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// Copyright 2018 Google Inc.
//
// 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 (
"os"
"os/signal"
"reflect"
"syscall"
"gvisor.googlesource.com/gvisor/pkg/abi/linux"
"gvisor.googlesource.com/gvisor/pkg/sentry/arch"
"gvisor.googlesource.com/gvisor/pkg/sentry/kernel"
)
// numSignals is the number of normal (non-realtime) signals on Linux.
const numSignals = 32
// forwardSignals listens for incoming signals and delivers them to k. It starts
// when the start channel is closed and stops when the stop channel is closed.
func forwardSignals(k *kernel.Kernel, sigchans []chan os.Signal, start, stop 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 {
// stop channel
break
} else {
// start channel
started = true
sc[0] = reflect.SelectCase{Dir: reflect.SelectRecv, Chan: reflect.ValueOf(stop)}
}
}
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.
if !started || !k.SendExternalSignal(&arch.SignalInfo{Signo: int32(index)}, "sentry") {
// Kernel is not ready to receive signals.
//
// 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: Convert Go's runtime.raise from
// tkill to tgkill so PrepareForwarding doesn't need to
// be called until after filter installation.
switch linux.Signal(index) {
case linux.SIGHUP, linux.SIGINT, linux.SIGTERM:
dieFromSignal(linux.Signal(index))
}
}
}
// Close all individual channels.
for _, sigchan := range sigchans {
signal.Stop(sigchan)
close(sigchan)
}
}
// PrepareForwarding ensures that synchronous signals are forwarded to k and
// returns a callback that starts signal delivery, which itself returns a
// callback that stops signal forwarding.
func PrepareForwarding(k *kernel.Kernel) func() func() {
start := make(chan struct{})
stop := 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)
// SignalPanic is handled by Run.
if linux.Signal(sig) == kernel.SignalPanic {
continue
}
signal.Notify(sigchan, syscall.Signal(sig))
}
// Start up our listener.
go forwardSignals(k, sigchans, start, stop) // S/R-SAFE: synchronized by Kernel.extMu
return func() func() {
close(start)
return func() {
close(stop)
}
}
}
// StartForwarding ensures that synchronous signals are forwarded to k and
// returns a callback that stops signal forwarding.
func StartForwarding(k *kernel.Kernel) func() {
return PrepareForwarding(k)()
}
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