cgroup2: do not parse /proc/cgroups

/proc/cgroups is meaningless for v2 and should be ignored.

https://github.com/torvalds/linux/blob/v5.3/Documentation/admin-guide/cgroup-v2.rst#deprecated-v1-core-features

* Now GetAllSubsystems() parses /sys/fs/cgroup/cgroup.controller, not /proc/cgroups.
  The function result also contains "pseudo" controllers: {"devices", "freezer"}.
  As it is hard to detect availability of pseudo controllers, pseudo controllers
  are always assumed to be available.

* Now IOGroupV2.Name() returns "io", not "blkio"

Fix #2155 #2156

Signed-off-by: Akihiro Suda <akihiro.suda.cz@hco.ntt.co.jp>

libcontainer/cgroups/fs/io_v2.go

@@ -17,12 +17,11 @@ type IOGroupV2 struct {
 }
 
 func (s *IOGroupV2) Name() string {
-	// for compatibility with v1 blkio controller
-	return "blkio"
+	return "io"
 }
 
 func (s *IOGroupV2) Apply(d *cgroupData) error {
-	_, err := d.join("blkio")
+	_, err := d.join("io")
 	if err != nil && !cgroups.IsNotFound(err) {
 		return err
 	}
@@ -62,7 +61,7 @@ func (s *IOGroupV2) Set(path string, cgroup *configs.Cgroup) error {
 }
 
 func (s *IOGroupV2) Remove(d *cgroupData) error {
-	return removePath(d.path("blkio"))
+	return removePath(d.path("io"))
 }
 
 func readCgroup2MapFile(path string, name string) (map[string][]string, error) {

libcontainer/cgroups/utils.go

@@ -266,6 +266,21 @@ func GetCgroupMounts(all bool) ([]Mount, error) {
 
 // GetAllSubsystems returns all the cgroup subsystems supported by the kernel
 func GetAllSubsystems() ([]string, error) {
+	// /proc/cgroups is meaningless for v2
+	// https://github.com/torvalds/linux/blob/v5.3/Documentation/admin-guide/cgroup-v2.rst#deprecated-v1-core-features
+	if IsCgroup2UnifiedMode() {
+		// "pseudo" controllers do not appear in /sys/fs/cgroup/cgroup.controllers.
+		// - devices: implemented in kernel 4.15
+		// - freezer: implemented in kernel 5.2
+		// We assume these are always available, as it is hard to detect availability.
+		pseudo := []string{"devices", "freezer"}
+		data, err := ioutil.ReadFile("/sys/fs/cgroup/cgroup.controllers")
+		if err != nil {
+			return nil, err
+		}
+		subsystems := append(pseudo, strings.Fields(string(data))...)
+		return subsystems, nil
+	}
 	f, err := os.Open("/proc/cgroups")
 	if err != nil {
 		return nil, err