Assembly - GNU/Linux - Hello World!

Fri, 18 Mar 2011 00:00:00 +0000

In this tutorial, we are going to see how to create an executable from an assembly file.

The following example uses a 32-bit version of Ubuntu.
Notice that the version is important when you compiling your assembly file.
It is the same for the linkage operation.

Let’s see it in detail.

1. The code

This file is named hello.asm.

section .text
global _start ;must be declared for linker (ld)
_syscall: ;system call
int 0x25
ret
_start: ;tell linker entry point
push dword len ;message length
push dword msg ;message to write
push dword 1 ;file descriptor (stdout)
mov eax,0x3 ;system call number (sys_write)
call _syscall ;call kernel
add esp,12 ;clean stack (3 * 4)
push dword 0 ;exit code
mov eax,0x3f ;system call number (sys_exit)
call _syscall ;call kernel
;no need to clean stack
section .data
msg db "Hello, world!",0xa ;our dear string
len equ $ - msg ;length of our dear string

2. Compilation

The following command will create a new file: hello.o.
We tell to the compiler that we want an elf file, standing for Executable and Linkage Format.
We use nasm tool to compile it.

Assembly - objdump - -p command

Fri, 25 Feb 2011 00:00:00 +0000

To see the header of a binary, in our example “ls”, we have to use the -p command of the objdump tool.

Let’s try it:

$ objdump -p /bin/ls

Result:

/bin/ls: file format elf32-i386
Program Header:
PHDR off 0x00000034 vaddr 0x08048034 paddr 0x08048034 align 2**2
filesz 0x00000100 memsz 0x00000100 flags r-x
INTERP off 0x00000134 vaddr 0x08048134 paddr 0x08048134 align 2**0
filesz 0x00000013 memsz 0x00000013 flags r--
LOAD off 0x00000000 vaddr 0x08048000 paddr 0x08048000 align 2**12
filesz 0x0001a8d0 memsz 0x0001a8d0 flags r-x
LOAD off 0x0001a8d0 vaddr 0x080638d0 paddr 0x080638d0 align 2**12
filesz 0x00001b8e memsz 0x00001b8e flags rw-
DYNAMIC off 0x0001ae44 vaddr 0x08063e44 paddr 0x08063e44 align 2**2
filesz 0x000000e8 memsz 0x000000e8 flags rw-
NOTE off 0x00000148 vaddr 0x08048148 paddr 0x08048148 align 2**2
filesz 0x00000044 memsz 0x00000044 flags r--
EH_FRAME off 0x000177a8 vaddr 0x0805f7a8 paddr 0x0805f7a8 align 2**2
filesz 0x000006ec memsz 0x000006ec flags r--
STACK off 0x00000000 vaddr 0x00000000 paddr 0x00000000 align 2**2
filesz 0x00000000 memsz 0x00000000 flags rw-
Dynamic Section:
NEEDED libselinux.so.1
NEEDED librt.so.1
NEEDED libcap.so.2
NEEDED libacl.so.1
NEEDED libc.so.6
INIT 0x08049558
FINI 0x0805bb4c
GNU_HASH 0x0804818c
STRTAB 0x08064e40
SYMTAB 0x080481f4
STRSZ 0x000005e3
SYMENT 0x00000010
DEBUG 0x00000000
PLTGOT 0x08063f78
PLTRELSZ 0x00000330
PLTREL 0x00000011
JMPREL 0x08049228
REL 0x08049198
RELSZ 0x00000090
RELENT 0x00000008
VERNEED 0x080490c8
VERNEEDNUM 0x00000003
VERSYM 0x08048fc8
0x6ffffef9 0x080489e4
0x6ffffdf7 0x000000b4
0x6ffffef8 0x08048a98
0x6ffffdf6 0x00000354
Version References:
required from librt.so.1:
0x0d696912 0x00 09 GLIBC_2.2
required from libacl.so.1:
0x05822450 0x00 08 ACL_1.0
required from libc.so.6:
0x09691a73 0x00 11 GLIBC_2.2.3
0x0d696914 0x00 10 GLIBC_2.4
0x0d696913 0x00 07 GLIBC_2.3
0x09691974 0x00 06 GLIBC_2.3.4
0x0d696911 0x00 05 GLIBC_2.1
0x09691f73 0x00 04 GLIBC_2.1.3
0x0d696912 0x00 03 GLIBC_2.2
0x0d696910 0x00 02 GLIBC_2.0

Assemblies on BadproG

Assembly - GNU/Linux - Hello World!

1. The code

2. Compilation

Assembly - objdump - -p command