274 lines
10 KiB
Python
Executable File
274 lines
10 KiB
Python
Executable File
#!/usr/bin/env python
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# Copyright (c) 2011 The Chromium Authors. All rights reserved.
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# Use of this source code is governed by a BSD-style license that can be
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# found in the LICENSE file.
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"""Usage: change_mach_o_flags.py [--executable-heap] [--no-pie] <executablepath>
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Arranges for the executable at |executable_path| to have its data (heap)
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pages protected to prevent execution on Mac OS X 10.7 ("Lion"), and to have
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the PIE (position independent executable) bit set to enable ASLR (address
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space layout randomization). With --executable-heap or --no-pie, the
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respective bits are cleared instead of set, making the heap executable or
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disabling PIE/ASLR.
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This script is able to operate on thin (single-architecture) Mach-O files
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and fat (universal, multi-architecture) files. When operating on fat files,
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it will set or clear the bits for each architecture contained therein.
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NON-EXECUTABLE HEAP
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Traditionally in Mac OS X, 32-bit processes did not have data pages set to
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prohibit execution. Although user programs could call mprotect and
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mach_vm_protect to deny execution of code in data pages, the kernel would
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silently ignore such requests without updating the page tables, and the
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hardware would happily execute code on such pages. 64-bit processes were
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always given proper hardware protection of data pages. This behavior was
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controllable on a system-wide level via the vm.allow_data_exec sysctl, which
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is set by default to 1. The bit with value 1 (set by default) allows code
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execution on data pages for 32-bit processes, and the bit with value 2
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(clear by default) does the same for 64-bit processes.
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In Mac OS X 10.7, executables can "opt in" to having hardware protection
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against code execution on data pages applied. This is done by setting a new
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bit in the |flags| field of an executable's |mach_header|. When
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MH_NO_HEAP_EXECUTION is set, proper protections will be applied, regardless
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of the setting of vm.allow_data_exec. See xnu-1699.22.73/osfmk/vm/vm_map.c
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override_nx and xnu-1699.22.73/bsd/kern/mach_loader.c load_machfile.
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The Apple toolchain has been revised to set the MH_NO_HEAP_EXECUTION when
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producing executables, provided that -allow_heap_execute is not specified
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at link time. Only linkers shipping with Xcode 4.0 and later (ld64-123.2 and
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later) have this ability. See ld64-123.2.1/src/ld/Options.cpp
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Options::reconfigureDefaults() and
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ld64-123.2.1/src/ld/HeaderAndLoadCommands.hpp
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HeaderAndLoadCommandsAtom<A>::flags().
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This script sets the MH_NO_HEAP_EXECUTION bit on Mach-O executables. It is
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intended for use with executables produced by a linker that predates Apple's
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modifications to set this bit itself. It is also useful for setting this bit
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for non-i386 executables, including x86_64 executables. Apple's linker only
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sets it for 32-bit i386 executables, presumably under the assumption that
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the value of vm.allow_data_exec is set in stone. However, if someone were to
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change vm.allow_data_exec to 2 or 3, 64-bit x86_64 executables would run
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without hardware protection against code execution on data pages. This
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script can set the bit for x86_64 executables, guaranteeing that they run
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with appropriate protection even when vm.allow_data_exec has been tampered
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with.
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POSITION-INDEPENDENT EXECUTABLES/ADDRESS SPACE LAYOUT RANDOMIZATION
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This script sets or clears the MH_PIE bit in an executable's Mach-O header,
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enabling or disabling position independence on Mac OS X 10.5 and later.
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Processes running position-independent executables have varying levels of
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ASLR protection depending on the OS release. The main executable's load
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address, shared library load addresess, and the heap and stack base
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addresses may be randomized. Position-independent executables are produced
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by supplying the -pie flag to the linker (or defeated by supplying -no_pie).
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Executables linked with a deployment target of 10.7 or higher have PIE on
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by default.
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This script is never strictly needed during the build to enable PIE, as all
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linkers used are recent enough to support -pie. However, it's used to
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disable the PIE bit as needed on already-linked executables.
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"""
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import optparse
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import os
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import struct
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import sys
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# <mach-o/fat.h>
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FAT_MAGIC = 0xcafebabe
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FAT_CIGAM = 0xbebafeca
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# <mach-o/loader.h>
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MH_MAGIC = 0xfeedface
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MH_CIGAM = 0xcefaedfe
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MH_MAGIC_64 = 0xfeedfacf
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MH_CIGAM_64 = 0xcffaedfe
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MH_EXECUTE = 0x2
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MH_PIE = 0x00200000
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MH_NO_HEAP_EXECUTION = 0x01000000
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class MachOError(Exception):
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"""A class for exceptions thrown by this module."""
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pass
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def CheckedSeek(file, offset):
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"""Seeks the file-like object at |file| to offset |offset| and raises a
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MachOError if anything funny happens."""
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file.seek(offset, os.SEEK_SET)
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new_offset = file.tell()
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if new_offset != offset:
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raise MachOError, \
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'seek: expected offset %d, observed %d' % (offset, new_offset)
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def CheckedRead(file, count):
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"""Reads |count| bytes from the file-like |file| object, raising a
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MachOError if any other number of bytes is read."""
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bytes = file.read(count)
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if len(bytes) != count:
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raise MachOError, \
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'read: expected length %d, observed %d' % (count, len(bytes))
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return bytes
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def ReadUInt32(file, endian):
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"""Reads an unsinged 32-bit integer from the file-like |file| object,
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treating it as having endianness specified by |endian| (per the |struct|
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module), and returns it as a number. Raises a MachOError if the proper
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length of data can't be read from |file|."""
