libcontainer/intelrdt: support explicit assignment to root CLOS

Makes it possible e.g. to enable monitoring
(linux.intelRdt.enableMonitoring) without creating a CLOS (resctrl
group) for the container.

Implements opencontainers/runtime-spec#1289.

Signed-off-by: Markus Lehtonen <[email protected]>

Author: marquiz

libcontainer/configs/validate/validator.go

@@ -287,8 +287,9 @@ func intelrdtCheck(config *configs.Config) error {
 			return fmt.Errorf("intelRdt is specified in config, but Intel RDT is not enabled")
 		}
 
-		if config.IntelRdt.ClosID == "." || config.IntelRdt.ClosID == ".." || strings.Contains(config.IntelRdt.ClosID, "/") {
-			return fmt.Errorf("invalid intelRdt.ClosID %q", config.IntelRdt.ClosID)
+		switch clos := config.IntelRdt.ClosID; {
+		case clos == ".", clos == "..", len(clos) > 1 && strings.Contains(clos, "/"):
+			return fmt.Errorf("invalid intelRdt.ClosID %q", clos)
 		}
 
 		if !intelrdt.IsCATEnabled() && config.IntelRdt.L3CacheSchema != "" {