channel: Check for channel type in kernel cmdline options

With a recent kernel change, the agent can no longer rely on
/dev/vsock and AF_VSOCK in socket(), to detect if vhost-vsock
channel has been passed by the runtime.
These are not created when the vhost-vsock driver is initialised.

The runtime should now pass this information explicilty.
Based on the channel type passed, the agent now checks for that
partcular channel type.
In case it is not passed, check for both serial and vsock channel.
This also introduces a change in the way vsock channel is detected
by checking for devices under the vhost-vsock driver path.

Fixes kata-containers#506

Signed-off-by: Archana Shinde <archana.m.shinde@intel.com>

agent.go

@@ -156,6 +156,22 @@ var collatedTrace = false
 // if true, coredump when an internal error occurs or a fatal signal is received
 var crashOnError = false
 
+// commType is used to denote the communication channel type used.
+type commType int
+
+const (
+	// virtio-serial channel
+	serialCh commType = iota
+
+	// vsock channel
+	vsockCh
+
+	// channel type not passed explicitly
+	unknownCh
+)
+
+var commCh = unknownCh
+
 // This is the list of file descriptors we can properly close after the process
 // has been started. When the new process is exec(), those file descriptors are
 // duplicated and it is our responsibility to close them since we have opened

channel.go

@@ -50,24 +50,29 @@ func newChannel(ctx context.Context) (channel, error) {
 	defer span.Finish()
 
 	var serialErr error
-	var serialPath string
 	var vsockErr error
-	var vSockSupported bool
+	var ch channel
 
 	for i := 0; i < channelExistMaxTries; i++ {
-		// check vsock path
-		if _, err := os.Stat(vSockDevPath); err == nil {
-			if vSockSupported, vsockErr = isAFVSockSupportedFunc(); vSockSupported && vsockErr == nil {
-				span.SetTag("channel-type", "vsock")
-				return &vSockChannel{}, nil
+		switch commCh {
+		case serialCh:
+			if ch, serialErr = checkForSerialChannel(ctx); serialErr == nil && ch.(*serialChannel) != nil {
+				return ch, nil
+			}
+		case vsockCh:
+			if ch, vsockErr = checkForVsockChannel(ctx); vsockErr == nil && ch.(*vSockChannel) != nil {
+				return ch, nil
 			}
-		}
 
-		// Check serial port path
-		if serialPath, serialErr = findVirtualSerialPath(serialChannelName); serialErr == nil {
-			span.SetTag("channel-type", "serial")
-			span.SetTag("serial-path", serialPath)
-			return &serialChannel{serialPath: serialPath}, nil
+		case unknownCh:
+			// If we have not been explicitly passed if vsock is used or not, maybe due to
+			// an older runtime, try to check for vsock support.
+			if ch, vsockErr = checkForVsockChannel(ctx); vsockErr == nil && ch.(*vSockChannel) != nil {
+				return ch, nil
+			}
+			if ch, serialErr = checkForSerialChannel(ctx); serialErr == nil && ch.(*serialChannel) != nil {
+				return ch, nil
+			}
 		}
 
 		time.Sleep(channelExistWaitTime)
@@ -84,6 +89,44 @@ func newChannel(ctx context.Context) (channel, error) {
 	return nil, fmt.Errorf("Neither vsocks nor serial ports were found")
 }
 
