Windows System Call Sequence and Simulation

There are hundreds of documents telling how Windows implements its system call, using int 2e or sysenter. But I can find no code to run to learn how exactly it works. And I managed to write it for my own.

The C code requires only SDK to compile, for I have copied all DDK definitions inline. It opens a C:\test.txt file and write Hello World! to it. Quite simple. I’ve tried a HelloWorld console application. But its call sequence is far more complex than I have expected, after I have made some reverse engineering and read some code from ReactOS project(Wine does not help, since it does not implement a Win32 compatible call sequence in the console case). The code is the basis of our further investigation. It invokes NtCreateFile(), NtWriteFile() and NtClose() in ntdll.dll with dynamic loading:

I found the handle value and all three function pointers are fixed, at least on my Windows XP(SP3). It may be caused by the preferred base address of ntdll.dll. The code should work on all Windows platforms, since it has no hardcoded values.

Now, translate the C code into assembly. Error handling is ommitted:

Compile the code with:

The assembly code of NtCreateFile(), NtWriteFile() and NtClose() are copied directly from ntdll.dll. For NtCreate(), 25h is the system service number that will be used to index into the KiServiceTable(SSDT, System Service Dispatch Table) to locate the kernel function that handles the call.

System service numbers vary between Windows versions. This is why they are not recommend to be used directly to invoke system calls. I only demonstrate the approach here. For Windows XP, the values of the three numbers are 25h, 112h and 19h. While for Windows 7, they are 42h, 18ch and 32h. Change them yourself if you’re running Windows 7. For a complete list of system service numbers, refer here or dissemble your ntdll.dll manually :). The output executable is a tiny one, only 3KB in size, since it eliminates the usage of CRT. Moreover, it has an empty list of import functions!

At 7ffe0300h is a pointer to the following code:

NOTE: The assembly code may work only when compiled to a 32-bit application. 64-bit mode is not tested and need modification to work.

One last point, it seems the STR_HELLO string is required to be aligned to 8 byte border. Otherwise, you will get 0x80000002 error code(STATUS_DATATYPE_MISALIGNMENT).

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