Flipcall refinements.

Note that some of these changes affect the protocol in
backward-incompatible ways.

- Replace use of "initially-active" and "initially-inactive" with
"client" and "server" respectively for clarity.

- Fix a race condition involving Endpoint.Shutdown() by repeatedly
invoking FUTEX_WAKE until it is confirmed that no local thread is
blocked in FUTEX_WAIT.

- Drop flipcall.ControlMode.

PiperOrigin-RevId: 260981382

1 files changed, 146 insertions, 0 deletions

diff --git a/pkg/flipcall/ctrl_futex.go b/pkg/flipcall/ctrl_futex.go
new file mode 100644
index 000000000..865b6f640
--- /dev/null
+++ b/pkg/flipcall/ctrl_futex.go
@@ -0,0 +1,146 @@
+// Copyright 2019 The gVisor Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package flipcall
+
+import (
+	"encoding/json"
+	"fmt"
+	"math"
+	"sync/atomic"
+
+	"gvisor.dev/gvisor/pkg/log"
+)
+
+type endpointControlImpl struct {
+	state int32
+}
+
+// Bits in endpointControlImpl.state.
+const (
+	epsBlocked = 1 << iota
+	epsShutdown
+)
+
+func (ep *Endpoint) ctrlInit(opts ...EndpointOption) error {
+	if len(opts) != 0 {
+		return fmt.Errorf("unknown EndpointOption: %T", opts[0])
+	}
+	return nil
+}
+
+type ctrlHandshakeRequest struct{}
+
+type ctrlHandshakeResponse struct{}
+
+func (ep *Endpoint) ctrlConnect() error {
+	if err := ep.enterFutexWait(); err != nil {
+		return err
+	}
+	_, err := ep.futexConnect(&ctrlHandshakeRequest{})
+	ep.exitFutexWait()
+	return err
+}
+
+func (ep *Endpoint) ctrlWaitFirst() error {
+	if err := ep.enterFutexWait(); err != nil {
+		return err
+	}
+	defer ep.exitFutexWait()
+
+	// Wait for the handshake request.
+	if err := ep.futexSwitchFromPeer(); err != nil {
+		return err
+	}
+
+	// Read the handshake request.
+	reqLen := atomic.LoadUint32(ep.dataLen())
+	if reqLen > ep.dataCap {
+		return fmt.Errorf("invalid handshake request length %d (maximum %d)", reqLen, ep.dataCap)
+	}
+	var req ctrlHandshakeRequest
+	if err := json.NewDecoder(ep.NewReader(reqLen)).Decode(&req); err != nil {
+		return fmt.Errorf("error reading handshake request: %v", err)
+	}
+
+	// Write the handshake response.
+	w := ep.NewWriter()
+	if err := json.NewEncoder(w).Encode(ctrlHandshakeResponse{}); err != nil {
+		return fmt.Errorf("error writing handshake response: %v", err)
+	}
+	*ep.dataLen() = w.Len()
+
+	// Return control to the client.
+	if err := ep.futexSwitchToPeer(); err != nil {
+		return err
+	}
+
+	// Wait for the first non-handshake message.
+	return ep.futexSwitchFromPeer()
+}
+
+func (ep *Endpoint) ctrlRoundTrip() error {
+	if err := ep.futexSwitchToPeer(); err != nil {
+		return err
+	}
+	if err := ep.enterFutexWait(); err != nil {
+		return err
+	}
+	err := ep.futexSwitchFromPeer()
+	ep.exitFutexWait()
+	return err
+}
+
+func (ep *Endpoint) ctrlWakeLast() error {
+	return ep.futexSwitchToPeer()
+}
+
+func (ep *Endpoint) enterFutexWait() error {
+	switch eps := atomic.AddInt32(&ep.ctrl.state, epsBlocked); eps {
+	case epsBlocked:
+		return nil
+	case epsBlocked | epsShutdown:
+		atomic.AddInt32(&ep.ctrl.state, -epsBlocked)
+		return shutdownError{}
+	default:
+		// Most likely due to ep.enterFutexWait() being called concurrently
+		// from multiple goroutines.
+		panic(fmt.Sprintf("invalid flipcall.Endpoint.ctrl.state before flipcall.Endpoint.enterFutexWait(): %v", eps-epsBlocked))
+	}
+}
+
+func (ep *Endpoint) exitFutexWait() {
+	atomic.AddInt32(&ep.ctrl.state, -epsBlocked)
+}
+
+func (ep *Endpoint) ctrlShutdown() {
+	// Set epsShutdown to ensure that future calls to ep.enterFutexWait() fail.
+	if atomic.AddInt32(&ep.ctrl.state, epsShutdown)&epsBlocked != 0 {
+		// Wake the blocked thread. This must loop because it's possible that
+		// FUTEX_WAKE occurs after the waiter sets epsBlocked, but before it
+		// blocks in FUTEX_WAIT.
+		for {
+			// Wake MaxInt32 threads to prevent a broken or malicious peer from
+			// swallowing our wakeup by FUTEX_WAITing from multiple threads.
+			if err := ep.futexWakeConnState(math.MaxInt32); err != nil {
+				log.Warningf("failed to FUTEX_WAKE Endpoints: %v", err)
+				break
+			}
+			yieldThread()
+			if atomic.LoadInt32(&ep.ctrl.state)&epsBlocked == 0 {
+				break
+			}
+		}
+	}
+}