How to disassemble a binary executable in Linux to get the assembly code?

Question

I was told to use a disassembler. Does gcc have anything built in? What is the easiest way to do this?

Solution 1

I don't think gcc has a flag for it, since it's primarily a compiler, but another of the GNU development tools does. objdump takes a -d/--disassemble flag:

$ objdump -d /path/to/binary

The disassembly looks like this:

080483b4 <main>:
 80483b4:   8d 4c 24 04             lea    0x4(%esp),%ecx
 80483b8:   83 e4 f0                and    $0xfffffff0,%esp
 80483bb:   ff 71 fc                pushl  -0x4(%ecx)
 80483be:   55                      push   %ebp
 80483bf:   89 e5                   mov    %esp,%ebp
 80483c1:   51                      push   %ecx
 80483c2:   b8 00 00 00 00          mov    $0x0,%eax
 80483c7:   59                      pop    %ecx
 80483c8:   5d                      pop    %ebp
 80483c9:   8d 61 fc                lea    -0x4(%ecx),%esp
 80483cc:   c3                      ret    
 80483cd:   90                      nop
 80483ce:   90                      nop
 80483cf:   90                      nop

Solution 2

An interesting alternative to objdump is gdb. You don't have to run the binary or have debuginfo.

$ gdb -q ./a.out 
Reading symbols from ./a.out...(no debugging symbols found)...done.
(gdb) info functions 
All defined functions:

Non-debugging symbols:
0x00000000004003a8  _init
0x00000000004003e0  __libc_start_main@plt
0x00000000004003f0  __gmon_start__@plt
0x0000000000400400  _start
0x0000000000400430  deregister_tm_clones
0x0000000000400460  register_tm_clones
0x00000000004004a0  __do_global_dtors_aux
0x00000000004004c0  frame_dummy
0x00000000004004f0  fce
0x00000000004004fb  main
0x0000000000400510  __libc_csu_init
0x0000000000400580  __libc_csu_fini
0x0000000000400584  _fini
(gdb) disassemble main
Dump of assembler code for function main:
   0x00000000004004fb <+0>:     push   %rbp
   0x00000000004004fc <+1>:     mov    %rsp,%rbp
   0x00000000004004ff <+4>:     sub    $0x10,%rsp
   0x0000000000400503 <+8>:     callq  0x4004f0 <fce>
   0x0000000000400508 <+13>:    mov    %eax,-0x4(%rbp)
   0x000000000040050b <+16>:    mov    -0x4(%rbp),%eax
   0x000000000040050e <+19>:    leaveq 
   0x000000000040050f <+20>:    retq   
End of assembler dump.
(gdb) disassemble fce
Dump of assembler code for function fce:
   0x00000000004004f0 <+0>:     push   %rbp
   0x00000000004004f1 <+1>:     mov    %rsp,%rbp
   0x00000000004004f4 <+4>:     mov    $0x2a,%eax
   0x00000000004004f9 <+9>:     pop    %rbp
   0x00000000004004fa <+10>:    retq   
End of assembler dump.
(gdb)

With full debugging info it's even better.

(gdb) disassemble /m main
Dump of assembler code for function main:
9       {
   0x00000000004004fb <+0>:     push   %rbp
   0x00000000004004fc <+1>:     mov    %rsp,%rbp
   0x00000000004004ff <+4>:     sub    $0x10,%rsp

10        int x = fce ();
   0x0000000000400503 <+8>:     callq  0x4004f0 <fce>
   0x0000000000400508 <+13>:    mov    %eax,-0x4(%rbp)

11        return x;
   0x000000000040050b <+16>:    mov    -0x4(%rbp),%eax

12      }
   0x000000000040050e <+19>:    leaveq 
   0x000000000040050f <+20>:    retq   

End of assembler dump.
(gdb)

objdump has a similar option (-S)

Solution 3

This answer is specific to x86. Portable tools that can disassemble AArch64, MIPS, or whatever machine code include objdump and llvm-objdump.

---

Agner Fog's disassembler, objconv, is quite nice. It will add comments to the disassembly output for performance problems (like the dreaded LCP stall from instructions with 16bit immediate constants, for example).

objconv  -fyasm a.out /dev/stdout | less

(It doesn't recognize - as shorthand for stdout, and defaults to outputting to a file of similar name to the input file, with .asm tacked on.)

It also adds branch targets to the code. Other disassemblers usually disassemble jump instructions with just a numeric destination, and don't put any marker at a branch target to help you find the top of loops and so on.

It also indicates NOPs more clearly than other disassemblers (making it clear when there's padding, rather than disassembling it as just another instruction.)

It's open source, and easy to compile for Linux. It can disassemble into NASM, YASM, MASM, or GNU (AT&T) syntax.

