Merge pull request mdlayher#32 from mdlayher/mdl-contextid

vsock: export ContextID for checking host context ID and vsock support

ioctl_linux.go

@@ -20,12 +20,23 @@ type fs interface {
 	Ioctl(fd uintptr, request int, argp unsafe.Pointer) error
 }
 
-// localContextID retrieves the local context ID for this system, using the
-// methods from fs.  The context ID is stored in cid for later use.
+// contextID retrieves the local context ID for this system.
+func contextID() (uint32, error) {
+	// Fetch the context ID using a real filesystem.
+	var cid uint32
+	if err := sysContextID(sysFS{}, &cid); err != nil {
+		return 0, err
+	}
+
+	return cid, nil
+}
+
+// sysContextID retrieves the local context ID for this system, using the
+// methods from fs. The context ID is stored in cid for later use.
 //
 // This method uses this signature to enable easier testing without unsafe
 // usage of unsafe.Pointer.
-func localContextID(fs fs, cid *uint32) error {
+func sysContextID(fs fs, cid *uint32) error {
 	f, err := fs.Open(devVsock)
 	if err != nil {
 		return err

ioctl_linux_test.go

@@ -55,7 +55,7 @@ func Test_localContextIDGuest(t *testing.T) {
 		ioctl: ioctl,
 	}
 
-	if err := localContextID(fs, &cid); err != nil {
+	if err := sysContextID(fs, &cid); err != nil {
 		t.Fatalf("failed to retrieve host's context ID: %v", err)
 	}
 
@@ -70,11 +70,13 @@ func Test_localContextIDGuestIntegration(t *testing.T) {
 		t.Skip("machine is not a guest, skipping")
 	}
 
-	var cid uint32
-	if err := localContextID(sysFS{}, &cid); err != nil {
+	cid, err := ContextID()
+	if err != nil {
 		t.Fatalf("failed to retrieve guest's context ID: %v", err)
 	}
 
+	t.Logf("guest context ID: %d", cid)
+
 	// Guests should always have a context ID of 3 or more, since
 	// 0-2 are invalid or reserved.
 	if cid < 3 {
@@ -87,11 +89,13 @@ func Test_localContextIDHostIntegration(t *testing.T) {
 		t.Skip("machine is not a hypervisor, skipping")
 	}
 
-	var cid uint32
-	if err := localContextID(sysFS{}, &cid); err != nil {
+	cid, err := ContextID()
+	if err != nil {
 		t.Fatalf("failed to retrieve host's context ID: %v", err)
 	}
 
+	t.Logf("host context ID: %d", cid)
+
 	if want, got := uint32(Host), cid; want != got {
 		t.Fatalf("unexpected host context ID:\n- want: %d\n-  got: %d",
 			want, got)
@@ -106,8 +110,8 @@ func isHypervisor(t *testing.T) bool {
 		t.Skipf("device %q not available, kernel module not loaded?", devVsock)
 	}
 
-	var cid uint32
-	if err := localContextID(sysFS{}, &cid); err != nil {
+	cid, err := ContextID()
+	if err != nil {
 		if os.IsPermission(err) {
 			t.Skipf("permission denied, make sure user has access to %q", devVsock)
 		}

listener_linux.go

@@ -42,8 +42,8 @@ func (l *listener) Accept() (net.Conn, error) {
 
 // listenStream is the entry point for ListenStream on Linux.
 func listenStream(port uint32) (*Listener, error) {
-	var cid uint32
-	if err := localContextID(sysFS{}, &cid); err != nil {
+	cid, err := ContextID()
+	if err != nil {
 		return nil, err
 	}
 

vsock.go

@@ -170,3 +170,13 @@ func (a *Addr) String() string {
 func (a *Addr) fileName() string {
 	return fmt.Sprintf("%s:%s", a.Network(), a.String())
 }
+
+// ContextID retrieves the local VM sockets context ID for this system.
+// ContextID can be used to directly determine if a system is capable of using
+// VM sockets.
+//
+// If the kernel module is unavailable, access to the kernel module is denied,
+// or VM sockets are unsupported on this system, it returns an error.
+func ContextID() (uint32, error) {
+	return contextID()
+}

vsock_others.go

@@ -36,3 +36,5 @@ func (*connFD) Read(_ []byte) (int, error)                    { return 0, errUni
 func (*connFD) Write(_ []byte) (int, error)                   { return 0, errUnimplemented }
 func (*connFD) Close() error                                  { return errUnimplemented }
 func (*connFD) Shutdown(_ int) error                          { return errUnimplemented }
+
+func contextID() (uint32, error) { return 0, errUnimplemented }

vsock_others_test.go

@@ -7,6 +7,11 @@ import "testing"
 func TestUnimplemented(t *testing.T) {
 	want := errUnimplemented
 
+	if _, got := ContextID(); want != got {
+		t.Fatalf("unexpected error from ContextID:\n- want: %v\n-  got: %v",
+			want, got)
+	}
+
 	if _, got := listenStream(0); want != got {
 		t.Fatalf("unexpected error from listenStream:\n- want: %v\n-  got: %v",
 			want, got)