*: introduce pidfd-socket flag

The container manager like containerd-shim can't use cgroup.kill feature or
freeze all the processes in cgroup to terminate the exec init process.
It's unsafe to call kill(2) since the pid can be recycled. It's good to
provide the pidfd of init process through the pidfd-socket. It's similar to
the console-socket. With the pidfd, the container manager like containerd-shim
can send the signal to target process safely.

And for the standard init process, we can have polling support to get
exit event instead of blocking on wait4.

Signed-off-by: Wei Fu <[email protected]>

create.go

@@ -34,6 +34,10 @@ command(s) that get executed on start, edit the args parameter of the spec. See
 			Value: "",
 			Usage: "path to an AF_UNIX socket which will receive a file descriptor referencing the master end of the console's pseudoterminal",
 		},
+		cli.StringFlag{
+			Name:  "pidfd-socket",
+			Usage: "path to an AF_UNIX socket which will receive a file descriptor referencing the init process",
+		},
 		cli.StringFlag{
 			Name:  "pid-file",
 			Value: "",

exec.go

@@ -33,6 +33,10 @@ following will output a list of processes running in the container:
 			Name:  "console-socket",
 			Usage: "path to an AF_UNIX socket which will receive a file descriptor referencing the master end of the console's pseudoterminal",
 		},
+		cli.StringFlag{
+			Name:  "pidfd-socket",
+			Usage: "path to an AF_UNIX socket which will receive a file descriptor referencing the init process",
+		},
 		cli.StringFlag{
 			Name:  "cwd",
 			Usage: "current working directory in the container",
@@ -181,6 +185,7 @@ func execProcess(context *cli.Context) (int, error) {
 		shouldDestroy:   false,
 		container:       container,
 		consoleSocket:   context.String("console-socket"),
+		pidfdSocket:     context.String("pidfd-socket"),
 		detach:          context.Bool("detach"),
 		pidFile:         context.String("pid-file"),
 		action:          CT_ACT_RUN,

libcontainer/container_linux.go

@@ -588,6 +588,13 @@ func (c *Container) newParentProcess(p *Process) (parentProcess, error) {
 		cmd.Env = append(cmd.Env, "_LIBCONTAINER_LOGLEVEL="+p.LogLevel)
 	}
 
+	if p.PidfdSocket != nil {
+		cmd.ExtraFiles = append(cmd.ExtraFiles, p.PidfdSocket)
+		cmd.Env = append(cmd.Env,
+			"_LIBCONTAINER_PIDFD="+strconv.Itoa(stdioFdCount+len(cmd.ExtraFiles)-1),
+		)
+	}
+
 	if safeExe != nil {
 		// Due to a Go stdlib bug, we need to add safeExe to the set of
 		// ExtraFiles otherwise it is possible for the stdlib to clobber the fd

libcontainer/init_linux.go

@@ -170,6 +170,16 @@ func startInitialization() (retErr error) {
 		defer consoleSocket.Close()
 	}
 
+	var pidfdSocket *os.File
+	if envConsole := os.Getenv("_LIBCONTAINER_PIDFD"); envConsole != "" {
+		console, err := strconv.Atoi(envConsole)
+		if err != nil {
+			return fmt.Errorf("unable to convert _LIBCONTAINER_PIDFD: %w", err)
+		}
+		pidfdSocket = os.NewFile(uintptr(console), "pidfd-socket")
+		defer pidfdSocket.Close()
+	}
+
 	// Get mount files (O_PATH).
 	mountSrcFds, err := parseFdsFromEnv("_LIBCONTAINER_MOUNT_FDS")
 	if err != nil {
@@ -208,10 +218,10 @@ func startInitialization() (retErr error) {
 	}()
 
 	// If init succeeds, it will not return, hence none of the defers will be called.
-	return containerInit(it, pipe, consoleSocket, fifofd, logFD, dmzExe, mountFds{sourceFds: mountSrcFds, idmapFds: idmapFds})
+	return containerInit(it, pipe, consoleSocket, pidfdSocket, fifofd, logFD, dmzExe, mountFds{sourceFds: mountSrcFds, idmapFds: idmapFds})
 }
 
-func containerInit(t initType, pipe *os.File, consoleSocket *os.File, fifoFd, logFd int, dmzExe *os.File, mountFds mountFds) error {
+func containerInit(t initType, pipe, consoleSocket, pidfdSocket *os.File, fifoFd, logFd int, dmzExe *os.File, mountFds mountFds) error {
 	var config *initConfig
 	if err := json.NewDecoder(pipe).Decode(&config); err != nil {
 		return err
@@ -230,6 +240,7 @@ func containerInit(t initType, pipe *os.File, consoleSocket *os.File, fifoFd, lo
 		i := &linuxSetnsInit{
 			pipe:          pipe,
 			consoleSocket: consoleSocket,
+			pidfdSocket:   pidfdSocket,
 			config:        config,
 			logFd:         logFd,
 			dmzExe:        dmzExe,
@@ -239,6 +250,7 @@ func containerInit(t initType, pipe *os.File, consoleSocket *os.File, fifoFd, lo
 		i := &linuxStandardInit{
 			pipe:          pipe,
 			consoleSocket: consoleSocket,
+			pidfdSocket:   pidfdSocket,
 			parentPid:     unix.Getppid(),
 			config:        config,
 			fifoFd:        fifoFd,
@@ -666,3 +678,17 @@ func signalAllProcesses(m cgroups.Manager, s unix.Signal) error {
 
