Hi Bardur,
Maybe I've missed something reading through this thread, but *assuming* (yeah, I know) that packages can't run arbitrary scripts at install time (which I think is a valid assumption for pacman),
Is this so? I don't know since I've only scratched the surface of arch until now. But I'm not quite sure about this, since, for example, there must be a way to add new users like http after installing apache. How should this be done without a post-install-script?
Of course an attacker can still (via the build executables) delete all the files you actually care about ($HOME) or install trojans into your $HOME/bin (etc.), but still... If you discover such a comprosmise you'd "only" have to delete your $HOME and restore from backup[0], whereas a root compromise would require a full reinstall of everything.
Even if your assumption about pacman is correct: Just let the malicious PKGBUILD write a file into /etc/cron.d/, /etc/systemd or something like that and you're doomed. No need for privilege escalation. Regards, Roland