91  Assembler

Assembling is the action to take a computer instruction in human readable form (using mnemonics) and convert that into a bunch of bytes that can be executed by a machine.

In rizin, the assembler and disassembler logic is implemented in the rz_asm_* API, and can be used with the pa and pad commands from the commandline as well as using rz-asm.

Rz-asm can be used to quickly copy-paste hexpairs that represent a given machine instruction. The following line is assembling this mov instruction for x86/32.

$ rz-asm -a x86 -b 32 'mov eax, 33'

Apart from the specifying the input as an argument, you can also pipe it to rz-asm:

$ echo 'push eax;nop;nop' | rz-asm -f -

As you have seen, rz-asm can assemble one or many instructions. In line by separating them with a semicolon ;, but can also read that from a file, using generic nasm/gas/.. syntax and directives. You can check the rz-asm manpage for more details on this.

The pa and pad are a subcommands of print, what means they will only print assembly or disassembly. In case you want to actually write the instruction it is required to use wa or wx commands with the assembly string or bytes appended.

The assembler understands the following input languages and their flavors: x86 (Intel and AT&T variants), olly (OllyDBG syntax), powerpc (PowerPC), arm and java. For Intel syntax, rz-asm tries to mimic NASM or GAS.

There are several examples in the rz-asm source code directory. Consult them to understand how you can assemble a raw binary file from a rz-asm description.

Lets create an assembly file called selfstop.rzasm:

; Self-Stop shellcode written in rz-asm for x86
; --pancake

.arch x86
.equ base 0x8048000
.org 0x8048000  ; the offset where we inject the 5 byte jmp

  push 0x8048000
  mov eax, 20
  int 0x80

  mov ebx, eax
  mov ecx, 19
  mov eax, 37
  int 0x80
; The call injection


Now we can assemble it in place:

[0x00000000]> e asm.bits=32
[0x00000000]> wx `!rz-asm -f a.rzasm`
[0x00000000]> pd 20
       0x00000000    6800800408   push 0x8048000 ;  0x08048000
       0x00000005    60           pushad
       0x00000006    b814000000   mov eax, 0x14 ;  0x00000014
       0x0000000b    cd80         int 0x80
       0x0000000d    89c3         mov ebx, eax
       0x0000000f    b913000000   mov ecx, 0x13 ;  0x00000013
       0x00000014    b825000000   mov eax, 0x25 ;  0x00000025
       0x00000019    cd80         int 0x80
       0x0000001b    61           popad
       0x0000001c    c3           ret
       0x0000001d    c3           ret

91.0.1 Visual mode

Assembling also is accessible in rizin visual mode through pressing A key to insert the assembly in the current offset.

The cool thing of writing assembly using the visual assembler interface that the changes are done in memory until you press enter.

So you can check the size of the code and which instructions is overlapping before committing the changes.