Why does a shebang script run as init= have an euid of 0 when run from an initrd, but not otherwise?

Question

I am trying to create my own PID 1 init script, to be called from the boot cmdline with init=/myscript. How can I make it work on a real filesystem, with any kernel?

When it runs in an initrd, it works fine and can mount things, etc. - but when I use it on my filesystem without an initrd, it fails to mount things, because:

mount: only root can do that (effective UID is 1000)

When I strace any command that fails, it inevitably issues geteuid32() and that returns 1000. Why? How can I run as euid 0?

Answer 1

There's no special treatment for init on initrd, so there must be some other issue.

If run as root, the euid will match the owner of the binary if the setuid bit is set.

Check the ownership on /bin/mount.

Answer 2

Use Sudo from Unprivileged Accounts

The error is telling you what the problem is: you're using a "mortal" account with UID 1000, not the root account with UID 0, to call mount. Many system functions need to run as root. Many processes like initrd run as root, so the commands have sufficient privilege to run. However, when you're running things from your everyday user account, you need su or sudo to perform actions as a different user.

For mount, you'd want to call it like this:

sudo mount /mnt/foo

Unless specified by the -u flag, the user for sudo commands defaults to root. Running your command through sudo should resolve the problem you've described, provided the account with UID 1000 is set up properly in the appropriate /etc/sudoers or /etc/sudoers.d file.

See Also

• man 5 sudo
• man 5 sudoers
• man 8 visudo