runc kill: add support for cgroup.kill

cgroup.kill inteface was added to Linux kernel 5.14 (see [1], [2]).
Use it if we can.

[1] https://lwn.net/Articles/855049/
[2] https://lwn.net/Articles/855924/

Signed-off-by: Kir Kolyshkin <[email protected]>

libcontainer/signal_all_linux.go

@@ -2,6 +2,7 @@ package libcontainer
 
 import (
 	"errors"
+	"fmt"
 	"os"
 	"unsafe"
 
@@ -34,36 +35,57 @@ func isWaitable(pid int) (bool, error) {
 	return si.pid != 0, nil
 }
 
-// signalAllProcesses freezes then iterates over all the processes inside the
-// manager's cgroups sending the signal s to them.
-// If s is SIGKILL and subreaper is not enabled then it will wait for each
-// process to exit.
-// For all other signals it will check if the process is ready to report its
-// exit status and only if it is will a wait be performed.
+// signalAllProcesses sends signal to all the process inside the manager's cgroup.
+// In case the signal is SIGKILL, and cgroup.kill is available, it is used. Otherwise,
+// the cgroup is frozen, then the signal is sent to all the processes one by one.
+//
+// If s is SIGKILL and subreaper is not enabled, this function waits for each
+// process to exit. For all other signals it will check if the process is ready
+// to report its exit status and only if it is will a wait be performed.
 func signalAllProcesses(m cgroups.Manager, s os.Signal) error {
 	sig, ok := s.(unix.Signal)
 	if !ok {
 		return errors.New("unsupported signal type")
 	}
 
-	if err := m.Freeze(configs.Frozen); err != nil {
-		logrus.Warn(err)
+	haveCgroupKill := false
+
+	// Use cgroup.kill, if available.
+	if s == unix.SIGKILL {
+		if p := m.Path(""); p != "" { // Either cgroup v2 or hybrid.
+			if err := cgroupKillAll(p); err == nil {
+				haveCgroupKill = true
+			} else if !errors.Is(err, unix.ENOENT) {
+				logrus.Warnf("cgroupKillAll: %v", err)
+			}
+		}
+	}
+
+	if !haveCgroupKill {
+		if err := m.Freeze(configs.Frozen); err != nil {
+			logrus.Warn(err)
+		}
 	}
+
 	pids, err := m.GetAllPids()
 	if err != nil {
-		if err := m.Freeze(configs.Thawed); err != nil {
-			logrus.Warn(err)
+		if !haveCgroupKill {
+			if err := m.Freeze(configs.Thawed); err != nil {
+				logrus.Warn(err)
+			}
 		}
 		return err
 	}
-	for _, pid := range pids {
-		if err := unix.Kill(pid, sig); err != nil && err != unix.ESRCH {
+	if !haveCgroupKill {
+		for _, pid := range pids {
+			if err := unix.Kill(pid, sig); err != nil && err != unix.ESRCH {
+				logrus.Warn(err)
+			}
+		}
+		if err := m.Freeze(configs.Thawed); err != nil {
 			logrus.Warn(err)
 		}
 	}
-	if err := m.Freeze(configs.Thawed); err != nil {
-		logrus.Warn(err)
-	}
 
 	subreaper, err := system.GetSubreaper()
 	if err != nil {
@@ -103,3 +125,58 @@ func signalAllProcesses(m cgroups.Manager, s os.Signal) error {
 	}
 	return nil
 }
+
+func prepareCgWait(dir string) (int, error) {
+	fd, err := unix.InotifyInit()
+	if err != nil {
+		return -1, fmt.Errorf("unable to init inotify: %w", err)
+	}
+	_, err = unix.InotifyAddWatch(fd, dir+"/cgroup.events", unix.IN_MODIFY)
+	if err != nil {
+		unix.Close(fd)
+		return -1, fmt.Errorf("unable to add inotify watch: %w", err)
+	}
+	return fd, nil
+}
+
+func cgWait(fd int) error {
+	fds := []unix.PollFd{{
+		Fd:     int32(fd),
+		Events: unix.POLLIN,
+	}}
+	for {
+		res, err := unix.Poll(fds, 10000)
+		if err == unix.EINTR {
+			continue
+		}
+		if err != nil {
+			return &os.SyscallError{Syscall: "poll", Err: err}
+		}
+		if res == 0 { // Timeout.
+			return &os.SyscallError{Syscall: "poll", Err: unix.ETIMEDOUT}
+		}
+		if res > 0 && fds[0].Revents&unix.POLLIN != 0 {
+			return nil
+		}
+	}
+}
+
+func cgroupKillAll(path string) error {
+	const file = "cgroup.kill"
+	if err := unix.Access(path+"/"+file, unix.F_OK); err != nil {
+		return &os.PathError{Op: "access", Path: path + "/" + file, Err: err}
+	}
+
+	fd, err := prepareCgWait(path)
+	if err != nil {
+		return err
+	}
+
+	err = cgroups.WriteFile(path, file, "1")
+	if err == nil {
+		err = cgWait(fd)
+	}
+	_ = unix.Close(fd)
+
+	return err
+}