devices: add emulator.Rules shorthand

The devices cgroup emulator is also useful for removing unneeded rules
as well as computing what the final default-allow state of the filter
will be (allow-list or deny-list).

Signed-off-by: Aleksa Sarai <cyphar@cyphar.com>

libcontainer/cgroups/devices/devices_emulator.go

@@ -371,3 +371,12 @@ func (source *Emulator) Transition(target *Emulator) ([]*devices.Rule, error) {
 	}
 	return transitionRules, nil
 }
+
+// Rules returns the minimum set of rules necessary to convert a *deny-all*
+// cgroup to the emulated filter state (note that this is not the same as a
+// default cgroupv1 cgroup -- which is allow-all). This is effectively just a
+// wrapper around Transition() with the source emulator being an empty cgroup.
+func (e *Emulator) Rules() ([]*devices.Rule, error) {
+	defaultCgroup := &Emulator{defaultAllow: false}
+	return defaultCgroup.Transition(e)
+}

libcontainer/cgroups/systemd/common.go

@@ -203,8 +203,7 @@ func generateDeviceProperties(rules []*devices.Rule) ([]systemdDbus.Property, er
 	// Now generate the set of rules we actually need to apply. Unlike the
 	// normal devices cgroup, in "strict" mode systemd defaults to a deny-all
 	// whitelist which is the default for devices.Emulator.
-	baseEmu := &cgroupdevices.Emulator{}
-	finalRules, err := baseEmu.Transition(configEmu)
+	finalRules, err := configEmu.Rules()
 	if err != nil {
 		return nil, errors.Wrap(err, "get simplified rules for systemd")
 	}