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#!/usr/bin/python3
# Copyright 2024 Helmut Grohne <helmut@subdivi.de>
# SPDX-License-Identifier: LGPL-2.0-or-later
"""Extract a given tarball into a temporary location and chroot into it inside
a user and mount namespace.
"""
import argparse
import os
import pathlib
import re
import socket
import sys
import tempfile
if __file__.split("/")[-2:-1] == ["examples"]:
sys.path.insert(0, "/".join(__file__.split("/")[:-2]))
import linuxnamespaces
import linuxnamespaces.tarutils
class TarFile(
linuxnamespaces.tarutils.ZstdTarFile, linuxnamespaces.tarutils.XAttrTarFile
):
pass
def main() -> None:
parser = argparse.ArgumentParser()
parser.add_argument(
"--save",
action="store_true",
help="save and replace the tarball at the end of the session",
)
parser.add_argument(
"--same-uid",
action="store_true",
help="map the current user to itself in the namespace",
)
parser.add_argument(
"--bind",
action="append",
help="bind mount the given location after extraction",
default=[],
)
parser.add_argument(
"basetar",
type=pathlib.Path,
action="store",
help="location of the tarball containing the chroot",
)
parser.add_argument(
"command",
nargs=argparse.REMAINDER,
help="command to run inside the chroot",
)
args = parser.parse_args()
assert args.basetar.exists()
myuid = os.getuid()
mygid = os.getgid()
uidrange = linuxnamespaces.IDAllocation.loadsubid("uid").allocatemap(65536)
gidrange = linuxnamespaces.IDAllocation.loadsubid("gid").allocatemap(65536)
if args.same_uid:
uidmaps = [
uidrange[:myuid],
linuxnamespaces.IDMapping(uidrange.innerstart + myuid, myuid, 1),
uidrange[myuid + 1:],
]
gidmaps = [
gidrange[:mygid],
linuxnamespaces.IDMapping(gidrange.innerstart + mygid, mygid, 1),
gidrange[mygid + 1:],
]
else:
uidmaps = [uidrange]
gidmaps = [gidrange]
bindmounts = []
for conf in args.bind:
confparts = conf.split(":")
if not os.path.exists(confparts[0]):
raise ValueError(
f"cannot bind mount {confparts[0]}: does not exist"
)
if len(confparts) > 2:
raise ValueError(f"bind mount {conf} not understood")
if len(confparts) < 2:
confparts.append(confparts[0])
bindmounts.append(
(
os.path.abspath(confparts[0]),
os.path.normpath(
os.path.join("/", confparts[1])
).removeprefix("/"),
),
)
with tempfile.TemporaryDirectory() as tdir:
parentsock, childsock = socket.socketpair()
pid = os.fork()
if pid == 0:
parentsock.close()
# Once we drop privileges via setreuid and friends, we may become
# unable to open basetar or to chdir to tdir, so do those early.
with TarFile.open(args.basetar, "r:*") as tarf:
os.chdir(tdir)
linuxnamespaces.unshare(
linuxnamespaces.CloneFlags.NEWUSER
| linuxnamespaces.CloneFlags.NEWNS
)
childsock.send(
linuxnamespaces.tarutils.get_comptype(
tarf
).encode("ascii") + b"\0",
)
childsock.recv(1)
childsock.close()
# The other process will now have set up our id mapping and
# will have changed ownership of our working directory.
os.setreuid(0, 0)
os.setregid(0, 0)
os.setgroups([])
for tmem in tarf:
name = re.sub(r"^/*(\.{1,2}/+)*", "", tmem.name)
if name.startswith("dev/"):
continue
tarf.extract(tmem, numeric_owner=True)
linuxnamespaces.bind_mount(".", "/mnt", recursive=True)
os.chdir("/mnt")
linuxnamespaces.bind_mount("/proc", "proc", recursive=True)
linuxnamespaces.bind_mount("/sys", "sys", recursive=True)
linuxnamespaces.populate_dev("/", ".", pts="host", tun=False)
for source, target in bindmounts:
os.makedirs(target, exist_ok=True)
linuxnamespaces.bind_mount(source, target, recursive=True)
linuxnamespaces.pivot_root(".", ".")
linuxnamespaces.umount(".", linuxnamespaces.UmountFlags.DETACH)
if args.command:
os.execvp(args.command[0], args.command)
else:
os.execlp(os.environ["SHELL"], os.environ["SHELL"])
os._exit(1)
childsock.close()
comptype = parentsock.recv(10).split(b"\0", 1)[0].decode("ascii")
linuxnamespaces.newidmaps(pid, uidmaps, gidmaps)
# We still had to be in the initial namespace to call newidmaps and
# now we transition to a namespace that can access both the container
# and the files of the invoking user.
if not args.same_uid:
uidmaps.append(linuxnamespaces.IDMapping(65536, myuid, 1))
gidmaps.append(linuxnamespaces.IDMapping(65536, mygid, 1))
linuxnamespaces.unshare_user_idmap(uidmaps, gidmaps)
os.chown(tdir, 0, 0)
os.chmod(tdir, 0o755)
parentsock.send(b"\0")
parentsock.close()
_, ret = os.waitpid(pid, 0)
if args.save and ret == 0:
tmptar = f"{args.basetar}.new"
try:
with TarFile.open(tmptar, "x:" + comptype) as tout:
tout.add(tdir, ".")
os.rename(tmptar, args.basetar)
except:
os.unlink(tmptar)
raise
sys.exit(ret)
if __name__ == "__main__":
main()
|
