NetBoot/resizehfs.py

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#!/usr/bin/env python3
# MIT License
# Copyright (c) 2018 Elliot Nunn
# Permission is hereby granted, free of charge, to any person obtaining a copy
# of this software and associated documentation files (the "Software"), to deal
# in the Software without restriction, including without limitation the rights
# to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
# copies of the Software, and to permit persons to whom the Software is
# furnished to do so, subject to the following conditions:
# The above copyright notice and this permission notice shall be included in all
# copies or substantial portions of the Software.
# THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
# IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
# FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
# AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
# LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
# OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
# SOFTWARE.
import struct
def alignment(num):
# count trailing zero bits to a max of 32
for i in reversed(range(33)):
if num & ((1 << i) - 1) == 0: return i
def bitmap_len(drNmAlBlks):
# how many bytes occupied by the 512b blocks required to make up n bits?
num_512s = (drNmAlBlks + 4095) // 4096
return num_512s * 512
def resize(img, freespace):
drVBMSt, = struct.unpack_from('>H', img, 0x40E)
drNmAlBlks, = struct.unpack_from('>H', img, 0x412)
drAlBlkSiz, = struct.unpack_from('>L', img, 0x414)
drAlBlSt, = struct.unpack_from('>H', img, 0x41C)
drFreeBks, = struct.unpack_from('>H', img, 0x422)
align = alignment(drAlBlSt)
header = bytearray(img[:0x600])
bitmap = img[drVBMSt*512:drVBMSt*512+bitmap_len(drNmAlBlks)]
guts = img[drAlBlSt*512:drAlBlSt*512+drNmAlBlks*drAlBlkSiz]
# Decide how much to shrink
drFreeBks -= drNmAlBlks
for drNmAlBlks in range(drNmAlBlks, 0, -1):
byte = bitmap[(drNmAlBlks-1) >> 3]
mask = 0x80 >> ((drNmAlBlks-1) & 7)
if byte & mask: break
drFreeBks += drNmAlBlks
# Decide how much to expand
while drNmAlBlks < 0xffff and drFreeBks * drAlBlkSiz < freespace:
drFreeBks += 1
drNmAlBlks += 1
# Resize components
bitmap = bitmap[:bitmap_len(drNmAlBlks)].ljust(bitmap_len(drNmAlBlks), b'\0')
guts = guts[:drNmAlBlks*drAlBlkSiz].ljust(drNmAlBlks*drAlBlkSiz, b'\0')
# Reposition the guts after the bitmap, preserving original alignment
drAlBlSt = 3 + len(bitmap)//512
while alignment(drAlBlSt) < align: drAlBlSt += 1
struct.pack_into('>H', header, 0x40E, len(header)//512) # drVBMSt
struct.pack_into('>H', header, 0x412, drNmAlBlks)
struct.pack_into('>H', header, 0x41C, drAlBlSt)
struct.pack_into('>H', header, 0x422, drFreeBks)
accum = bytearray()
accum.extend(header)
accum.extend(bitmap)
accum.extend(bytes(drAlBlSt*512 - len(accum)))
accum.extend(guts)
accum.extend(header[0x400:0x800])
accum.extend(bytes(512))
return accum
if __name__ == '__main__':
import sys
import argparse
parser = argparse.ArgumentParser(description='''
Resize an HFS disk image by adding/removing allocation blocks.
Warning: only images from `machfs` can be shrunk, because the
real MacOS uses the allocation blocks at the end of the disk.
''')
parser.add_argument('diskimage', metavar='PATH', action='store', help='disk image')
parser.add_argument('--freespace', metavar='BYTES', action='store', type=int, default=0, help='free space target (0 means shrink)')
parser.add_argument('--verbose', '-v', action='store_true', help='print stats')
args = parser.parse_args()
with open(args.diskimage, 'rb') as f:
oldimage = f.read()
if oldimage[1024:1026] != b'BD': sys.exit('Not an HFS volume')
newimage = resize(oldimage, args.freespace)
if args.verbose:
for kind, img in (('old', oldimage), ('new', newimage)):
drNmAlBlks, = struct.unpack_from('>H', img, 0x412)
drFreeBks, = struct.unpack_from('>H', img, 0x422)
print(kind, 'bytes: ', len(img))
print(kind, 'drNmAlBlks: ', drNmAlBlks)
print(kind, 'drFreeBks: ', drFreeBks)
print('percentage size:', 100 * len(newimage) // len(oldimage))
print('changed: ', ('no' if newimage == oldimage else 'yes'))
if newimage != oldimage:
with open(args.diskimage, 'wb') as f:
f.write(newimage)