#!/usr/bin/python3
# Copyright 2024 Helmut Grohne <helmut@subdivi.de>
# SPDX-License-Identifier: GPL-3

"""Mount a given ext4 filesystem image inside a user and mount namespace using
unprivileged fuse2fs and chroot into the mouned filesystem.

This requires a fuse package with support for /dev/fd/N. Either with fuse2 via
https://bugs.debian.org/1055222 or when e2fsprogs has been ported to fuse3.
"""

import os
import pathlib
import socket
import sys

if __file__.split("/")[-2:-1] == ["examples"]:
    sys.path.insert(0, "/".join(__file__.split("/")[:-2]))

import linuxnamespaces


def main() -> None:
    fsimage = pathlib.Path(sys.argv[1])
    assert fsimage.exists()
    uidmap = linuxnamespaces.IDAllocation.loadsubid("uid").allocatemap(65536)
    gidmap = linuxnamespaces.IDAllocation.loadsubid("gid").allocatemap(65536)
    mainpid = os.getpid()
    mainsock, childsock = socket.socketpair()
    @linuxnamespaces.run_in_fork
    def setup() -> None:
        mainsock.close()
        linuxnamespaces.newidmaps(mainpid, [uidmap], [gidmap])
        childsock.send(b"\0")
        _, fds, _, _ = socket.recv_fds(childsock, 1, 1, 0)
        childsock.close()
        os.set_inheritable(fds[0], True)
        os.execlp("fuse2fs", "fuse2fs", str(fsimage), "/dev/fd/%d" % fds[0])
    linuxnamespaces.unshare(
        linuxnamespaces.CloneFlags.NEWUSER | linuxnamespaces.CloneFlags.NEWNS
    )
    setup.start()
    mainsock.recv(1)
    os.setreuid(0, 0)
    os.setregid(0, 0)
    fusefd = os.open("/dev/fuse", os.O_RDWR)
    socket.send_fds(mainsock, [b"\0"], [fusefd])
    mainsock.close()
    linuxnamespaces.mount(
        str(fsimage),
        "/mnt",
        "fuse.ext4",
        linuxnamespaces.MountFlags.NONE,
        "fd=%d,rootmode=040755,user_id=0,group_id=0,allow_other" % fusefd,
    )
    os.chdir("/mnt")
    linuxnamespaces.bind_mount("/proc", "proc", recursive=True)
    linuxnamespaces.bind_mount("/sys", "sys", recursive=True)
    linuxnamespaces.populate_dev("/", ".", pidns=False, tun=False)
    linuxnamespaces.pivot_root(".", ".")
    linuxnamespaces.umount(".", linuxnamespaces.UmountFlags.DETACH)
    os.close(fusefd)
    os.execlp(os.environ["SHELL"], os.environ["SHELL"])


if __name__ == "__main__":
    main()