Opcode Script

While working through some of the corelan.be tutorials I came across the “assemble” functionality in windbg which allows you to lookup the opcodes of assembly instructions. I have previously used an online assembler for this sort of functionality before located at defuse.ca but I wanted the ability to lookup the instructions on the command line without going to the web. I looked into the functionality of gdb and nasm but it doesn’t appear that either provide this sort of functionality so…

Here is a little shell function I put in my .bashrc:

function asmop {
DIR=/tmp/asmop
mkdir $DIR
cd $DIR
FILE=tmpasm
while read x ; do
  touch $FILE.nasm
cat << EOF > $FILE.nasm
global _start
section .text
_start:
EOF
  echo $x >> $FILE.nasm;
  nasm -f elf32 -o $FILE.o $FILE.nasm
  ld -o $FILE $FILE.o
  objdump -D $FILE -M intel
  rm $FILE*
done
cd ../
rmdir $DIR
}

This lets us lookup opcodes for assembly like:

~/$ asmop
jmp esp

tmpasm:     file format elf32-i386


Disassembly of section .text:

08048060 <_start>:
 8048060:	ff e4                	jmp    esp
push eax

tmpasm:     file format elf32-i386


Disassembly of section .text:

08048060 <_start>:
 8048060:	50                   	push   eax