22 files changed, 581 insertions, 39 deletions

diff --git a/pkg/sentry/kernel/BUILD b/pkg/sentry/kernel/BUILD
index 41bee9a22..aba2414d4 100644
--- a/pkg/sentry/kernel/BUILD
+++ b/pkg/sentry/kernel/BUILD
@@ -1,9 +1,11 @@
 load("@io_bazel_rules_go//proto:def.bzl", "go_proto_library")
+load("@io_bazel_rules_go//go:def.bzl", "go_test")
+load("@rules_cc//cc:defs.bzl", "cc_proto_library")
 
 package(licenses = ["notice"])
 
 load("//tools/go_generics:defs.bzl", "go_template_instance")
-load("//tools/go_stateify:defs.bzl", "go_library", "go_test")
+load("//tools/go_stateify:defs.bzl", "go_library")
 
 go_template_instance(
     name = "pending_signals_list",
@@ -83,6 +85,12 @@ proto_library(
     deps = ["//pkg/sentry/arch:registers_proto"],
 )
 
+cc_proto_library(
+    name = "uncaught_signal_cc_proto",
+    visibility = ["//visibility:public"],
+    deps = [":uncaught_signal_proto"],
+)
+
 go_proto_library(
     name = "uncaught_signal_go_proto",
     importpath = "gvisor.dev/gvisor/pkg/sentry/kernel/uncaught_signal_go_proto",
diff --git a/pkg/sentry/kernel/epoll/BUILD b/pkg/sentry/kernel/epoll/BUILD
index f46c43128..65427b112 100644
--- a/pkg/sentry/kernel/epoll/BUILD
+++ b/pkg/sentry/kernel/epoll/BUILD
@@ -1,7 +1,9 @@
+load("@io_bazel_rules_go//go:def.bzl", "go_test")
+
 package(licenses = ["notice"])
 
 load("//tools/go_generics:defs.bzl", "go_template_instance")
-load("//tools/go_stateify:defs.bzl", "go_library", "go_test")
+load("//tools/go_stateify:defs.bzl", "go_library")
 
 go_template_instance(
     name = "epoll_list",
diff --git a/pkg/sentry/kernel/eventfd/BUILD b/pkg/sentry/kernel/eventfd/BUILD
index 1c5f979d4..983ca67ed 100644
--- a/pkg/sentry/kernel/eventfd/BUILD
+++ b/pkg/sentry/kernel/eventfd/BUILD
@@ -1,6 +1,8 @@
+load("@io_bazel_rules_go//go:def.bzl", "go_test")
+
 package(licenses = ["notice"])
 
-load("//tools/go_stateify:defs.bzl", "go_library", "go_test")
+load("//tools/go_stateify:defs.bzl", "go_library")
 
 go_library(
     name = "eventfd",
diff --git a/pkg/sentry/kernel/futex/BUILD b/pkg/sentry/kernel/futex/BUILD
index 6a31dc044..41f44999c 100644
--- a/pkg/sentry/kernel/futex/BUILD
+++ b/pkg/sentry/kernel/futex/BUILD
@@ -1,7 +1,9 @@
+load("@io_bazel_rules_go//go:def.bzl", "go_test")
+
 package(licenses = ["notice"])
 
 load("//tools/go_generics:defs.bzl", "go_template_instance")
-load("//tools/go_stateify:defs.bzl", "go_library", "go_test")
+load("//tools/go_stateify:defs.bzl", "go_library")
 
 go_template_instance(
     name = "atomicptr_bucket",
diff --git a/pkg/sentry/kernel/kernel.go b/pkg/sentry/kernel/kernel.go
index 8c1f79ab5..3cda03891 100644
--- a/pkg/sentry/kernel/kernel.go
+++ b/pkg/sentry/kernel/kernel.go
@@ -24,6 +24,7 @@
 //       TaskSet.mu
 //         SignalHandlers.mu
 //           Task.mu
+//       runningTasksMu
 //
 // Locking SignalHandlers.mu in multiple SignalHandlers requires locking
 // TaskSet.mu exclusively first. Locking Task.mu in multiple Tasks at the same
@@ -135,6 +136,22 @@ type Kernel struct {
 	// syslog is the kernel log.
 	syslog syslog
 
