Hacking & Security with Gaurav

POSTED BY GAURAV BAPLAWAT ON OCT 9, 2011

continuing...

Introduction to Memory and Stacks:

There are three main sections of memory:

1. Stack Section - Where the stack is located, stores local variables and function arguments.

2. Data Section - Where the heap is located, stores static and dynamic variables.
3. Code Section - Where the actual program instructions are located.

The stack section starts at the high memory addresses and grows downwards, towards the lower memory addresses; conversely, the data section (heap) starts at the lower memory addresses and grows upwards, towards the high memory addresses. Therefore, the stack and the heap grow towards each other as more variables are placed in each of those sections. I have shown that in below Figure..

 High Memory Addresses (0xFFFFFFFF)
---------------------- <-----Bottom of the stack
|                          |
|                          |   |
|         Stack        |   | Stack grows down
|                          |   v
|                          |
|---------------------| <----Top of the stack (ESP points here)
|                          |
|                          |
|                          |
|                          |
|                          |
|---------------------|  <----Top of the heap
|                          |
|                          |    ^
|       Heap          |     |   Heap grows up
|                          |    |
|                          |
|---------------------| <-----Bottom of the heap
|                          |
|    Instructions    |
|                          |
|                          |
-----------------------
Low Memory Addresses (0x00000000)  

 Some Essential Assembly Instructions for Reverse Engineering:

*The zero flag (ZF) is a 1 bit indicator which records the result of a cmp or test instruction

Each instruction performs one specific task, and can deal directly with registers, memory addresses, and the contents thereof. It is easiest to understand exactly what these functions are used for when seen in the context of a simple hello world program and try to relate assembly language with high level language such as C language.

Here is simple C program that displays Hello World:

 int main(int argc, char *argv[])                    {                     printf("Hello World!\n");                 return 0;           }    

Save this program as helloworld.c and compile it with 'gcc -o helloworld helloworld.c'; run the resulting binary and it should print "Hello World!" on the screen and exit. Ahhah... It looks quite simple. Now let's look how it will look in assembly language.

0x8048384     push ebp                      <--- Save the EBP value on the stack
0x8048385     mov ebp,esp               <--- Create a new EBP value for this function
0x8048387     sub esp,0x8                 <---Allocate 8 bytes on the stack for local variables
0x804838a     and esp,0xfffffff0          <---Clear the last byte of the ESP register
0x804838d     mov eax,0x0                 <---Place a zero in the EAX register
0x8048392     sub esp,eax                  <---Subtract EAX (0) from the value in ESP
0x8048394     mov DWORD PTR [esp],0x80484c4     <---Place our argument for the printf() (at address 0x08048384) onto the stack
0x804839b     call 0x80482b0 <_init+56>                     <---Call printf()
0x80483a0     mov eax,0x0                 <---Put our return value (0) into EAX
0x80483a5     leave                              <---Clean up the local variables and restore the EBP value
0x80483a6     ret                                  <---Pop the saved EIP value back into the EIP register

As you can easily figure out these instructions are similar to that of C program. You can easily note that flow of program is same. Off course it will be same as its a assembly code of same binary (exe) obtained from executing above C program.

I hope you all like it. We will continue our discussion tomorrow where i will explain how to analyze assembly language codes for those binaries whose high level source code we don't have.

A quick tip for all users how to learn assembly language better...  Pick a already made code and generate its binary or exe file and now obtains the assembly code of that binary and try to relate assembly code with high language code. I guarantee that will surely help you to understand better as I always used to do understand things like these ways only.