agent: support debug console

Start a debug console if the `agent.debug_console` option is part of the kernel
command line and a shell (bash or sh) is installed in the rootfs.
The debug console is useful when the developer wants to debug the agent process
or any other processes (even containers). The current way to do this is by
modifying a systemd target + creating a new system unit, the problem with this
approach is that it's not user-friendly and we depend on systemd, since it must
be installed in the rootfs.

fixes kata-containers#546

Signed-off-by: Julio Montes <julio.montes@intel.com>

README.md

@@ -6,6 +6,7 @@
 * [Debug mode](#debug-mode)
 * [Developer mode](#developer-mode)
 * [Enable trace support](#enable-trace-support)
+* [Enable debug console](#enable-debug-console)
 
 This project implements an agent called `kata-agent` that runs inside a virtual machine (VM).
 
@@ -27,3 +28,10 @@ enables [debug mode](#debug-mode).
 ## Enable trace support
 
 See [the tracing guide](TRACING.md).
+
+## Enable debug console
+
+Add `agent.debug_console` to the guest kernel command line to
+allow the agent process to start a debug console. Debug console is only available if `bash`
+or `sh` is installed in the rootfs or initrd image. Developers can [connect to the virtual
+machine using the debug console](https://github.com/kata-containers/documentation/blob/master/Developer-Guide.md#connect-to-the-virtual-machine-using-the-debug-console)

agent.go

@@ -43,6 +43,10 @@ import (
 const (
 	procCgroups = "/proc/cgroups"
 	meminfo     = "/proc/meminfo"
+
+	bashPath         = "/bin/bash"
+	shPath           = "/bin/sh"
+	debugConsolePath = "/dev/console"
 )
 
 var (
@@ -156,6 +160,9 @@ var collatedTrace = false
 // if true, coredump when an internal error occurs or a fatal signal is received
 var crashOnError = false
 
+// if true, a shell (bash or sh) is started only if it's available in the rootfs.
+var debugConsole = false
+
 // commType is used to denote the communication channel type used.
 type commType int
 
@@ -911,6 +918,8 @@ func (s *sandbox) initLogger() error {
 
 	agentLog.Logger.SetLevel(config.logLevel)
 
+	agentLog = agentLog.WithField("debug_console", debugConsole)
+
 	return announce()
 }
 
@@ -1196,6 +1205,56 @@ func cgroupsMount() error {
 	return ioutil.WriteFile(cgroupMemoryUseHierarchyPath, []byte{'1'}, cgroupMemoryUseHierarchyMode)
 }
 
+func setupDebugConsole() error {
+	if !debugConsole {
+		return nil
+	}
+
+	var shellPath string
+	for _, s := range []string{bashPath, shPath} {
+		var err error
+		if _, err = os.Stat(s); err == nil {
+			shellPath = s
+			break
+		}
+		agentLog.WithError(err).WithField("shell", s).Warn("Shell not found")
+	}
+
+	if shellPath == "" {
+		return errors.New("Shell not found")
+	}
+
+	cmd := exec.Command(shellPath)
+	cmd.Env = os.Environ()
+	f, err := os.OpenFile(debugConsolePath, os.O_RDWR, 0600)
+	if err != nil {
+		return err
+	}
+
+	cmd.Stdin = f
+	cmd.Stdout = f
+	cmd.Stderr = f
+
+	cmd.SysProcAttr = &syscall.SysProcAttr{
+		// Create Session
+		Setsid: true,
+		// Set Controlling terminal to Ctty
+		Setctty: true,
+		Ctty:    int(f.Fd()),
+	}
+
+	go func() {
+		for {
+			dcmd := *cmd
+			if err := dcmd.Run(); err != nil {
+				agentLog.WithError(err).Warn("failed to start debug console")
+			}
+		}
+	}()
+
+	return nil
+}
+
 // initAgentAsInit will do the initializations such as setting up the rootfs
 // when this agent has been run as the init process.
 func initAgentAsInit() error {
@@ -1271,6 +1330,10 @@ func realMain() error {
 		return fmt.Errorf("failed to setup logger: %v", err)
 	}
 
+	if err := setupDebugConsole(); err != nil {
+		agentLog.WithError(err).Error("failed to setup debug console")
+	}
+
 	rootSpan, rootContext, err = setupTracing(agentName)
 	if err != nil {
 		return fmt.Errorf("failed to setup tracing: %v", err)

config.go

@@ -22,6 +22,7 @@ const (
 	devModeFlag       = optionPrefix + "devmode"
 	traceModeFlag     = optionPrefix + "trace"
 	useVsockFlag      = optionPrefix + "use_vsock"
+	debugConsoleFlag  = optionPrefix + "debug_console"
 	kernelCmdlineFile = "/proc/cmdline"
 	traceModeStatic   = "static"
 	traceModeDynamic  = "dynamic"
@@ -79,6 +80,11 @@ func (c *agentConfig) parseCmdlineOption(option string) error {
 		return nil
 	}
 
+	if option == debugConsoleFlag {
+		debugConsole = true
+		return nil
+	}
+
 	if option == traceModeFlag {
 		enableTracing(traceModeStatic, defaultTraceType)
 		return nil