cgroup2: implement `runc ps`

Implemented `runc ps` for cgroup v2 , using a newly added method `m.GetUnifiedPath()`.
Unlike the v1  implementation that checks `m.GetPaths()["devices"]`, the v2 implementation does not require the device controller to be available.

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

libcontainer/cgroups/cgroups.go

@@ -37,6 +37,13 @@ type Manager interface {
 	// restore the object later.
 	GetPaths() map[string]string
 
+	// GetUnifiedPath returns the unified path when running in unified mode.
+	// The value corresponds to the all values of GetPaths() map.
+	//
+	// GetUnifiedPath returns error when running in hybrid mode as well as
+	// in legacy mode.
+	GetUnifiedPath() (string, error)
+
 	// Sets the cgroup as configured.
 	Set(container *configs.Config) error
 }

libcontainer/cgroups/fs/apply_raw.go

@@ -224,6 +224,28 @@ func (m *Manager) GetPaths() map[string]string {
 	return paths
 }
 
+func (m *Manager) GetUnifiedPath() (string, error) {
+	if !cgroups.IsCgroup2UnifiedMode() {
+		return "", errors.New("unified path is only supported when running in unified mode")
+	}
+	unifiedPath := ""
+	m.mu.Lock()
+	defer m.mu.Unlock()
+	for k, v := range m.Paths {
+		if unifiedPath == "" {
+			unifiedPath = v
+		} else if v != unifiedPath {
+			return unifiedPath,
+				errors.Errorf("expected %q path to be unified path %q, got %q", k, unifiedPath, v)
+		}
+	}
+	if unifiedPath == "" {
+		// FIXME: unified path could be detected even when no controller is available
+		return unifiedPath, errors.New("cannot detect unified path")
+	}
+	return unifiedPath, nil
+}
+
 func (m *Manager) GetStats() (*cgroups.Stats, error) {
 	m.mu.Lock()
 	defer m.mu.Unlock()
@@ -302,11 +324,25 @@ func (m *Manager) Freeze(state configs.FreezerState) error {
 }
 
 func (m *Manager) GetPids() ([]int, error) {
+	if cgroups.IsCgroup2UnifiedMode() {
+		path, err := m.GetUnifiedPath()
+		if err != nil {
+			return nil, err
+		}
+		return cgroups.GetPids(path)
+	}
 	paths := m.GetPaths()
 	return cgroups.GetPids(paths["devices"])
 }
 
 func (m *Manager) GetAllPids() ([]int, error) {
+	if cgroups.IsCgroup2UnifiedMode() {
+		path, err := m.GetUnifiedPath()
+		if err != nil {
+			return nil, err
+		}
+		return cgroups.GetAllPids(path)
+	}
 	paths := m.GetPaths()
 	return cgroups.GetAllPids(paths["devices"])
 }

libcontainer/cgroups/systemd/apply_nosystemd.go

@@ -42,6 +42,10 @@ func (m *Manager) GetPaths() map[string]string {
 	return nil
 }
 
+func (m *Manager) GetUnifiedPath() (string, error) {
+	return "", fmt.Errorf("Systemd not supported")
+}
+
 func (m *Manager) GetStats() (*cgroups.Stats, error) {
 	return nil, fmt.Errorf("Systemd not supported")
 }

libcontainer/cgroups/systemd/apply_systemd.go

@@ -297,6 +297,10 @@ func (m *LegacyManager) GetPaths() map[string]string {
 	return paths
 }
 
+func (m *LegacyManager) GetUnifiedPath() (string, error) {
+	return "", errors.New("unified path is only supported when running in unified mode")
+}
+
 func join(c *configs.Cgroup, subsystem string, pid int) (string, error) {
 	path, err := getSubsystemPath(c, subsystem)
 	if err != nil {

libcontainer/cgroups/systemd/unified_hierarchy.go

@@ -16,6 +16,7 @@ import (
 	"github.com/opencontainers/runc/libcontainer/cgroups"
 	"github.com/opencontainers/runc/libcontainer/cgroups/fs"
 	"github.com/opencontainers/runc/libcontainer/configs"
+	"github.com/pkg/errors"
 	"github.com/sirupsen/logrus"
 )
 
@@ -195,7 +196,24 @@ func (m *UnifiedManager) GetPaths() map[string]string {
 	m.mu.Unlock()
 	return paths
 }
-
+func (m *UnifiedManager) GetUnifiedPath() (string, error) {
+	unifiedPath := ""
+	m.mu.Lock()
+	defer m.mu.Unlock()
+	for k, v := range m.Paths {
+		if unifiedPath == "" {
+			unifiedPath = v
+		} else if v != unifiedPath {
+			return unifiedPath,
+				errors.Errorf("expected %q path to be unified path %q, got %q", k, unifiedPath, v)
+		}
+	}
+	if unifiedPath == "" {
+		// FIXME: unified path could be detected even when no controller is available
+		return unifiedPath, errors.New("cannot detect unified path")
+	}
+	return unifiedPath, nil
+}
 func createCgroupsv2Path(path string) (Err error) {
 	content, err := ioutil.ReadFile("/sys/fs/cgroup/cgroup.controllers")
 	if err != nil {
@@ -270,7 +288,7 @@ func (m *UnifiedManager) Freeze(state configs.FreezerState) error {
 }
 
 func (m *UnifiedManager) GetPids() ([]int, error) {
-	path, err := getSubsystemPath(m.Cgroups, "devices")
+	path, err := m.GetUnifiedPath()
 	if err != nil {
 		return nil, err
 	}
@@ -278,7 +296,7 @@ func (m *UnifiedManager) GetPids() ([]int, error) {
 }
 
 func (m *UnifiedManager) GetAllPids() ([]int, error) {
-	path, err := getSubsystemPath(m.Cgroups, "devices")
+	path, err := m.GetUnifiedPath()
 	if err != nil {
 		return nil, err
 	}

libcontainer/container_linux_test.go

@@ -54,6 +54,10 @@ func (m *mockCgroupManager) GetPaths() map[string]string {
 	return m.paths
 }
 
+func (m *mockCgroupManager) GetUnifiedPath() (string, error) {
+	return "", fmt.Errorf("unimplemented")
+}
+
 func (m *mockCgroupManager) Freeze(state configs.FreezerState) error {
 	return nil
 }