Check if path accessible by non-root user

Question

I have an installation script written in Python (in Linux) that runs as root and needs to check whether certain files are readable by a non-root user.

For this reason I can't use os.path.exists() or open(filename) (and catch any exceptions).

Currently I'm thinking of checking the permission bit on each of the files, but the only problem is that I will have to check the permission bits on the path leading up to the filename as well (directories need r+x bits set), which could be very slow process if I have thousands of files.

Is my solution the best one, or are there better alternatives?

edit: I will need the script run as root after the files are checked, so dropping root permissions is not an option unfortunately.

Answer 1

You could use os.seteuid to change the effective user to some non-root user. Then try opening the file. An IOError will be raised if permission is denied.

import os
os.seteuid(65534)  # user 65534 is `nobody`
filename='/etc/passwd-'
try:
    open(filename,'r')
except IOError as err:
    print(err)

# [Errno 13] Permission denied: '/etc/passwd-'