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 radare2, the assembler and disassembler logic is implemented in the rasm* API, and can be used with the pa and pad commands from the commandline as well as using
Rasm2 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.
$ rasm2 -a x86 -b 32 'mov eax, 33' b821000000
Apart from the specifying the input as an argument, you can also pipe it to rasm2:
$ echo 'push eax;nop;nop' | rasm2 -f - 5090
As you have seen, rasm2 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 rasm2 manpage for more details on this.
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
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),
java. For Intel syntax, rasm2 tries to mimic NASM or GAS.
There are several examples in the rasm2 source code directory. Consult them to understand how you can assemble a raw binary file from a rasm2 description.
Lets create an assembly file called
; ; Self-Stop shellcode written in rasm for x86 ; ; --pancake ; .arch x86 .equ base 0x8048000 .org 0x8048000 ; the offset where we inject the 5 byte jmp selfstop: push 0x8048000 pusha mov eax, 20 int 0x80 mov ebx, eax mov ecx, 19 mov eax, 37 int 0x80 popa ret ; ; The call injection ; ret
Now we can assemble it in place:
[0x00000000]> e asm.bits = 32 [0x00000000]> wx `!rasm2 -f a.rasm` [0x00000000]> pd 20 0x00000000 6800800408 push 0x8048000 ; 0x08048000 0x00000005 60 pushad 0x00000006 b814000000 mov eax, 0x14 ; 0x00000014 0x0000000b cd80 int 0x80 syscall[0x80]=? 0x0000000d 89c3 mov ebx, eax 0x0000000f b913000000 mov ecx, 0x13 ; 0x00000013 0x00000014 b825000000 mov eax, 0x25 ; 0x00000025 0x00000019 cd80 int 0x80 syscall[0x80]=? 0x0000001b 61 popad 0x0000001c c3 ret 0x0000001d c3 ret
Assembling also is accessible in radare2 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 commiting the changes.