cgroup2: Add Kill method to manager

This adds in support for killing all of the processes in a cgroup. In
5.14+ this is very simple, a cgroup.kill file exists that all you need
to do is write "1" to https://lwn.net/Articles/855924/. On kernels prior,
or if the file doesn't exist to be more pedantic to account for potential
backports, I've taken the approach runc currently uses which is a manual
process of freezing the cgroup -> sending a signal to all of the processes
-> thawing the cgroup.

This also adds in a simple test for this that should work on 5.15+ and prior
kernels.

Signed-off-by: Danny Canter <[email protected]>

Author: dcantah

README.md

@@ -188,6 +188,19 @@ if err != nil {
 }
 ```
 
+### Kill all processes in a cgroup
+
+```go
+m, err := cgroup2.LoadSystemd("/", "my-cgroup-abc.slice")
+if err != nil {
+	return err
+}
+err = m.Kill()
+if err != nil {
+	return err
+}
+```
+
 ### Attention
 
 All static path should not include `/sys/fs/cgroup/` prefix, it should start with your own cgroups name

cgroup2/manager.go

@@ -42,6 +42,7 @@ import (
 const (
 	subtreeControl     = "cgroup.subtree_control"
 	controllersFile    = "cgroup.controllers"
+	killFile           = "cgroup.kill"
 	defaultCgroup2Path = "/sys/fs/cgroup"
 	defaultSlice       = "system.slice"
 )
@@ -366,6 +367,86 @@ func (c *Manager) AddThread(tid uint64) error {
 	return writeValues(c.path, []Value{v})
 }
 
+// Kill will try to forcibly exit all of the processes in the cgroup. This is
+// equivalent to sending a SIGKILL to every process. On kernels 5.14 and greater
+// this will use the cgroup.kill file, on anything that doesn't have the cgroup.kill
+// file, a manual process of freezing -> sending a SIGKILL to every process -> thawing
+// will be used.
+func (c *Manager) Kill() error {
+	v := Value{
+		filename: killFile,
+		value:    "1",
+	}
+	err := writeValues(c.path, []Value{v})
+	if err == nil {
+		return nil
+	}
+	logrus.Warnf("falling back to slower kill implementation: %s", err)
+	// Fallback to slow method.
+	return c.fallbackKill()
+}
+
+// fallbackKill is a slower fallback to the more modern (kernels 5.14+)
+// approach of writing to the cgroup.kill file. This is heavily pulled
+// from runc's same approach (in signalAllProcesses), with the only differences
+// being this is just tailored to the API exposed in this library, and we don't
+// need to care about signals other than SIGKILL.
+//
+// https://github.com/opencontainers/runc/blob/8da0a0b5675764feaaaaad466f6567a9983fcd08/libcontainer/init_linux.go#L523-L529
+func (c *Manager) fallbackKill() error {
+	if err := c.Freeze(); err != nil {
+		logrus.Warn(err)
+	}
+	pids, err := c.Procs(true)
+	if err != nil {
+		if err := c.Thaw(); err != nil {
+			logrus.Warn(err)
+		}
+		return err
+	}
+	var procs []*os.Process
+	for _, pid := range pids {
+		p, err := os.FindProcess(int(pid))
+		if err != nil {
+			logrus.Warn(err)
+			continue
+		}
+		procs = append(procs, p)
+		if err := p.Signal(unix.SIGKILL); err != nil {
+			logrus.Warn(err)
+		}
+	}
+	if err := c.Thaw(); err != nil {
+		logrus.Warn(err)
+	}
+
+	subreaper, err := getSubreaper()
+	if err != nil {
+		// The error here means that PR_GET_CHILD_SUBREAPER is not
+		// supported because this code might run on a kernel older
+		// than 3.4. We don't want to throw an error in that case,
