1 files changed, 140 insertions, 0 deletions
diff --git a/pkg/sentry/sighandling/sighandling.go b/pkg/sentry/sighandling/sighandling.go
new file mode 100644
index 000000000..659b43363
--- /dev/null
+++ b/pkg/sentry/sighandling/sighandling.go
@@ -0,0 +1,140 @@
+// Copyright 2018 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 sighandling contains helpers for handling signals to applications.
+package sighandling
+
+import (
+	"fmt"
+	"os"
+	"os/signal"
+	"reflect"
+	"syscall"
+
+	"gvisor.googlesource.com/gvisor/pkg/abi/linux"
+)
+
+// numSignals is the number of normal (non-realtime) signals on Linux.
+const numSignals = 32
+
+// handleSignals listens for incoming signals and calls the given handler
+// function.
+//
+// It starts when the start channel is closed, stops when the stop channel
+// is closed, and closes done once it will no longer deliver signals to k.
+func handleSignals(sigchans []chan os.Signal, handler func(linux.Signal), start, stop, done chan struct{}) {
+	// Build a select case.
+	sc := []reflect.SelectCase{{Dir: reflect.SelectRecv, Chan: reflect.ValueOf(start)}}
+	for _, sigchan := range sigchans {
+		sc = append(sc, reflect.SelectCase{Dir: reflect.SelectRecv, Chan: reflect.ValueOf(sigchan)})
+	}
+
+	started := false
+	for {
+		// Wait for a notification.
+		index, _, ok := reflect.Select(sc)
+
+		// Was it the start / stop channel?
+		if index == 0 {
+			if !ok {
+				if !started {
+					// start channel; start forwarding and
+					// swap this case for the stop channel
+					// to select stop requests.
+					started = true
+					sc[0] = reflect.SelectCase{Dir: reflect.SelectRecv, Chan: reflect.ValueOf(stop)}
+				} else {
+					// stop channel; stop forwarding and
+					// clear this case so it is never
+					// selected again.
+					started = false
+					close(done)
+					sc[0].Chan = reflect.Value{}
+				}
+			}
+			continue
+		}
+
+		// How about a different close?
+		if !ok {
+			panic("signal channel closed unexpectedly")
+		}
+
+		// Otherwise, it was a signal on channel N. Index 0 represents the stop
+		// channel, so index N represents the channel for signal N.
+		signal := linux.Signal(index)
+
+		if !started {
+			// Kernel cannot receive signals, either because it is
+			// not ready yet or is shutting down.
+			//
+			// Kill ourselves if this signal would have killed the
+			// process before PrepareForwarding was called. i.e., all
+			// _SigKill signals; see Go
+			// src/runtime/sigtab_linux_generic.go.
+			//
+			// Otherwise ignore the signal.
+			//
+			// TODO(b/114489875): Drop in Go 1.12, which uses tgkill
+			// in runtime.raise.
+			switch signal {
+			case linux.SIGHUP, linux.SIGINT, linux.SIGTERM:
+				dieFromSignal(signal)
+				panic(fmt.Sprintf("Failed to die from signal %d", signal))
+			default:
+				continue
+			}
+		}
+
+		// Pass the signal to the handler.
+		handler(signal)
+	}
+}
+
+// PrepareHandler ensures that synchronous signals are passed to the given
+// handler function and returns a callback that starts signal delivery, which
+// itself returns a callback that stops signal handling.
+//
+// Note that this function permanently takes over signal handling. After the
+// stop callback, signals revert to the default Go runtime behavior, which
+// cannot be overridden with external calls to signal.Notify.
+func PrepareHandler(handler func(linux.Signal)) func() func() {
+	start := make(chan struct{})
+	stop := make(chan struct{})
+	done := make(chan struct{})
+
+	// Register individual channels. One channel per standard signal is
+	// required as os.Notify() is non-blocking and may drop signals. To avoid
+	// this, standard signals have to be queued separately. Channel size 1 is
+	// enough for standard signals as their semantics allow de-duplication.
+	//
+	// External real-time signals are not supported. We rely on the go-runtime
+	// for their handling.
+	var sigchans []chan os.Signal
+	for sig := 1; sig <= numSignals+1; sig++ {
+		sigchan := make(chan os.Signal, 1)
+		sigchans = append(sigchans, sigchan)
+		signal.Notify(sigchan, syscall.Signal(sig))
+	}
+	// Start up our listener.
+	go handleSignals(sigchans, handler, start, stop, done) // S/R-SAFE: synchronized by Kernel.extMu.
+
+	return func() func() {
+		close(start)
+		return func() {
+			close(stop)
+			<-done
+		}
+	}
+}