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.
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:
Next week, the 16th Working Conference on Reverse Engineering (WCRE) will be held in Lille, France. I will be there presenting NTrace: Function Boundary Tracing for Windows on IA-32. NTrace is a dynamic function boundary tracing toolkit for IA-32/x86 that can be used to trace both kernel and user mode Windows components – examples for components that can be traced include the kernel itself (ntoskrnl), drivers like NTFS as well as user mode components such as kernel32, shell32 or even explorer.
It is common practice to embed a version resource (VS_VERSIONINFO) into PE images such as DLL and EXE files. While this resource mainly serves informational purposes, the version information is occasionaly used to perform certain checks, such as verifying the module’s suitability for a particular purpose. Under certain circumstances, however, this versioning information may be too imprecise: Versions are not necessarily incremented after each build, so it is possible that two copies of a module carry the same versioning information, yet differ significantly in their implementation.
The cfix 1.2 package as released last week contained a rather stupid bug that the new build, 18.104.22.16844, now fixes: the amd64 binaries cfix64.exe and cfixkr64.sys were wrongly installed as cfix32.exe and cfixkr32.sys, respectively. Not only did this stand in contrast to what the documenation stated, it also resulted in cfix being unable to load the cfixkr driver on AMD64 platforms. The new MSI package is now available for download on Sourceforge.
cfix 1.2, which has been released today, introduces a number of new features, the most prominent being improved support for C++ and additional execution options. New C++ API To date, cfix has primarily focussed on C as the programming language to write unit tests in. Although C++ has always been supported, cfix has not made use of the additional capabilities C++ provides. With version 1.2, cfix makes C++ a first class citizen and introduces an additional API that leverages the benefits of C++ and allows writing test cases in a more convenient manner.
The Windows kernel maintains two types of threads – Non-GUI threads, and GUI threads. Non-GUI threads threads use the default stack size of 12KB (on i386, which this this discussion applies to) and the default System Service Descriptor table (SSDT), KeServiceDescriptorTable. GUI threads, in contrast, are expected to have much larger stack requirements and thus use an extended stack size of 60 KB (Note: these are the numbers for Svr03 and may vary among releases).
While I still use VisualStudio 2005 Team System for most of my development, I want to make sure that cfix works properly with VisualStudio 2008 as well. To test that, I recently started a Windows 2003 Server VM, installed VCExpress 2008 and cfix and attempted to run an example project in the VC debugger. As long as no assertions fired, everything seemed fine. I then altered the example’s source code so that one of the assertion would fail, ran it in the debugger – and waited.
Thread IDs uniquely identify a thread – this certainly holds for user mode threads and should also hold for kernel mode threads. But there is one kind of thread where the ID does not uniquely identify a KTHREAD – the Idle thread. On a uniprocessor system, there is only one Idle thread and this idle thread will have the thread ID 0 (in process 0). On a multiprocessor system, however, Windows creates one Idle thread per CPU.
cfix 1.1 introduces a number of new features. The most important among these is the additional ability to write kernel mode unit tests, i.e. unit tests that are run in kernel mode. Needless to say, cfix 1.1 still supports user mode unit tests. All contemporary unit testing frameworks focus on unit testing in user mode. Certainly, the vast majority of testing code can be assumed to be targeting user mode, so this does not come at a surprise.