Attachment "fix_macho" to
ticket [113be1991e]
added by
marc_culler
2021-01-30 17:43:44.
#!/usr/bin/env python3
import os
import sys
from macholib.MachO import MachO
class MachOExtender(MachO):
"""
Adds a method which adjusts the Mach-O header to make it appear as
if arbitrary data added to the end of the binary is a string within
the SYMTAB section of the LINKEDIT segment.
"""
def __init__(self, binary_file):
MachO.__init__(self, binary_file)
self.binary_file = binary_file
self.filesize = os.path.getsize(binary_file)
def update_header(self):
# A fat binary has a header for each architecture, but we can only
# operate on one architecture at a time. Fat binaries must be split
# appart with the lipo tool.
if len(self.headers) > 1:
raise RuntimeError(
'This tool only works on single-architecture binaries')
header = self.headers[0]
# The Mach-O header is followed by a sequence of load commands, some of
# which define segments. For a 64-bit macOS system, the commands which
# define a segment have command name LC_SEGMENT_64. The LINKEDIT segment
# is always the last segment. Our goal is to expand the LINKEDIT segment
# to include the extra data appended to the mach-O binary.
for command in header.commands:
if command[0].get_cmd_name() == 'LC_SEGMENT_64':
if command[1].segname.startswith(b'__LINKEDIT'):
linkedit_cmd = command
linkedit_info = linkedit_cmd[1]
data_size = self.filesize - linkedit_info.fileoff - linkedit_info.filesize
assert data_size > 0 , "You must append your extra data first!"
linkedit_info.filesize += data_size
# Apple says that he virtual memory size of the segment must be rounded
# up to a multiple of the page size. But we round to a multiple of
# 16384 like Apple does.
linkedit_info.vmsize = (linkedit_info.filesize + 0x3fff) & 0x4000
# Search for the LC_SYMTAB section. The extra data is made to appear as
# if it were a long string at the end of the string table within the
# LC_SYMTAB section. So we need to adjust the size of that section.
for command in header.commands[:]:
if command[0].get_cmd_name() == 'LC_SYMTAB':
symtab_info = command[1]
break
symtab_info.strsize += data_size
# Save the patched mach-O header.
with open(self.binary_file, 'rb+') as the_binary:
self.write(the_binary)
if __name__ == '__main__':
if len(sys.argv) != 2 or not os.path.exists(sys.argv[1]):
print("Usage: fix_macho path_to_binary")
sys.exit(1)
extender = MachOExtender(sys.argv[1])
extender.update_header()