Sample output:

; Filling space: 0FH
; Filler type: Multi-byte NOP
;       db 0FH, 1FH, 44H, 00H, 00H, 66H, 2EH, 0FH
;       db 1FH, 84H, 00H, 00H, 00H, 00H, 00H

ALIGN   16

foo:    ; Function begin
        cmp     rdi, 1                                  ; 00400620 _ 48: 83. FF, 01
        jbe     ?_026                                   ; 00400624 _ 0F 86, 00000084
        mov     r11d, 1                                 ; 0040062A _ 41: BB, 00000001
?_020:  mov     r8, r11                                 ; 00400630 _ 4D: 89. D8
        imul    r8, r11                                 ; 00400633 _ 4D: 0F AF. C3
        add     r8, rdi                                 ; 00400637 _ 49: 01. F8
        cmp     r8, 3                                   ; 0040063A _ 49: 83. F8, 03
        jbe     ?_029                                   ; 0040063E _ 0F 86, 00000097
        mov     esi, 1                                  ; 00400644 _ BE, 00000001
; Filling space: 7H
; Filler type: Multi-byte NOP
;       db 0FH, 1FH, 80H, 00H, 00H, 00H, 00H

ALIGN   8
?_021:  add     rsi, rsi                                ; 00400650 _ 48: 01. F6
        mov     rax, rsi                                ; 00400653 _ 48: 89. F0
        imul    rax, rsi                                ; 00400656 _ 48: 0F AF. C6
        shl     rax, 2                                  ; 0040065A _ 48: C1. E0, 02
        cmp     r8, rax                                 ; 0040065E _ 49: 39. C0
        jnc     ?_021                                   ; 00400661 _ 73, ED
        lea     rcx, [rsi+rsi]                          ; 00400663 _ 48: 8D. 0C 36
...

Note that this output is ready to be assembled back into an object file, so you can tweak the code at the asm source level, rather than with a hex-editor on the machine code. (So you aren't limited to keeping things the same size.) With no changes, the result should be near-identical. It might not be, though, since disassembly of stuff like

  (from /lib/x86_64-linux-gnu/libc.so.6)

SECTION .plt    align=16 execute                        ; section number 11, code

?_00001:; Local function
        push    qword [rel ?_37996]                     ; 0001F420 _ FF. 35, 003A4BE2(rel)
        jmp     near [rel ?_37997]                      ; 0001F426 _ FF. 25, 003A4BE4(rel)

...    
ALIGN   8
?_00002:jmp     near [rel ?_37998]                      ; 0001F430 _ FF. 25, 003A4BE2(rel)

; Note: Immediate operand could be made smaller by sign extension
        push    11                                      ; 0001F436 _ 68, 0000000B
; Note: Immediate operand could be made smaller by sign extension
        jmp     ?_00001                                 ; 0001F43B _ E9, FFFFFFE0

doesn't have anything in the source to make sure it assembles to the longer encoding that leaves room for relocations to rewrite it with a 32bit offset.

---

If you don't want to install it objconv, GNU binutils objdump -Mintel -d is very usable, and will already be installed if you have a normal Linux gcc setup.

Solution 4

there's also ndisasm, which has some quirks, but can be more useful if you use nasm. I agree with Michael Mrozek that objdump is probably best.

[later] you might also want to check out Albert van der Horst's ciasdis: http://home.hccnet.nl/a.w.m.van.der.horst/forthassembler.html. it can be hard to understand, but has some interesting features you won't likely find anywhere else.

Solution 5

You might find ODA useful. It's a web-based disassembler that supports tons of architectures.

http://onlinedisassembler.com/

View more solutions

Comments

[Michael Mrozek]

IDA seems a bit overkill for this, especially considering it's rather expensive

[Adrien Plisson]

the free version is not available for Linux, only the limited demo version. (too bad because, on windows, that's the best disassembler i have ever used)

[Peter Cordes]

For intel-syntax: objdump -Mintel -d. Or Agner Fog's objconv disassembler is the nicest one I've tried yet (see my answer). Adding numbered labels to branch-targets is really really nice.

[cfernandezlinux]

IDA is good but the problem of IDA is you get lazy if you used for small tasks.. gdb does the job for most of everything, gdb easier? no, but possible.

[Peter Cordes]

Useful options: objdump -drwC -Mintel. -r shows relocations from the symbol table. -C demangles C++ names. -W avoids line wrapping for long instructions. If you use it often, this is handy: alias disas='objdump -drwC -Mintel'.

[jouell]

great idea. getting Server Error (500) to onlinedisassembler.com/odaweb - hope it's transient.

[Alexander Pozdneev]

Add -S to display source code intermixed with disassembly. (As pointed in another answer.)

[Albert van der Horst]

In particular: home.hccnet.nl/a.w.m.van.der.horst/ciasdis.html contains under "latest developments" a debian package that you can install easily. With proper instructions (it does scripting) it will generate a source file that will reassemble again to the exact same binary. I'm not aware of any package that can do that. It may be hard to use from the instructions, I intend to publish in github with extensive examples.

[Peter Cordes]

This is only reliable with optimization disabled, otherwise parts of the operations inside the region could optimize into stuff outside, or be optimized away. So you can only see the clunky -O0 asm.

[Bat]

Wow! This is fantastic. Btw, regarding your problem, why don't you use an alias for it to skip typing this huge command?

[Akib Azmain]

IDA is proprietary software, it doesn't respect the user's freedom. It contains DRM which restricts the user from using many features. Moreover, that's a paid software. See gnu.org/proprietary/proprietary.html.

[user135142]

can i know is there a disassembler which will output only AT&A assembly? not all the addresses, binary encodings, etc...