Since I was looking at this already and had a few things to investigate and fix in our systemd-using hosts, I checked how plausible it is to insert a molly-guard-like password prompt as part of the reboot/shutdown process on CentOS 7 (i.e. using systemd).

Problems encountered include:

• Asking for a password from a service/unit in systemd — Use systemd-ask-password and needs some agent setup to reply to this correctly?
• The reboot command always walls a message to all logged in users before it even runs the new reboot-molly unit, as it expects a reboot to happen. The argument --no-wall stops this but that requires a change to the reboot command. Hence back to the original problem of replacing packaged files/symlinks with RPM
• The reboot.target unit is a “systemd.special” unit, which means that it has some special behaviour and cannot be renamed. We can modify it, of course, by editing the reboot.target file.
• How do we get a systemd unit to run first and block anything later from running until it is complete? (In fact to abort the reboot but just for this time rather than being set as permanently failed. Reboot failing is a bit of a strange situation for it to be in…) The dependencies appear to work but the reboot target is quite keen on running other items from the dependency list — I’m more than likely doing something wrong here!

So for now this is shelved. It would be nice to have a solution though, so any hints from systemd experts are greatfully received!

(Note that CentOS 7 uses systemd 208, so new features in later versions which help won’t be available to us)