PASSING DATA TO THE STACK

Stack Overview

Let’s take this simple code as an example:

addTwo:
    add eax, ebx
    ret

main:
    push 4
    push 1
    call addTwo
    mov ebx, eax
    mov eax, 1
    int 80h

The stack layout will look like this:

Stack
ret (sp)
1
4

ret = return address sp = stack pointer

If I call addTwo, how does it know that the first parameter is 4 and the second parameter is 1!? One of the most common ways is to create a base pointer to reference the parameters.

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Creating a Base Pointer

Before accessing the parameters, I need to set up a base pointer ebp to reference the stack frame.

addTwo:
    push ebp
    mov ebp, esp
    add eax, ebx
    ret

main:
    push 4
    push 1
    call addTwo
    mov ebx, eax
    mov eax, 1
    int 80h

• push ebp - Save the base pointer;
• mov ebp, esp - Set the base pointer to the stack pointer;

The stack layout will look like this:

Stack
ebp (sp)
ret
1
4

ret = return address sp = stack pointer

The ebp acts like the base of my stack frame and the divider between the stack frames. It helps me reference the parameters and local variables. If I had procedures nested in procedures, I could use the base pointer to reference the parameters and local variables of the outer procedure.

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Accessing and Cleaning the Stack

Parameters are accessed relative to the base pointer ebp.

addTwo:
    push ebp
    mov ebp, esp
    mov eax, [ebp + 8]
    mov ebx, [ebp + 12]
    add eax, ebx
    pop ebp
    ret

main:
    push 4
    push 1
    call addTwo
    mov ebx, eax
    mov eax, 1
    int 80h

Stack	Value
ebp (sp)	+0
ret	+4
1	+8
4	+12

ret = return address sp = stack pointer

I can use x/x or x/4x to inspect the stack in GDB. This will help me understand how the stack is laid out and how I can access the parameters.