+		// and we simplify things, considering there is no subreaper
+		// set.
+		subreaper = 0
+	}
+
+	for _, p := range procs {
+		// In case a subreaper has been setup, this code must not
+		// wait for the process. Otherwise, we cannot be sure the
+		// current process will be reaped by the subreaper, while
+		// the subreaper might be waiting for this process in order
+		// to retrieve its exit code.
+		if subreaper == 0 {
+			if _, err := p.Wait(); err != nil {
+				if !errors.Is(err, unix.ECHILD) {
+					logrus.Warnf("wait on pid %d failed: %s", p.Pid, err)
+				}
+			}
+		}
+	}
+	return nil
+}
+
 func (c *Manager) Delete() error {
 	// kernel prevents cgroups with running process from being removed, check the tree is empty
 	processes, err := c.Procs(true)
@@ -763,7 +844,8 @@ func setDevices(path string, devices []specs.LinuxDeviceCgroup) error {
 // the reason this is necessary is because the "-" character has a special meaning in
 // systemd slice. For example, when creating a slice called "my-group-112233.slice",
 // systemd will create a hierarchy like this:
-//      /sys/fs/cgroup/my.slice/my-group.slice/my-group-112233.slice
+//
+//	/sys/fs/cgroup/my.slice/my-group.slice/my-group-112233.slice
 func getSystemdFullPath(slice, group string) string {
 	return filepath.Join(defaultCgroup2Path, dashesToPath(slice), dashesToPath(group))
 }

cgroup2/manager_test.go

@@ -142,6 +142,54 @@ func TestSystemdFullPath(t *testing.T) {
 	}
 }
 
+func TestKill(t *testing.T) {
+	checkCgroupMode(t)
+	manager, err := NewManager(defaultCgroup2Path, "/test1", ToResources(&specs.LinuxResources{}))
+	if err != nil {
+		t.Fatal(err)
+	}
+	var procs []*exec.Cmd
+	for i := 0; i < 5; i++ {
+		cmd := exec.Command("sleep", "infinity")
+		if err := cmd.Start(); err != nil {
+			t.Fatal(err)
+		}
+		if cmd.Process == nil {
+			t.Fatal("Process is nil")
+		}
+		if err := manager.AddProc(uint64(cmd.Process.Pid)); err != nil {
+			t.Fatal(err)
+		}
+		procs = append(procs, cmd)
+	}
+	// Verify we have 5 pids before beginning Kill below.
+	pids, err := manager.Procs(true)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if len(pids) != 5 {
+		t.Fatalf("expected 5 pids, got %d", len(pids))
+	}
+	// Now run kill, and check that nothing is running after.
+	if err := manager.Kill(); err != nil {
+		t.Fatal(err)
+	}
+
+	done := make(chan struct{})
+	go func() {
+		for _, proc := range procs {
+			_ = proc.Wait()
+		}
+		done <- struct{}{}
+	}()
+
+	select {
+	case <-time.After(time.Second * 3):
+		t.Fatal("timed out waiting for processes to exit")
+	case <-done:
+	}
+}
+
 func TestMoveTo(t *testing.T) {
 	checkCgroupMode(t)
 	manager, err := NewManager(defaultCgroup2Path, "/test1", ToResources(&specs.LinuxResources{}))

cgroup2/utils.go

@@ -26,12 +26,14 @@ import (
 	"strconv"
 	"strings"
 	"time"
+	"unsafe"
 
 	"github.com/containerd/cgroups/v3/cgroup2/stats"
 
 	"github.com/godbus/dbus/v5"
 	"github.com/opencontainers/runtime-spec/specs-go"
 	"github.com/sirupsen/logrus"
+	"golang.org/x/sys/unix"
 )
 
 const (
@@ -434,3 +436,11 @@ func readHugeTlbStats(path string) []*stats.HugeTlbStat {
 	}
 	return usage
 }
+
+func getSubreaper() (int, error) {
+	var i uintptr
+	if err := unix.Prctl(unix.PR_GET_CHILD_SUBREAPER, uintptr(unsafe.Pointer(&i)), 0, 0, 0); err != nil {
+		return -1, err
+	}
+	return int(i), nil
+}