Sudo no longer available and users can't install or modify most files

Question

I recently started messing with some files on a virtual hosted machine and I was logged in a normal/non-root user. At first everything was fine, I was able to install anything I needed using:

sudo apt-get install tree

But I somehow messed up a file where now when I do that I get the following error:

sudo: can't open /etc/sudoers: Permission denied

I'm not sure how to diagnose this, I've googled for solutions but everything's about what sudo is and nothing related to what to do about re-enabling users to use it.

Let me know if you'd like any other details in comments :)

Edit:

ls -l /etc/sudoers
-r--r----- 1 root root 577 Apr 20 03:42 /etc/sudoers

Answer 1

There's a lot of things that could be failing, but given your output of ls -l /etc/sudoers, most of them are ruled out.

First a couple of quick checks:

1. what does type sudo print?
2. what does getent group 0 print?

Then, to properly see what's going on, can you log in as root using su -, then run this?

# apt-get install ltrace
# ltrace -u <username> sudo true 2>&1  | egrep '(sudo|stat|set.*[ug].*id)'

and/or

# apt-get install strace
# strace -u <yourusername> sudo true 2>&1 | egrep '(sudo|set.*[ug].*id)'

And add the output to your question.

Answer 2

As a last resort, you can copy the /etc/sudoers file to a backup location, then apt-get --reinstall install sudo