How to disassemble fasm-generated binary?

Question

I am writing program in FASM assembler, and want to see what code is generated after all macro expansions. One usually can disasseble binary with objdump -d, but for binary, generated by fasm, it outputs only following:

$ cat true.fasm
format ELF64 executable
sys_exit = 60
entry $
      mov eax, sys_exit
      xor edi, edi
      syscall
$ fasm true.fasm
$ objdum -d ./true
out/true:     file format elf64-x86-64

What I can do is to load binary into gdb, start it with starti and decode instructions with x/10i $rip, which is sub-optimal. Is there non-interactive command that can do the same?

FASM doesn't create ELF section info (so there is no `.text` section), only program headers that tell the OS how to map it into memory. I don't know a good convenient way to disassemble it, other than treating it like a flat binary and disassembling everything (e.g. `ndisasm -b32`). Then finding the start of the actual instructions yourself. — Peter Cordes, Oct 20 '19 at 18:03
You can try using the `-D` (capital D, disassemble all) option of objdump to see if that works better. — Ross Ridge, Oct 20 '19 at 20:56

score 4 · Accepted Answer · answered Jan 07 '20 at 11:49

You can easly using radare2, using pdf command that means disassemble :

% cat test.asm 
format ELF64 executable
sys_exit = 60
entry $
  mov rax, sys_exit
  xor rdi, rdi
  syscall
% ./fasm test.asm
flat assembler  version 1.73.04  (16384 kilobytes memory) 1 passes, 132 bytes.
% file test
test: ELF 64-bit LSB executable, x86-64, version 1 (SYSV), statically linked, stripped
% r2 -AA test
[x] Analyze all flags starting with sym. and entry0 (aa)
[x] Analyze function calls (aac)
[x] Analyze len bytes of instructions for references (aar)
[x] Check for objc references
[x] Check for vtables
[x] Type matching analysis for all functions (aaft)
[x] Propagate noreturn information
[x] Use -AA or aaaa to perform additional experimental analysis.
[x] Finding function preludes
[x] Enable constraint types analysis for variables
-- In visual mode press 'c' to toggle the cursor mode. Use tab to navigate
[0x00400078]> pdf
        ;-- segment.LOAD0:
        ;-- rip:
┌ 12: entry0 ();
│           0x00400078      48c7c03c0000.  mov rax, 0x3c               ; '<' ; 60 ; [00] -rwx segment size 12 named LOAD0
│           0x0040007f      4831ff         xor rdi, rdi
└           0x00400082      0f05           syscall
[0x00400078]>

How to disassemble fasm-generated binary?

1 Answers1