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Tuts 4 You

Reversing Microsoft Visual C++


About This File

Microsoft Visual C++ is the most widely used compiler for Win32 so it is important for the Win32 reverser to be familiar with its inner working. Being able to recognize the compiler-generated glue code helps to quickly concentrate on the actual code written by the programmer. It also helps in recovering the high-level structure of the program.

In part I of this 2-part article (see also: Part II: Classes, Methods and RTTI), I will concentrate on the stack layout, exception handling and related structures in MSVC-compiled programs. Some familiarity with assembler, registers, calling conventions etc. is assumed.

Terms:
Stack frame: A fragment of the stack segment used by a function. Usually contains function arguments, return-to-caller address, saved registers, local variables and other data specific to this function. On x86 (and most other architectures) caller and callee stack frames are contiguous.
Frame pointer: A register or other variable that points to a fixed location inside the stack frame. Usually all data inside the stack frame is addressed relative to the frame pointer. On x86 it's usually ebp and it usually points just below the return address.
Object: An instance of a (C++) class.
Unwindable Object: A local object with auto storage-class specifier that is allocated on the stack and needs to be destructed when it goes out of scope.
Stack UInwinding: Automatic destruction of such objects that happens when the control leaves the scope due to an exception.
There are two types of exceptions that can be used in a C or C++ program.
SEH exceptions (from "Structured Exception Handling"). Also known as Win32 or system exceptions. These are exhaustively covered in the famous Matt Pietrek article[1]. They are the only exceptions available to C programs. The compiler-level support includes keywords __try, __except, __finally and a few others.
C++ exceptions (sometimes referred to as "EH"). Implemented on top of SEH, C++ exceptions allow throwing and catching of arbitrary types. A very important feature of C++ is automatic stack unwinding during exception processing, and MSVC uses a pretty complex underlying framework to ensure that it works properly in all cases.
In the following diagrams memory addresses increase from top to bottom, so the stack grows "up". It's the way the stack is represented in IDA and opposite to the most other publications.


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