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PiperOrigin-RevId: 194583126
Change-Id: Ica1d8821a90f74e7e745962d71801c598c652463

1 files changed, 126 insertions, 0 deletions

diff --git a/pkg/sentry/kernel/pending_signals.go b/pkg/sentry/kernel/pending_signals.go
new file mode 100644
index 000000000..d8701f47a
--- /dev/null
+++ b/pkg/sentry/kernel/pending_signals.go
@@ -0,0 +1,126 @@
+// 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 kernel
+
+import (
+	"gvisor.googlesource.com/gvisor/pkg/abi/linux"
+	"gvisor.googlesource.com/gvisor/pkg/bits"
+	"gvisor.googlesource.com/gvisor/pkg/sentry/arch"
+)
+
+const (
+	// stdSignalCap is the maximum number of instances of a given standard
+	// signal that may be pending. ("[If] multiple instances of a standard
+	// signal are delivered while that signal is currently blocked, then only
+	// one instance is queued.") - signal(7)
+	stdSignalCap = 1
+
+	// rtSignalCap is the maximum number of instances of a given realtime
+	// signal that may be pending.
+	//
+	// TODO: In Linux, the minimum signal queue size is
+	// RLIMIT_SIGPENDING, which is by default max_threads/2.
+	rtSignalCap = 32
+)
+
+// pendingSignals holds a collection of pending signals. The zero value of
+// pendingSignals is a valid empty collection. pendingSignals is thread-unsafe;
+// users must provide synchronization.
+type pendingSignals struct {
+	// signals contains all pending signals.
+	//
+	// Note that signals is zero-indexed, but signal 1 is the first valid
+	// signal, so signals[0] contains signals with signo 1 etc. This offset is
+	// usually handled by using Signal.index().
+	signals [linux.SignalMaximum]pendingSignalQueue
+
+	// Bit i of pendingSet is set iff there is at least one signal with signo
+	// i+1 pending.
+	pendingSet linux.SignalSet
+}
+
+// pendingSignalQueue holds a pendingSignalList for a single signal number.
+type pendingSignalQueue struct {
+	pendingSignalList
+	length int
+}
+
+type pendingSignal struct {
+	// pendingSignalEntry links into a pendingSignalList.
+	pendingSignalEntry
+	*arch.SignalInfo
+}
+
+// enqueue enqueues the given signal. enqueue returns true on success and false
+// on failure (if the given signal's queue is full).
+//
+// Preconditions: info represents a valid signal.
+func (p *pendingSignals) enqueue(info *arch.SignalInfo) bool {
+	sig := linux.Signal(info.Signo)
+	q := &p.signals[sig.Index()]
+	if sig.IsStandard() {
+		if q.length >= stdSignalCap {
+			return false
+		}
+	} else if q.length >= rtSignalCap {
+		return false
+	}
+	q.pendingSignalList.PushBack(&pendingSignal{SignalInfo: info})
+	q.length++
+	p.pendingSet |= linux.SignalSetOf(sig)
+	return true
+}
+
+// dequeue dequeues and returns any pending signal not masked by mask. If no
+// unmasked signals are pending, dequeue returns nil.
+func (p *pendingSignals) dequeue(mask linux.SignalSet) *arch.SignalInfo {
+	// "Real-time signals are delivered in a guaranteed order. Multiple
+	// real-time signals of the same type are delivered in the order they were
+	// sent. If different real-time signals are sent to a process, they are
+	// delivered starting with the lowest-numbered signal. (I.e., low-numbered
+	// signals have highest priority.) By contrast, if multiple standard
+	// signals are pending for a process, the order in which they are delivered
+	// is unspecified. If both standard and real-time signals are pending for a
+	// process, POSIX leaves it unspecified which is delivered first. Linux,
+	// like many other implementations, gives priority to standard signals in
+	// this case." - signal(7)
+	lowestPendingUnblockedBit := bits.TrailingZeros64(uint64(p.pendingSet &^ mask))
+	if lowestPendingUnblockedBit >= linux.SignalMaximum {
+		return nil
+	}
+	return p.dequeueSpecific(linux.Signal(lowestPendingUnblockedBit + 1))
+}
+
+func (p *pendingSignals) dequeueSpecific(sig linux.Signal) *arch.SignalInfo {
+	q := &p.signals[sig.Index()]
+	ps := q.pendingSignalList.Front()
+	if ps == nil {
+		return nil
+	}
+	q.pendingSignalList.Remove(ps)
+	q.length--
+	if q.length == 0 {
+		p.pendingSet &^= linux.SignalSetOf(sig)
+	}
+	return ps.SignalInfo
+}
+
+// discardSpecific causes all pending signals with number sig to be discarded.
+func (p *pendingSignals) discardSpecific(sig linux.Signal) {
+	q := &p.signals[sig.Index()]
+	q.pendingSignalList.Reset()
+	q.length = 0
+	p.pendingSet &^= linux.SignalSetOf(sig)
+}