loopback device setup/teardown/offset in python

losetup.py

import ctypes
import fcntl

SetStatus64 = 0x4C04
GetStatus64 = 0x4C05
CLR_FD   = 0x4C01
SET_FD   = 0x4C00
NameSize = 64
KeySize  = 32

class Info(ctypes.Structure):
    _fields_ = (
            ('Device',          ctypes.c_uint64),
            ('INode',           ctypes.c_uint64),
            ('RDevice',         ctypes.c_uint64),
            ('Offset',          ctypes.c_uint64),
            ('SizeLimit',       ctypes.c_uint64),
            ('Number',          ctypes.c_uint32),
            ('EncryptType',     ctypes.c_uint32),
            ('EncryptKeySize',  ctypes.c_uint32),
            ('Flags',           ctypes.c_uint32),
            ('FileName',        ctypes.c_byte),
            ('CryptName',       NameSize*ctypes.c_byte),
            ('EncryptKey',      KeySize*ctypes.c_byte),
            ('Init',            2*ctypes.c_uint64),
    )
    def __repr__(self):
        ret = []
        for n, v in a._fields_:
            ret.append(f'{n}: {getattr(self, n)}')
        return '\n'.join(ret)

def set_offset(loop, offset):
    a = Info()
    fcntl.ioctl(loop, GetStatus64, a)
    a.Offset = offset
    fcntl.ioctl(loop, SetStatus64, a)

fd = open('disk.img')
loop = open('/dev/loop7')

# mount fd to loopback device
fcntl.ioctl(loop, SET_FD, fd.fileno())
set_offset(loop, 512*2048)

a = Info()
fcntl.ioctl(loop, GetStatus64, a)
print(a)

# unmount
fcntl.ioctl(loop, CLR_FD, 0)