systemd $HOME environment variable different from expected

Question

I have a service that runs as root:

[Unit]
Description=my service
After=docker.service network-online.target
Requires=docker.service network-online.target

[Service]
TimeoutStartSec=0
RestartSec=60
Restart=always
User=root
Group=root

ExecStart=/usr/bin/myservice

[Install]
WantedBy=multi-user.target

I am getting some unexpected behavior with this:

The environment variable $HOME is set to /root when I look at the environment variable that myservice is getting which is causing it to fail.

However, as the root user I am getting:

$ echo $HOME
/home/root

This is in line with what I see in /etc/passwd

root:x:0:0:root:/home/root:/bin/sh

When I run myservice manually, it gets the correct $HOME env variable and it works.

I would like $HOME to be /home/root, not /root. So...

1. Where is systemd getting /root from, and why is $HOME getting overwritten?
2. How do I prevent systemd from doing this globally, aside from explicitly setting the $HOME environment variable in the service file? <-- upon reviewing the source code this doesn't seem like it'll make a difference

Answer 1

After examining the source code, I found out systemd actually hardcodes the home directory for root users. I have not found any documentation on configs that can change this so it's just something to be aware of for Yocto projects using the default home directory.

I confirmed this by modifying the hard coded /root values to /home/root in https://github.com/systemd/systemd/blob/main/src/basic/user-util.c and observed the modified systemd was spawning services with the correct $HOME variable.

As of now, symlinking /root with /home/root is a good workaround. Switching the default ROOT_HOME to /root may not be a bad idea to be consistent with the rest of the linux world.