unit: drop ProtectClock=yes from systemd-udevd.service

This partially reverts cabc1c6.

The setting ProtectClock= implies DeviceAllow=, which is not suitable
for udevd. Although we are slowly removing cgropsv1 support, but
DeviceAllow= with cgroupsv1 is necessarily racy, and reloading PID1
during the early boot process may cause issues like #24668.

Let's disable ProtectClock= for udevd. And, if necessary, let's
explicitly drop CAP_SYS_TIME and CAP_WAKE_ALARM (and possibly others)
by using CapabilityBoundingSet= later.

Fixes #24668.

Author: yuwata

units/systemd-udevd.service.in

@@ -17,8 +17,6 @@ ConditionPathIsReadWrite=/sys
 
 [Service]
 Delegate=pids
-DeviceAllow=block-* rwm
-DeviceAllow=char-* rwm
 Type=notify
 # Note that udev will reset the value internally for its workers
 OOMScoreAdjust=-1000
@@ -30,7 +28,6 @@ ExecReload=udevadm control --reload --timeout 0
 KillMode=mixed
 TasksMax=infinity
 PrivateMounts=yes
-ProtectClock=yes
 ProtectHostname=yes
 MemoryDenyWriteExecute=yes
 RestrictAddressFamilies=AF_UNIX AF_NETLINK AF_INET AF_INET6