IA-32 Assembly

IA32 Assembly Language

As I was working on picoCTF2021 - filtered-shellcode challenge, I realized that I need to write the shellcode in 32 bit assembly rather than in 64 bit. And compiling 32 bit assembly code is different than compiling 64 bit assembly code using gcc.

Great reference: https://academic.macewan.ca/boersn/images/quickref-20121215.pdf

First, to create an object file, use nasm.

nasm -f elf32 -o file.o file.s. This creates an object file file.o

Then link the object file with ld. The object file must have global _start.

ld -m elf_i386 -e _start -o file file.o

This will create a binary file. We can get the assembly code back by running this command objdump -M intel-mnemonic -D ./file

The assembly code would start as below:

global _start
section .text
_start:

      xor eax, eax
      ...

You can look at how many bytes each instruction takes up by using objdump, but if you want to get the string literal and use that to pass in as data stream, you can go here: https://defuse.ca/online-x86-assembler.htm#disassembly and enter your shellcode.