Drop-in replacement/wrapper around macOS' linker (ld) that respects segprot's `max_prot` value

ld64

#!/usr/bin/python2
import argparse
import sys
import subprocess
from itertools import takewhile
from macholib import MachO, ptypes

def parse_rwx(text):
    return ('r' in text and 1) | ('w' in text and 2) | ('x' in text and 4)

def apply_maxprots(path, maxprots):
    mach = MachO.MachO(path)
    header = mach.headers[0]
    offset = ptypes.sizeof(header.mach_header)

    for cload, ccmd, cdata in header.commands:
        if not hasattr(ccmd, 'segname'): break

        segname = ccmd.segname.to_str().strip(chr(0))

        if segname in maxprots and ccmd.maxprot != maxprots[segname]:
            fields = takewhile(lambda (n, _): n != 'maxprot', cload._fields_ + ccmd._fields_)
            index = offset + sum(ptypes.sizeof(typ) for _, typ in fields)

            with open(path, 'r+b') as fh:
                fh.seek(index)
                fh.write(chr(maxprots[segname]))

        offset += cload.cmdsize

try:
    subprocess.check_call(['/usr/bin/ld'] + sys.argv[1:])
except subprocess.CalledProcessError as ex:
    sys.exit(ex.returncode)

parser = argparse.ArgumentParser()
parser.add_argument('-o', default='a.out')
parser.add_argument('-segprot', nargs=3, action='append')
args, _ = parser.parse_known_args()

if args.segprot:
    segprots = {segment: parse_rwx(maxprot) for segment, maxprot, _ in args.segprot}
    apply_maxprots(args.o, segprots)

Run with -segprot __TEXT rwx rx to allow self-modifying code.

Thank you!
You helped me to find a solution for "syscall.Mprotect panic: permission denied" in Golang.

thanks both of you!
solved my problem on running test for monkey patching
tested & worked on mac os big sur 11.2.3

thanks both of you!
solved my problem on running test for monkey patching
tested & worked on mac os big sur 11.2.3

hi,i have same issue in my mac os big sur 11.2.3. i want learn how to run the file to solve my question.

it didn't work on Apple Silicon, mac os big sur 11.4

Yes, see https://developer.apple.com/documentation/apple-silicon/porting-just-in-time-compilers-to-apple-silicon.

RWX-pages will most likely never be allowed on Apple Silicon, the closest is to use mprotect & MAP_JIT in tandem with pthread_jit_write_protect_np (although this still leaves a ton of use cases outside the realm of possibility).

Mac OS Version 11.6 m1 does not work.
Could some one give a solution to help syscall.Mprotect works on m1 silicon?