Trips of a curious penguin.

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Making system calls from Assembly in Mac OS X

The next step in my playing with chroot escapes is crafting some shellcode. Recently my main dev machine is a MacBook running OS X, so it felt reasonable to fiddle with making system calls of that platform.

By the way, a system call is a function of the kernel invoked by a userspace program and it can be something like writing to a file descriptor, or even exiting. Usually, these are wrapped by C functions in the standard library.

The system calls

First, we need to know what system call we want to make, and what arguments it pretends.

A full list is hosted by Apple here. The header also hints at the fact that they are inherited from BSD. Yeah, that makes sense.

So, to write our proverbial Hello world we will pick the syscall 4

4   AUE_NULL    ALL { user_ssize_t write(int fd, user_addr_t cbuf, user_size_t nbyte); }


Let’s start easy. A cute 32-bit program, written in NASM assembler language. Compile with nasm or yasm, output format MachO, and link with ld.

I’m on a Intel machine, so what we are looking for is the x86 syscall calling conventions for the OS X or BSD platform. They are pretty simple:

  • arguments passed on the stack, pushed right-to-left
  • stack 16-bytes aligned
  • syscall number in the eax register
  • call by interrupt 0x80

So what we have to do to print a “Hello world” is:

  • push the length of the string (int) to the stack
  • push a pointer to the string to the stack
  • push the stdout file descriptor (1) to the stack
  • align the stack by moving the stack pointer 4 more bytes (16 - 4 * 3)
  • set the eax register to the write syscall number (4)
  • interrupt 0x80


64-bit is a bit cleaner, but completely different: OS X (and GNU/Linux and everyone except Windows) on 64 architectures adopt the System V AMD64 ABI reference. Jump to section A.2.1 for the syscall calling convention.

  • arguments are passed on the registers rdi, rsi, rdx, r10, r8 and r9
  • syscall number in the rax register
  • the call is done via the syscall instruction
  • what OS X contributes to the mix is that you have to add 0x20000000 to the syscall number (still have to figure out why)

So, here is the (IMHO) much more clean 64-bit “Hello world”. Ah, if you want to do this at home and have it actually run, generate a macho64 object with a new version of NASM or with YASM, and link with ld as usual.