libct/cg/sd/v2: fix ENOENT on cgroup delegation

Apparently, not all files listed in /sys/kernel/cgroup/delegate must
exist in every cgroup, so we should ignore ENOENT.

Dot not ignore ENOENT on the directory itself though.

Change cgroupFilesToChown to not return ".", and refactor it to not do
any dynamic slice appending in case we're using the default built-in
list of files.

Fixes: 35d20c4
Signed-off-by: Kir Kolyshkin <kolyshkin@gmail.com>
(cherry picked from commit 8c04b98)
Signed-off-by: Kir Kolyshkin <kolyshkin@gmail.com>

libcontainer/cgroups/systemd/v2.go

@@ -2,6 +2,7 @@ package systemd
 
 import (
 	"bufio"
+	"errors"
 	"fmt"
 	"math"
 	"os"
@@ -292,14 +293,21 @@ func (m *unifiedManager) Apply(pid int) error {
 	}
 
 	if c.OwnerUID != nil {
+		// The directory itself must be chowned.
+		err := os.Chown(m.path, *c.OwnerUID, -1)
+		if err != nil {
+			return err
+		}
+
 		filesToChown, err := cgroupFilesToChown()
 		if err != nil {
 			return err
 		}
 
 		for _, v := range filesToChown {
 			err := os.Chown(m.path+"/"+v, *c.OwnerUID, -1)
-			if err != nil {
+			// Some files might not be present.
+			if err != nil && !errors.Is(err, os.ErrNotExist) {
 				return err
 			}
 		}
@@ -312,21 +320,23 @@ func (m *unifiedManager) Apply(pid int) error {
 // uid in /sys/kernel/cgroup/delegate.  If the file is not present
 // (Linux < 4.15), use the initial values mentioned in cgroups(7).
 func cgroupFilesToChown() ([]string, error) {
-	filesToChown := []string{"."} // the directory itself must be chowned
 	const cgroupDelegateFile = "/sys/kernel/cgroup/delegate"
+
 	f, err := os.Open(cgroupDelegateFile)
-	if err == nil {
-		defer f.Close()
-		scanner := bufio.NewScanner(f)
-		for scanner.Scan() {
-			filesToChown = append(filesToChown, scanner.Text())
-		}
-		if err := scanner.Err(); err != nil {
-			return nil, fmt.Errorf("error reading %s: %w", cgroupDelegateFile, err)
-		}
-	} else {
-		filesToChown = append(filesToChown, "cgroup.procs", "cgroup.subtree_control", "cgroup.threads")
+	if err != nil {
+		return []string{"cgroup.procs", "cgroup.subtree_control", "cgroup.threads"}, nil
 	}
+	defer f.Close()
+
+	filesToChown := []string{}
+	scanner := bufio.NewScanner(f)
+	for scanner.Scan() {
+		filesToChown = append(filesToChown, scanner.Text())
+	}
+	if err := scanner.Err(); err != nil {
+		return nil, fmt.Errorf("error reading %s: %w", cgroupDelegateFile, err)
+	}
+
 	return filesToChown, nil
 }