libct/cg/sd: fix SkipDevices for systemd

Commit 108ee85 adds SkipDevices flag, which is used by kubernetes
to create cgroups for pods.

Unfortunately the above commit falls short, and systemd DevicePolicy and
DeviceAllow properties are still set, which requires kubernetes to set
"allow everything" rule.

This commit fixes this: if SkipDevices flag is set, no Device*
properties are set, meaning all devices are allowed.

NOTE that SkipDevices can only be set for non-container cgroup
(IOW it is not possible to start a container when this option is set).

Fixes: 108ee85
Signed-off-by: Kir Kolyshkin <kolyshkin@gmail.com>

libcontainer/cgroups/systemd/common.go

@@ -160,7 +160,15 @@ func findDeviceGroup(ruleType devices.Type, ruleMajor int64) (string, error) {
 
 // generateDeviceProperties takes the configured device rules and generates a
 // corresponding set of systemd properties to configure the devices correctly.
-func generateDeviceProperties(rules []*devices.Rule) ([]systemdDbus.Property, error) {
+func generateDeviceProperties(r *configs.Resources) ([]systemdDbus.Property, error) {
+	if r.SkipDevices {
+		// Not adding any Device* properties means the default of
+		// DevicePolicy=auto will be used by systemd, which allows
+		// access to all devices (if no explicit DeviceAllow= is
+		// present).
+		return []systemdDbus.Property{}, nil
+	}
+
 	// DeviceAllow is the type "a(ss)" which means we need a temporary struct
 	// to represent it in Go.
 	type deviceAllowEntry struct {
@@ -177,7 +185,7 @@ func generateDeviceProperties(rules []*devices.Rule) ([]systemdDbus.Property, er
 
 	// Figure out the set of rules.
 	configEmu := &cgroupdevices.Emulator{}
-	for _, rule := range rules {
+	for _, rule := range r.Devices {
 		if err := configEmu.Apply(*rule); err != nil {
 			return nil, errors.Wrap(err, "apply rule for systemd")
 		}

libcontainer/cgroups/systemd/v1.go

@@ -61,7 +61,7 @@ var legacySubsystems = []subsystem{
 func genV1ResourcesProperties(r *configs.Resources, cm *dbusConnManager) ([]systemdDbus.Property, error) {
 	var properties []systemdDbus.Property
 
-	deviceProperties, err := generateDeviceProperties(r.Devices)
+	deviceProperties, err := generateDeviceProperties(r)
 	if err != nil {
 		return nil, err
 	}

libcontainer/cgroups/systemd/v2.go

@@ -172,7 +172,7 @@ func genV2ResourcesProperties(r *configs.Resources, cm *dbusConnManager) ([]syst
 	//       aren't the end of the world, but it is a bit concerning. However
 	//       it's unclear if systemd removes all eBPF programs attached when
 	//       doing SetUnitProperties...
-	deviceProperties, err := generateDeviceProperties(r.Devices)
+	deviceProperties, err := generateDeviceProperties(r)
 	if err != nil {
 		return nil, err
 	}