libct/cg/sd: ignore UnitExists only for Apply(-1)

Commit d223e2a ("Ignore error when starting transient unit
that already exists" modified the code handling errors from startUnit
to ignore UnitExists error.

Apparently it was done so that kubelet can create the same pod slice
over and over without hitting an error (see [1]).

While it works for a pod slice to ensure it exists, it is a gross bug
to ignore UnitExists when creating a container. In this case, the
container init PID won't be added to the systemd unit (and to the
required cgroup), and as a result the container will successfully
run in a current user cgroup, without any cgroup limits applied.

So, fix the code to only ignore UnitExists if we're not adding a process
to the systemd unit. This way, kubelet will keep working as is, but
runc will refuse to create containers which are not placed into a
requested cgroup.

[1] opencontainers#1124

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

libcontainer/cgroups/systemd/common.go

@@ -124,7 +124,7 @@ func isUnitExists(err error) bool {
 	return isDbusError(err, "org.freedesktop.systemd1.UnitExists")
 }
 
-func startUnit(cm *dbusConnManager, unitName string, properties []systemdDbus.Property) error {
+func startUnit(cm *dbusConnManager, unitName string, properties []systemdDbus.Property, ignoreExist bool) error {
 	statusChan := make(chan string, 1)
 	err := cm.retryOnDisconnect(func(c *systemdDbus.Conn) error {
 		_, err := c.StartTransientUnitContext(context.TODO(), unitName, "replace", properties, statusChan)
@@ -134,7 +134,13 @@ func startUnit(cm *dbusConnManager, unitName string, properties []systemdDbus.Pr
 		if !isUnitExists(err) {
 			return err
 		}
-		return nil
+		if ignoreExist {
+			// TODO: remove this hack.
+			// This is kubelet making sure a slice exists (see
+			// https://github.com/opencontainers/runc/pull/1124).
+			return nil
+		}
+		return err
 	}
 
 	timeout := time.NewTimer(30 * time.Second)

libcontainer/cgroups/systemd/v1.go

@@ -204,7 +204,7 @@ func (m *LegacyManager) Apply(pid int) error {
 
 	properties = append(properties, c.SystemdProps...)
 
-	if err := startUnit(m.dbus, unitName, properties); err != nil {
+	if err := startUnit(m.dbus, unitName, properties, pid == -1); err != nil {
 		return err
 	}
 

libcontainer/cgroups/systemd/v2.go

@@ -291,7 +291,7 @@ func (m *UnifiedManager) Apply(pid int) error {
 
 	properties = append(properties, c.SystemdProps...)
 
-	if err := startUnit(m.dbus, unitName, properties); err != nil {
+	if err := startUnit(m.dbus, unitName, properties, pid == -1); err != nil {
 		return fmt.Errorf("unable to start unit %q (properties %+v): %w", unitName, properties, err)
 	}