Consider the following NASM code:
BITS 32
org 0x08048000
ehdr: ; Elf32_Ehdr
db 0x7F, "ELF", 1, 1, 1, 0 ; e_ident
times 8 db 0
dw 2 ; e_type
dw 3 ; e_machine
dd 1 ; e_version
dd _start ; e_entry
dd phdr - $$ ; e_phoff
dd 0 ; e_shoff
dd 0 ; e_flags
dw ehdrsize ; e_ehsize
dw phdrsize ; e_phentsize
dw 1 ; e_phnum
dw 0 ; e_shentsize
dw 0 ; e_shnum
dw 0 ; e_shstrndx
ehdrsize equ $ - ehdr
phdr: ; Elf32_Phdr
dd 1 ; p_type
dd 0 ; p_offset
dd $$ ; p_vaddr
dd $$ ; p_paddr
dd filesize ; p_filesz
dd filesize ; p_memsz
dd 5 ; p_flags
dd 0x1000 ; p_align
phdrsize equ $ - phdr
_start:
xor eax, eax ;now just return to system with ebx
inc eax
int 0x80
; your program here
filesize equ $ - $$
How to debug programs structured like this? Most preferrably I'm looking for a GDB solution that would let me step-run the assembly instructions, allowing to preview the registers every step. Here's what happens when I want to set a memory-based breakpoint there:
[localhost.localdomain][/tmp] $ gdb ./a.out
GNU gdb (GDB) Fedora (7.5.1-37.fc18)
Copyright (C) 2012 Free Software Foundation, Inc.
License GPLv3+: GNU GPL version 3 or later <http://gnu.org/licenses/gpl.html>
This is free software: you are free to change and redistribute it.
There is NO WARRANTY, to the extent permitted by law. Type "show copying"
and "show warranty" for details.
This GDB was configured as "x86_64-redhat-linux-gnu".
For bug reporting instructions, please see:
<http://www.gnu.org/software/gdb/bugs/>...
Reading symbols from /tmp/a.out...(no debugging symbols found)...done.
(gdb) break 0x08048054
No symbol table is loaded. Use the "file" command.
Make breakpoint pending on future shared library load? (y or [n]) n
(gdb) list breakpoints
No symbol table is loaded. Use the "file" command.