Python subprocess.run cannot execute in chroot

Question

I have two files, a.py:

import os, subprocess

os.chroot(".")
subprocess.run(["./b.sh"])

and b.sh:

#!/usr/bin/env bash
echo whateva

in an otherwise empty directory. b.sh has executable permission. Why does python3 a.py fail with:

Traceback (most recent call last):
  File "/yadda/yadda/a.py", line 5, in <module>
  File "/usr/lib/python3.11/subprocess.py", line 548, in run
  File "/usr/lib/python3.11/subprocess.py", line 1024, in __init__
  File "/usr/lib/python3.11/subprocess.py", line 1917, in _execute_child
FileNotFoundError: [Errno 2] No such file or directory: './b.sh'

This is running as root, uname -a is Linux me 6.4.1-arch2-1 #1 SMP PREEMPT_DYNAMIC Tue, 04 Jul 2023 08:39:40 +0000 x86_64 GNU/Linux (I get the same behavior in a Ubuntu Docker container). What am I not understanding? Do I need a shell binary in the chroot?

Thank you!

os.chroot(".") is changing the root directory to the current directory. So after that, Python/subprocess is looking for b.sh in the new root folder, not the original path. Try using an absolute path for "b.sh" — Moid
– Moid, Commented Jul 21, 2023 at 21:06
Does /usr/bin/env exist in the chroot? What about bash? If you're running things in a chroot you need to include all the necessary dependencies (which may also include shared libraries). — larsks
– larsks, Commented Jul 21, 2023 at 21:08
@larsks that makes sense-- in the context of my project, a.py is running in a directory with cross-compiled binaries and libraries for bash, binutils, gcc, coreutils a lot of standard stuff. Do you have any idea of how I could figure out what library or binary Python is expecting but missing? Or what component is not working as expected? Right now I'm looking at github.com/python/cpython/blob/main/Modules/_posixsubprocess.c, which I guess is a C module within subprocess.py in python3.10 — food
– food, Commented Jul 21, 2023 at 21:14
What is inside the chroot? Do dynamic libraries needed to run env and bash and echo are inside the chroot? Do I need a shell binary in the chroot? Yes, you definitely need many files in the chroot to run shell including the shell binary. — KamilCuk
– KamilCuk, Commented Jul 21, 2023 at 22:37

micromoses · Accepted Answer · 2023-07-21 22:55:24Z

You are missing the env executable, or it exist in a different path under your chrooted tree (i.e. not under /usr/env/). This can be easily demonstrated even without chroot. Let's remove the chroot, and change b.sh as follows:

#!/usr/bin/no_such_file bash
echo whateva

Unix (in my case MacOS, but also Linux) will fail executing no_such_file like so:

>>> subprocess.run(["./b.sh"])
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/Library/Developer/CommandLineTools/Library/Frameworks/Python3.framework/Versions/3.9/lib/python3.9/subprocess.py", line 505, in run
    with Popen(*popenargs, **kwargs) as process:
  File "/Library/Developer/CommandLineTools/Library/Frameworks/Python3.framework/Versions/3.9/lib/python3.9/subprocess.py", line 951, in __init__
    self._execute_child(args, executable, preexec_fn, close_fds,
  File "/Library/Developer/CommandLineTools/Library/Frameworks/Python3.framework/Versions/3.9/lib/python3.9/subprocess.py", line 1821, in _execute_child
    raise child_exception_type(errno_num, err_msg, err_filename)
FileNotFoundError: [Errno 2] No such file or directory: './b.sh'
>>>

But simply changing it to a valid executable (back to env) will load just fine:

>>> subprocess.run(["./b.sh"])
whateva
CompletedProcess(args=['./b.sh'], returncode=0)
>>>

If you don't need the shell script to be portable, and as you mentioned in a comment that you already have a bash cross-compiled binary, you can simply use that instead of env, such as #!/bin/bash. The path to the binary must be within the chroot environment, so in case it resides elsewhere - make sure to update it accordingly.

Diving a bit deeper, execve(2)'s man page somewhat explains that ENOENT is set if the executable cannot be found. I've also found this page which might provide a better explanation as to why the failure seems to be related to b.sh existence instead of the binary itself.

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