Exploring the MS-DOS Stub

A long time ago when I got my first computer, I accidentally opened a 32-bit demo with a nice chiptune inside MS-DOS and it worked. I was surprised by how this happens. I was curious to find out how this works behind the scenes. Back in the time I was a little kid and had no clue about programming. This curiosity leads me to discover amazing things I never imagined.
First, let us have a look at the PE header. It starts with the MS-DOS header and contains a 16-bit MS-DOS executable (stub program).

(source: https://commons.wikimedia.org/wiki/File:Portable_Executable_32_bit_Structure.png)

String Length Function in NASM

In certain situations when I want to print some string to stdout we need the length for the write syscall in linux. So we can’t always depend on the $-string macro, which is valid for a defined string.

We use the REPNE (REPeat while Not Equal) instruction which will loop as long as CX != 0. Along with REPNE we use SCASB (scan byte string). It compares the content of the accumulator (AL, AX, or EAX) against the current value pointed at by ES:[EDI]. In the end we calculate the difference between offsets of the scanned string (EDI) and the original string (EBX) to find the length.

push ebx
push ecx
mov ebx, edi
xor al, al
mov ecx, 0xffffffff
repne scasb ; REPeat while Not Equal [edi] != al
sub edi, ebx ; length = offset of (edi - ebx)
mov eax, edi
pop ebx
pop ecx


IsDebuggerPresent API

I was interested in learning about the anti-reversing techniques in the world of reverse engineering. There are so many techniques out there and I thought of trying few techniques and understanding them from the lowest level. I thought of sharing the things I’ve been experimenting these days.
IsDebuggerPresent is a Windows API that can be used to detect a debugger. Here’s an example code:

[code language=”C”]
* IsDebuggerPresent Example
* Author: Osanda Malith Jayathissa (@OsandaMalith)
* Website: http://osandamalith.wordpress.com

#include <windows.h>

int main() {
MessageBox(0, IsDebuggerPresent() ? "Debugger found" : "Debugger not found","Status",0x30);

If we open in a debugger “Debugger Found” text will get triggered in the MessageBox API. How this API works? Open the API in the debugger and you get the following piece of code.

View post on imgur.com

[code language=”C”]

Writing a Bootloader

What is a Bootloader?

A bootloader is a special program that is executed each time a bootable device is initialized by the computer during its power on or reset that will load the kernel image into the memory. This application is very close to hardware and to the architecture of the CPU. All x86 PCs boot in Real Mode. In this mode you have only 16-bit instructions. Our bootloader runs in Real Mode and our bootloader is a 16-bit program.


How this works?

When you switch on the PC the BIOS want to boot up an OS which must be found somewhere in hard disks, floppy disk, CDs, etc. The order in which BIOS searches an OS is user configurable. Next the BIOS reads the first 512 byte sector of the bootable disk. Usually a sector is 512 bytes in size. This is known as the Master Boot Record (MBR). BIOS simply loads the contents of the MBR into memory location “0x7c00” and jumps to that location to start executing whatever code is in the MBR. Our bootloader should be 512 bytes in size as well.