support cgroupsv2

To extract the PIDs in the container, we're looking at procs controller
of the cgroup.  Add an on-demand check for the cgroups mode and a pids
extractor for v2.

Fixes: containers/podman/issues/4192
Signed-off-by: Valentin Rothberg <rothberg@redhat.com>

internal/cgroups/cgroups.go

@@ -0,0 +1,44 @@
+// Copyright 2019 psgo authors
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package cgroups
+
+import (
+	"sync"
+	"syscall"
+)
+
+const (
+	CgroupRoot        = "/sys/fs/cgroup"
+	cgroup2SuperMagic = 0x63677270
+)
+
+var (
+	isUnifiedOnce sync.Once
+	isUnified     bool
+	isUnifiedErr  error
+)
+
+// IsCgroup2UnifiedMode returns whether we are running in cgroup or cgroupv2 mode.
+func IsCgroup2UnifiedMode() (bool, error) {
+	isUnifiedOnce.Do(func() {
+		var st syscall.Statfs_t
+		if err := syscall.Statfs(CgroupRoot, &st); err != nil {
+			isUnified, isUnifiedErr = false, err
+		} else {
+			isUnified, isUnifiedErr = st.Type == cgroup2SuperMagic, nil
+		}
+	})
+	return isUnified, isUnifiedErr
+}

internal/proc/pids.go

@@ -1,4 +1,4 @@
-// Copyright 2018 psgo authors
+// Copyright 2018-2019 psgo authors
 //
 // Licensed under the Apache License, Version 2.0 (the "License");
 // you may not use this file except in compliance with the License.
@@ -18,8 +18,11 @@ import (
 	"bufio"
 	"fmt"
 	"os"
+	"path/filepath"
 	"strconv"
 	"strings"
+
+	"github.com/containers/psgo/internal/cgroups"
 )
 
 // GetPIDs extracts and returns all PIDs from /proc.
@@ -49,45 +52,98 @@ func GetPIDs() ([]string, error) {
 	return pids, nil
 }
 
-// pidCgroupPath returns the path to the pid's pids cgroup.
-func pidCgroupPath(pid string) (string, error) {
+// GetPIDsFromCgroup returns a strings slice of all pids listesd in pid's pids
+// cgroup.  It automatically detects if we're running in unified mode or not.
+func GetPIDsFromCgroup(pid string) ([]string, error) {
+	unified, err := cgroups.IsCgroup2UnifiedMode()
+	if err != nil {
+		return nil, err
+	}
+	if unified {
+		return getPIDsFromCgroupV2(pid)
+	}
+	return getPIDsFromCgroupV1(pid)
+}
+
+// getPIDsFromCgroupV1 returns a strings slice of all pids listesd in pid's pids
+// cgroup.
+func getPIDsFromCgroupV1(pid string) ([]string, error) {
+	// First, find the corresponding path to the PID cgroup.
 	f, err := os.Open(fmt.Sprintf("/proc/%s/cgroup", pid))
 	if err != nil {
-		return "", err
+		return nil, err
 	}
 	defer f.Close()
 
 	scanner := bufio.NewScanner(f)
+	cgroupPath := ""
 	for scanner.Scan() {
 		fields := strings.Split(scanner.Text(), ":")
 		if len(fields) != 3 {
 			continue
 		}
 		if fields[1] == "pids" {
-			return fmt.Sprintf("/sys/fs/cgroup/pids/%s/cgroup.procs", fields[2]), nil
+			cgroupPath = fmt.Sprintf("/sys/fs/cgroup/pids/%s/cgroup.procs", fields[2])
 		}
 	}
-	return "", fmt.Errorf("couldn't find pids group for PID %s", pid)
+
+	if cgroupPath == "" {
+		return nil, fmt.Errorf("couldn't find v1 pids group for PID %s", pid)
+	}
+
+	// Second, extract the PIDs inside the cgroup.
+	f, err = os.Open(cgroupPath)
+	if err != nil {
+		return nil, err
+	}
+	defer f.Close()
+
+	pids := []string{}
+	scanner = bufio.NewScanner(f)
+	for scanner.Scan() {
+		pids = append(pids, scanner.Text())
+	}
+
+	return pids, nil
 }
 
-// GetPIDsFromCgroup returns a strings slice of all pids listesd in pid's pids
+// getPIDsFromCgroupV2 returns a strings slice of all pids listesd in pid's pids
 // cgroup.
-func GetPIDsFromCgroup(pid string) ([]string, error) {
-	cgroupPath, err := pidCgroupPath(pid)
+func getPIDsFromCgroupV2(pid string) ([]string, error) {
+	// First, find the corresponding path to the PID cgroup.
+	f, err := os.Open(fmt.Sprintf("/proc/%s/cgroup", pid))
 	if err != nil {
 		return nil, err
 	}
+	defer f.Close()
+
+	scanner := bufio.NewScanner(f)
+	cgroupSlice := ""
+	for scanner.Scan() {
+		fields := strings.Split(scanner.Text(), ":")
+		if len(fields) != 3 {
+			continue
+		}
+		cgroupSlice = fields[2]
+		break
+	}
+
+	if cgroupSlice == "" {
+		return nil, fmt.Errorf("couldn't find v2 pids group for PID %s", pid)
+	}
 
-	f, err := os.Open(cgroupPath)
+	// Second, extract the PIDs inside the cgroup.
+	f, err = os.Open(filepath.Join(cgroups.CgroupRoot, cgroupSlice, "cgroup.procs"))
 	if err != nil {
 		return nil, err
 	}
 	defer f.Close()
 
 	pids := []string{}
-	scanner := bufio.NewScanner(f)
+	scanner = bufio.NewScanner(f)
 	for scanner.Scan() {
 		pids = append(pids, scanner.Text())
 	}
+
 	return pids, nil
 }