cgroup: systemd: properly expand systemd slice names

Rather than using '/' to denote hierarchy in slice names, systemd uses
'-' in an odd way. This results in runC incorrectly assuming that
certain kernel features are missing (and using inconsistent paths for
the cgroups not supported by systemd), because the "subsystem path" used
is not the one that systemd has created. Fix all of this by properly
expanding slice names.

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

libcontainer/cgroups/systemd/apply_systemd.go

@@ -387,6 +387,28 @@ func joinPids(c *configs.Cgroup, pid int) error {
 	return nil
 }
 
+// systemd represents slice heirarchy using `-`, so we need to follow suit when
+// generating the path of slice. Essentially, test-a-b.slice becomes
+// test.slice/test-a.slice/test-a-b.slice.
+func expandSlice(slice string) (string, error) {
+	suffix := ".slice"
+	sliceName := strings.TrimSuffix(slice, suffix)
+
+	var path, prefix string
+	for _, component := range strings.Split(sliceName, "-") {
+		// test--a.slice isn't permitted, nor is -test.slice.
+		if component == "" {
+			return "", fmt.Errorf("invalid slice name: %s", slice)
+		}
+
+		// Append the component to the path and to the prefix.
+		path += prefix + component + suffix + "/"
+		prefix += component + "-"
+	}
+
+	return path, nil
+}
+
 func getSubsystemPath(c *configs.Cgroup, subsystem string) (string, error) {
 	mountpoint, err := cgroups.FindCgroupMountpoint(subsystem)
 	if err != nil {
@@ -403,6 +425,11 @@ func getSubsystemPath(c *configs.Cgroup, subsystem string) (string, error) {
 		slice = c.Parent
 	}
 
+	slice, err = expandSlice(slice)
+	if err != nil {
+		return "", err
+	}
+
 	return filepath.Join(mountpoint, initPath, slice, getUnitName(c)), nil
 }