import struct
import sys
SECTOR_SIZE = 2048
def find_image(fp):
# el-torito boot record descriptor
fp.seek(0x11 * SECTOR_SIZE)
data = fp.read(SECTOR_SIZE)
assert data[:0x47] == b'\x00CD001\x01EL TORITO SPECIFICATION' + b'\x0' * 41
boot_catalog_sector = struct.unpack('<L', data[0x47:0x4B])[0]
# check the validation entry in the catalog
fp.seek(boot_catalog_sector * SECTOR_SIZE)
data = fp.read(0x20)
assert data[0:1] == b'\x01'
assert data[0x1e:0x20] == b'\x55\xAA'
assert sum(struct.unpack('<16H', data)) % 0x10000 == 0
# Read the initial/default entry
data = fp.read(0x20)
(bootable, image_type, load_segment, system_type, sector_count,
image_sector) = struct.unpack('<BBHBxHL', data[:12])
image_offset = image_sector * SECTOR_SIZE
if image_type == 1:
# 1.2MB floppy
image_size = 1200 * 1024
elif image_type == 2:
# 1.44MB floppy
image_size = 1440 * 1024
elif image_type == 3:
# 2.88MB floppy
image_size = 2880 * 1024
elif image_type == 4:
# Hard disk image. Read the MBR partition table to locate file system
fp.seek(image_offset)
data = fp.read(512)
# Read the first partition entry
(bootable, part_type, part_start, part_size) = struct.unpack_from(
'<BxxxBxxxLL', data, 0x1BE)
assert bootable == 0x80 # is partition bootable?
image_offset += part_start * 512
image_size = part_size * 512
else:
raise AssertionError('unhandled image format: %d' % image_type)
fp.seek(image_offset)
return fp.read(image_size)
if __name__ == '__main__':
with open(sys.argv[1], 'rb') as iso, open(sys.argv[2], 'wb') as img:
img.write(find_image(iso))
