macOS Mach-O multiarch ("fat") binary: Python parsing library

This is a simple container format that encapsulates multiple Mach-O files, each generally for a different architecture. XNU can execute these files just like single-arch Mach-Os and will pick the appropriate entry.

KS implementation details

License: CC0-1.0

This page hosts a formal specification of macOS Mach-O multiarch ("fat") binary using Kaitai Struct. This specification can be automatically translated into a variety of programming languages to get a parsing library.

Usage

Runtime library

All parsing code for Python generated by Kaitai Struct depends on the Python runtime library. You have to install it before you can parse data.

The Python runtime library can be installed from PyPI:

python3 -m pip install kaitaistruct

Code

Parse a local file and get structure in memory:

data = MachOFat.from_file("path/to/local/file.bin")

Or parse structure from a bytes:

from kaitaistruct import KaitaiStream, BytesIO

raw = b"\x00\x01\x02..."
data = MachOFat(KaitaiStream(BytesIO(raw)))

After that, one can get various attributes from the structure by invoking getter methods like:

data.magic # => get magic

Python source code to parse macOS Mach-O multiarch ("fat") binary

mach_o_fat.py

# This is a generated file! Please edit source .ksy file and use kaitai-struct-compiler to rebuild

import kaitaistruct
from kaitaistruct import KaitaiStruct, KaitaiStream, BytesIO


if getattr(kaitaistruct, 'API_VERSION', (0, 9)) < (0, 9):
    raise Exception("Incompatible Kaitai Struct Python API: 0.9 or later is required, but you have %s" % (kaitaistruct.__version__))

import mach_o
class MachOFat(KaitaiStruct):
    """This is a simple container format that encapsulates multiple Mach-O files,
    each generally for a different architecture. XNU can execute these files just
    like single-arch Mach-Os and will pick the appropriate entry.
    
    .. seealso::
       Source - https://opensource.apple.com/source/xnu/xnu-7195.121.3/EXTERNAL_HEADERS/mach-o/fat.h.auto.html
    """
    def __init__(self, _io, _parent=None, _root=None):
        self._io = _io
        self._parent = _parent
        self._root = _root if _root else self
        self._read()

    def _read(self):
        self.magic = self._io.read_bytes(4)
        if not self.magic == b"\xCA\xFE\xBA\xBE":
            raise kaitaistruct.ValidationNotEqualError(b"\xCA\xFE\xBA\xBE", self.magic, self._io, u"/seq/0")
        self.num_fat_arch = self._io.read_u4be()
        self.fat_archs = []
        for i in range(self.num_fat_arch):
            self.fat_archs.append(MachOFat.FatArch(self._io, self, self._root))


    class FatArch(KaitaiStruct):
        def __init__(self, _io, _parent=None, _root=None):
            self._io = _io
            self._parent = _parent
            self._root = _root if _root else self
            self._read()

        def _read(self):
            self.cpu_type = KaitaiStream.resolve_enum(MachO.CpuType, self._io.read_u4be())
            self.cpu_subtype = self._io.read_u4be()
            self.ofs_object = self._io.read_u4be()
            self.len_object = self._io.read_u4be()
            self.align = self._io.read_u4be()

        @property
        def object(self):
            if hasattr(self, '_m_object'):
                return self._m_object

            _pos = self._io.pos()
            self._io.seek(self.ofs_object)
            self._raw__m_object = self._io.read_bytes(self.len_object)
            _io__raw__m_object = KaitaiStream(BytesIO(self._raw__m_object))
            self._m_object = mach_o.MachO(_io__raw__m_object)
            self._io.seek(_pos)
            return getattr(self, '_m_object', None)