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bytes = CheckedRead(file, 4)
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(uint32,) = struct.unpack(endian + 'I', bytes)
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return uint32
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def ReadMachHeader(file, endian):
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"""Reads an entire |mach_header| structure (<mach-o/loader.h>) from the
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file-like |file| object, treating it as having endianness specified by
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|endian| (per the |struct| module), and returns a 7-tuple of its members
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as numbers. Raises a MachOError if the proper length of data can't be read
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from |file|."""
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bytes = CheckedRead(file, 28)
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magic, cputype, cpusubtype, filetype, ncmds, sizeofcmds, flags = \
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struct.unpack(endian + '7I', bytes)
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return magic, cputype, cpusubtype, filetype, ncmds, sizeofcmds, flags
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def ReadFatArch(file):
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"""Reads an entire |fat_arch| structure (<mach-o/fat.h>) from the file-like
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|file| object, treating it as having endianness specified by |endian|
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(per the |struct| module), and returns a 5-tuple of its members as numbers.
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Raises a MachOError if the proper length of data can't be read from
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|file|."""
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bytes = CheckedRead(file, 20)
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cputype, cpusubtype, offset, size, align = struct.unpack('>5I', bytes)
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return cputype, cpusubtype, offset, size, align
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def WriteUInt32(file, uint32, endian):
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"""Writes |uint32| as an unsinged 32-bit integer to the file-like |file|
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object, treating it as having endianness specified by |endian| (per the
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|struct| module)."""
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bytes = struct.pack(endian + 'I', uint32)
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assert len(bytes) == 4
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file.write(bytes)
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def HandleMachOFile(file, options, offset=0):
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"""Seeks the file-like |file| object to |offset|, reads its |mach_header|,
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and rewrites the header's |flags| field if appropriate. The header's
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endianness is detected. Both 32-bit and 64-bit Mach-O headers are supported
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(mach_header and mach_header_64). Raises MachOError if used on a header that
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does not have a known magic number or is not of type MH_EXECUTE. The
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MH_PIE and MH_NO_HEAP_EXECUTION bits are set or cleared in the |flags| field
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according to |options| and written to |file| if any changes need to be made.
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If already set or clear as specified by |options|, nothing is written."""
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CheckedSeek(file, offset)
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magic = ReadUInt32(file, '<')
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if magic == MH_MAGIC or magic == MH_MAGIC_64:
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endian = '<'
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elif magic == MH_CIGAM or magic == MH_CIGAM_64:
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endian = '>'
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else:
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raise MachOError, \
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'Mach-O file at offset %d has illusion of magic' % offset
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CheckedSeek(file, offset)
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magic, cputype, cpusubtype, filetype, ncmds, sizeofcmds, flags = \
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ReadMachHeader(file, endian)
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assert magic == MH_MAGIC or magic == MH_MAGIC_64
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if filetype != MH_EXECUTE:
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raise MachOError, \
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'Mach-O file at offset %d is type 0x%x, expected MH_EXECUTE' % \
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(offset, filetype)
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original_flags = flags
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if options.no_heap_execution:
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flags |= MH_NO_HEAP_EXECUTION
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else:
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flags &= ~MH_NO_HEAP_EXECUTION
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if options.pie:
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flags |= MH_PIE
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else:
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flags &= ~MH_PIE
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if flags != original_flags:
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CheckedSeek(file, offset + 24)
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WriteUInt32(file, flags, endian)
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def HandleFatFile(file, options, fat_offset=0):
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"""Seeks the file-like |file| object to |offset| and loops over its
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|fat_header| entries, calling HandleMachOFile for each."""
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CheckedSeek(file, fat_offset)
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magic = ReadUInt32(file, '>')
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assert magic == FAT_MAGIC
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nfat_arch = ReadUInt32(file, '>')
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for index in xrange(0, nfat_arch):
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cputype, cpusubtype, offset, size, align = ReadFatArch(file)
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assert size >= 28
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# HandleMachOFile will seek around. Come back here after calling it, in
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# case it sought.
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fat_arch_offset = file.tell()
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HandleMachOFile(file, options, offset)
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CheckedSeek(file, fat_arch_offset)
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def main(me, args):
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parser = optparse.OptionParser('%prog [options] <executable_path>')
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parser.add_option('--executable-heap', action='store_false',
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dest='no_heap_execution', default=True,
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help='Clear the MH_NO_HEAP_EXECUTION bit')
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parser.add_option('--no-pie', action='store_false',
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dest='pie', default=True,
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help='Clear the MH_PIE bit')
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(options, loose_args) = parser.parse_args(args)
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if len(loose_args) != 1:
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parser.print_usage()
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return 1
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executable_path = loose_args[0]
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executable_file = open(executable_path, 'rb+')
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magic = ReadUInt32(executable_file, '<')
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if magic == FAT_CIGAM:
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# Check FAT_CIGAM and not FAT_MAGIC because the read was little-endian.
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HandleFatFile(executable_file, options)
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elif magic == MH_MAGIC or magic == MH_CIGAM or \
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magic == MH_MAGIC_64 or magic == MH_CIGAM_64:
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HandleMachOFile(executable_file, options)
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else:
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raise MachOError, '%s is not a Mach-O or fat file' % executable_file
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executable_file.close()
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return 0
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if __name__ == '__main__':
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sys.exit(main(sys.argv[0], sys.argv[1:]))
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