/analyze-disabled mode”.

If, however, you have VSTS or the Windows SDK installed on your machine and your %PATH% happens to include the right directories, cl’s search for c1xxast.dll will succeeded and — tada — /analyze suddenly works. On my development machine, this obviously was the case, whilst on the build machine, it was not.

Compiler version mish-mash

I added the Windows SDK’s bin directory to the build machine’s %PATH% and rerun the build. As I expected, /analyze now worked fine — what I did not quite expect though was that I was now getting dozens of compilation warnings like:

warning C6309: Argument '1' is null: this does not adhere to function 
specification of 'CfixCreateThread'

The reason for this was simple: The WDK cl.exe (remember, version 14.00.50727.220), thanks to a proper %PATH%, now used c1xxast.dll from SDK 7 to perform code analysis — despite the fact that c1xxast.dll actually “belonged” to cl version 15.00.30729.01. So the c1xxast.dll was one generation ahead of the WDK I was using.

The really, really cool thing about cl being able to work with a newer c1xxast.dll is that you can continue using WDK 6000 or 6001 (with W2K support!) and still benefit from the latest-and-greatest static code analysis features.

The reason for getting several warnings on the build machine while not getting similar warnings on my development machine was simply that on my development machine, the VS 2005 directory preceded the SDK directory in my %PATH%. Once I switched the order, I got the same wanings on both machines. This leads me to:

The ugly thing about this, however, is that a tiny change in the order of directories in %PATH% can suddenly make a huge difference w.r.t. code analysis. This is not quite what you’d normally expect.

(The additional compiler warnings, by the way, were a result of the improved analysis checks in cl 15: cl 14 routinely failed to verify the usage of __in vs. __in_opt parameters; cl 15 has become much more precise here and found several mis-attributed function signatures.)


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About me

Johannes Passing, M.Sc., living in Berlin, Germany.

Besides his consulting work, Johannes mainly focusses on Win32, COM, and NT kernel mode development, along with Java and .Net. He also is the author of cfix, a C/C++ unit testing framework for Win32 and NT kernel mode, Visual Assert, a Visual Studio Unit Testing-AddIn, and NTrace, a dynamic function boundary tracing toolkit for Windows NT/x86 kernel/user mode code.

Contact Johannes: jpassing (at) acm org

Johannes' GPG fingerprint is BBB1 1769 B82D CD07 D90A 57E8 9FE1 D441 F7A0 1BB1.

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