+func checkForSerialChannel(ctx context.Context) (*serialChannel, error) {
+	span, _ := trace(ctx, "channel", "checkForSerialChannel")
+	defer span.Finish()
+
+	// Check serial port path
+	serialPath, serialErr := findVirtualSerialPath(serialChannelName)
+	if serialErr == nil {
+		span.SetTag("channel-type", "serial")
+		span.SetTag("serial-path", serialPath)
+		agentLog.Debug("Serial channel type detected")
+		return &serialChannel{serialPath: serialPath}, nil
+	}
+
+	return nil, serialErr
+}
+
+func checkForVsockChannel(ctx context.Context) (*vSockChannel, error) {
+	span, _ := trace(ctx, "channel", "checkForVsockChannel")
+	defer span.Finish()
+
+	// check vsock path
+	var err error
+	_, err = os.Stat(vSockDevPath)
+
+	if err != nil {
+		return nil, err
+	}
+
+	vSockSupported, vsockErr := isAFVSockSupportedFunc()
+	if vSockSupported && vsockErr == nil {
+		span.SetTag("channel-type", "vsock")
+		agentLog.Debug("Vsock channel type detected")
+		return &vSockChannel{}, nil
+	}
+
+	return nil, fmt.Errorf("Vsock not found : %s", vsockErr)
+}
+
 type vSockChannel struct {
 }
 
@@ -228,23 +271,51 @@ func (c *serialChannel) teardown() error {
 	return c.serialConn.Close()
 }
 
+// isAFVSockSupported checks if vsock channel is used by the runtime
+// by checking for devices under the vhost-vsock driver path.
+// It returns true if more a device is found for the driver.
 func isAFVSockSupported() (bool, error) {
-	fd, err := unix.Socket(unix.AF_VSOCK, unix.SOCK_STREAM, 0)
-	if err != nil {
-		// This case is valid. It means AF_VSOCK is not a supported
-		// domain on this system.
-		if err == unix.EAFNOSUPPORT {
-			return false, nil
-		}
+	// Driver path for virtio-vsock
+	sysVsockPath := "/sys/bus/virtio/drivers/vmw_vsock_virtio_transport/"
 
+	files, err := ioutil.ReadDir(sysVsockPath)
+
+	// This should not happen for a hypervisor with vsock driver
+	if err != nil {
 		return false, err
 	}
 
-	if err := unix.Close(fd); err != nil {
-		return true, err
+	// standard driver files that should be ignored
+	driverFiles := []string{"bind", "uevent", "unbind"}
+
+	for _, file := range files {
+		for _, f := range driverFiles {
+			if file.Name() == f {
+				continue
+			}
+		}
+
+		fPath := filepath.Join(sysVsockPath, file.Name())
+		fInfo, err := os.Lstat(fPath)
+		if err != nil {
+			return false, err
+		}
+
+		if fInfo.Mode()&os.ModeSymlink == 0 {
+			continue
+		}
+
+		link, err := os.Readlink(fPath)
+		if err != nil {
+			return false, err
+		}
+
+		if strings.Contains(link, "devices") {
+			return true, nil
+		}
 	}
 
-	return true, nil
+	return false, nil
 }
 
 func findVirtualSerialPath(serialName string) (string, error) {

config.go

@@ -8,6 +8,7 @@ package main
 
 import (
 	"io/ioutil"
+	"strconv"
 	"strings"
 
 	"github.com/sirupsen/logrus"
@@ -20,6 +21,7 @@ const (
 	logLevelFlag       = optionPrefix + "log"
 	devModeFlag        = optionPrefix + "devmode"
 	traceModeFlag      = optionPrefix + "trace"
+	useVsockFlag       = optionPrefix + "use_vsock"
 	kernelCmdlineFile  = "/proc/cmdline"
 	traceValueIsolated = "isolated"
 	traceValueCollated = "collated"
@@ -102,6 +104,16 @@ func (c *agentConfig) parseCmdlineOption(option string) error {
 		case traceValueCollated:
 			enableTracing(true)
 		}
+	case useVsockFlag:
+		if flag, err := strconv.ParseBool(split[valuePosition]); err == nil {
+			if flag {
+				agentLog.Debug("Param passed to use vsock channel")
+				commCh = vsockCh
+			} else {
+				agentLog.Debug("Param passed to NOT use vsock channel")
+				commCh = serialCh
+			}
+		}
 	default:
 		if strings.HasPrefix(split[optionPosition], optionPrefix) {
 			return grpcStatus.Errorf(codes.NotFound, "Unknown option %s", split[optionPosition])