 	return nil
 }
+
+// setupPidfd opens a process file descriptor of init process, and sends the
+// file descriptor back to the socket.
+func setupPidfd(socket *os.File, initType string) error {
+	pidFd, err := unix.PidfdOpen(os.Getpid(), 0)
+	if err != nil {
+		return fmt.Errorf("failed to pidfd_open: %w", err)
+	}
+
+	if err := utils.SendRawFd(socket, initType, uintptr(pidFd)); err != nil {
+		return fmt.Errorf("failed to send pidfd on socket: %w", err)
+	}
+	return unix.Close(pidFd)
+}

libcontainer/process.go

@@ -77,6 +77,9 @@ type Process struct {
 	// ConsoleSocket provides the masterfd console.
 	ConsoleSocket *os.File
 
+	// PidfdSocket provides process file descriptor of it own.
+	PidfdSocket *os.File
+
 	// Init specifies whether the process is the first process in the container.
 	Init bool
 

libcontainer/setns_init_linux.go

@@ -22,6 +22,7 @@ import (
 type linuxSetnsInit struct {
 	pipe          *os.File
 	consoleSocket *os.File
+	pidfdSocket   *os.File
 	config        *initConfig
 	logFd         int
 	dmzExe        *os.File
@@ -56,6 +57,11 @@ func (l *linuxSetnsInit) Init() error {
 			return err
 		}
 	}
+	if l.pidfdSocket != nil {
+		if err := setupPidfd(l.pidfdSocket, "setns"); err != nil {
+			return fmt.Errorf("failed to setup pidfd: %w", err)
+		}
+	}
 	if l.config.NoNewPrivileges {
 		if err := unix.Prctl(unix.PR_SET_NO_NEW_PRIVS, 1, 0, 0, 0); err != nil {
 			return err

libcontainer/standard_init_linux.go

@@ -22,6 +22,7 @@ import (
 type linuxStandardInit struct {
 	pipe          *os.File
 	consoleSocket *os.File
+	pidfdSocket   *os.File
 	parentPid     int
 	fifoFd        int
 	logFd         int
@@ -114,6 +115,12 @@ func (l *linuxStandardInit) Init() error {
 		}
 	}
 
+	if l.pidfdSocket != nil {
+		if err := setupPidfd(l.pidfdSocket, "standard"); err != nil {
+			return fmt.Errorf("failed to setup pidfd: %w", err)
+		}
+	}
+
 	// Finish the rootfs setup.
 	if l.config.Config.Namespaces.Contains(configs.NEWNS) {
 		if err := finalizeRootfs(l.config.Config); err != nil {

run.go

@@ -35,6 +35,10 @@ command(s) that get executed on start, edit the args parameter of the spec. See
 			Value: "",
 			Usage: "path to an AF_UNIX socket which will receive a file descriptor referencing the master end of the console's pseudoterminal",
 		},
+		cli.StringFlag{
+			Name:  "pidfd-socket",
+			Usage: "path to an AF_UNIX socket which will receive a file descriptor referencing the init process",
+		},
 		cli.BoolFlag{
 			Name:  "detach, d",
 			Usage: "detach from the container's process",

utils_linux.go

@@ -194,6 +194,7 @@ type runner struct {
 	preserveFDs     int
 	pidFile         string
 	consoleSocket   string
+	pidfdSocket     string
 	container       *libcontainer.Container
 	action          CtAct
 	notifySocket    *notifySocket
@@ -250,6 +251,14 @@ func (r *runner) run(config *specs.Process) (int, error) {
 	}
 	defer tty.Close()
 
+	if r.pidfdSocket != "" {
+		connClose, err := setupPidfdSocket(process, r.pidfdSocket)
+		if err != nil {
+			return -1, err
+		}
+		defer connClose()
+	}
+
 	switch r.action {
 	case CT_ACT_CREATE:
 		err = r.container.Start(process)
@@ -385,6 +394,7 @@ func startContainer(context *cli.Context, action CtAct, criuOpts *libcontainer.C
 		listenFDs:       listenFDs,
 		notifySocket:    notifySocket,
 		consoleSocket:   context.String("console-socket"),
+		pidfdSocket:     context.String("pidfd-socket"),
 		detach:          context.Bool("detach"),
 		pidFile:         context.String("pid-file"),
 		preserveFDs:     context.Int("preserve-fds"),
@@ -394,3 +404,25 @@ func startContainer(context *cli.Context, action CtAct, criuOpts *libcontainer.C
 	}
 	return r.run(spec.Process)
 }
+
+func setupPidfdSocket(process *libcontainer.Process, sockpath string) (_clean func(), _ error) {
+	conn, err := net.Dial("unix", sockpath)
+	if err != nil {
+		return nil, fmt.Errorf("failed to dail %s: %w", sockpath, err)
+	}
+
+	uc, ok := conn.(*net.UnixConn)
+	if !ok {
+		return nil, errors.New("failed to cast to UnixConn")
+	}
+
+	socket, err := uc.File()
+	if err != nil {
+		return nil, fmt.Errorf("failed to dup socket: %w", err)
+	}
+
+	process.PidfdSocket = socket
+	return func() {
+		conn.Close()
+	}, nil
+}