If a service requests both ProtectSubset=pid and ProtectHostname=true then it will currently fail to start. The ProcSubset=pid option instructs systemd to mount procfs for the service with subset=pid which hides all entries other than /proc/<pid>. Consequently trying to interact with the two files /proc/sys/kernel/{hostname,domainname} covered by ProtectHostname=true will fail. Fix this by only performing this check when ProtectSubset=pid is not requested. Essentially ProtectSubset=pid implies/provides ProtectHostname=true.
System and Service Manager
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