1 files changed, 94 insertions, 0 deletions
diff --git a/pkg/flipcall/futex_linux.go b/pkg/flipcall/futex_linux.go
new file mode 100644
index 000000000..3f592ad16
--- /dev/null
+++ b/pkg/flipcall/futex_linux.go
@@ -0,0 +1,94 @@
+// 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.
+
+// +build linux
+
+package flipcall
+
+import (
+	"fmt"
+	"math"
+	"sync/atomic"
+	"syscall"
+
+	"gvisor.dev/gvisor/pkg/abi/linux"
+	"gvisor.dev/gvisor/pkg/log"
+)
+
+func (ep *Endpoint) doFutexRoundTrip() error {
+	ourSeq, err := ep.doFutexNotifySeq()
+	if err != nil {
+		return err
+	}
+	return ep.doFutexWaitSeq(ourSeq)
+}
+
+func (ep *Endpoint) doFutexWaitFirst() error {
+	return ep.doFutexWaitSeq(0)
+}
+
+func (ep *Endpoint) doFutexNotifyLast() error {
+	_, err := ep.doFutexNotifySeq()
+	return err
+}
+
+func (ep *Endpoint) doFutexNotifySeq() (uint32, error) {
+	ourSeq := atomic.AddUint32(ep.seq(), 1)
+	if err := ep.futexWake(1); err != nil {
+		return ourSeq, fmt.Errorf("failed to FUTEX_WAKE peer Endpoint: %v", err)
+	}
+	return ourSeq, nil
+}
+
+func (ep *Endpoint) doFutexWaitSeq(prevSeq uint32) error {
+	nextSeq := prevSeq + 1
+	for {
+		if ep.isShutdown() {
+			return endpointShutdownError{}
+		}
+		if err := ep.futexWait(prevSeq); err != nil {
+			return fmt.Errorf("failed to FUTEX_WAIT for peer Endpoint: %v", err)
+		}
+		seq := atomic.LoadUint32(ep.seq())
+		if seq == nextSeq {
+			return nil
+		}
+		if seq != prevSeq {
+			return fmt.Errorf("invalid packet sequence number %d (expected %d or %d)", seq, prevSeq, nextSeq)
+		}
+	}
+}
+
+func (ep *Endpoint) doFutexInterruptForShutdown() {
+	// Wake MaxInt32 threads to prevent a malicious or broken peer from
+	// swallowing our wakeup by FUTEX_WAITing from multiple threads.
+	if err := ep.futexWake(math.MaxInt32); err != nil {
+		log.Warningf("failed to FUTEX_WAKE Endpoint: %v", err)
+	}
+}
+
+func (ep *Endpoint) futexWake(numThreads int32) error {
+	if _, _, e := syscall.RawSyscall(syscall.SYS_FUTEX, uintptr(ep.packet), linux.FUTEX_WAKE, uintptr(numThreads)); e != 0 {
+		return e
+	}
+	return nil
+}
+
+func (ep *Endpoint) futexWait(seq uint32) error {
+	_, _, e := syscall.Syscall6(syscall.SYS_FUTEX, uintptr(ep.packet), linux.FUTEX_WAIT, uintptr(seq), 0, 0, 0)
+	if e != 0 && e != syscall.EAGAIN && e != syscall.EINTR {
+		return e
+	}
+	return nil
+}