+	// runningTasksMu synchronizes disable/enable of cpuClockTicker when
+	// the kernel is idle (runningTasks == 0).
+	//
+	// runningTasksMu is used to exclude critical sections when the timer
+	// disables itself and when the first active task enables the timer,
+	// ensuring that tasks always see a valid cpuClock value.
+	runningTasksMu sync.Mutex `state:"nosave"`
+
+	// runningTasks is the total count of tasks currently in
+	// TaskGoroutineRunningSys or TaskGoroutineRunningApp. i.e., they are
+	// not blocked or stopped.
+	//
+	// runningTasks must be accessed atomically. Increments from 0 to 1 are
+	// further protected by runningTasksMu (see incRunningTasks).
+	runningTasks int64
+
 	// cpuClock is incremented every linux.ClockTick. cpuClock is used to
 	// measure task CPU usage, since sampling monotonicClock twice on every
 	// syscall turns out to be unreasonably expensive. This is similar to how
@@ -150,6 +167,22 @@ type Kernel struct {
 	// cpuClockTicker increments cpuClock.
 	cpuClockTicker *ktime.Timer `state:"nosave"`
 
+	// cpuClockTickerDisabled indicates that cpuClockTicker has been
+	// disabled because no tasks are running.
+	//
+	// cpuClockTickerDisabled is protected by runningTasksMu.
+	cpuClockTickerDisabled bool
+
+	// cpuClockTickerSetting is the ktime.Setting of cpuClockTicker at the
+	// point it was disabled. It is cached here to avoid a lock ordering
+	// violation with cpuClockTicker.mu when runningTaskMu is held.
+	//
+	// cpuClockTickerSetting is only valid when cpuClockTickerDisabled is
+	// true.
+	//
+	// cpuClockTickerSetting is protected by runningTasksMu.
+	cpuClockTickerSetting ktime.Setting
+
 	// fdMapUids is an ever-increasing counter for generating FDTable uids.
 	//
 	// fdMapUids is mutable, and is accessed using atomic memory operations.
@@ -912,6 +945,102 @@ func (k *Kernel) resumeTimeLocked() {
 	}
 }
 
+func (k *Kernel) incRunningTasks() {
+	for {
+		tasks := atomic.LoadInt64(&k.runningTasks)
+		if tasks != 0 {
+			// Standard case. Simply increment.
+			if !atomic.CompareAndSwapInt64(&k.runningTasks, tasks, tasks+1) {
+				continue
+			}
+			return
+		}
+
+		// Transition from 0 -> 1. Synchronize with other transitions and timer.
+		k.runningTasksMu.Lock()
+		tasks = atomic.LoadInt64(&k.runningTasks)
+		if tasks != 0 {
+			// We're no longer the first task, no need to
+			// re-enable.
+			atomic.AddInt64(&k.runningTasks, 1)
+			k.runningTasksMu.Unlock()
+			return
+		}
+
+		if !k.cpuClockTickerDisabled {
+			// Timer was never disabled.
+			atomic.StoreInt64(&k.runningTasks, 1)
+			k.runningTasksMu.Unlock()
+			return
+		}
+
+		// We need to update cpuClock for all of the ticks missed while we
+		// slept, and then re-enable the timer.
+		//
+		// The Notify in Swap isn't sufficient. kernelCPUClockTicker.Notify
+		// always increments cpuClock by 1 regardless of the number of
+		// expirations as a heuristic to avoid over-accounting in cases of CPU
+		// throttling.
+		//
+		// We want to cover the normal case, when all time should be accounted,
+		// so we increment for all expirations. Throttling is less concerning
+		// here because the ticker is only disabled from Notify. This means
+		// that Notify must schedule and compensate for the throttled period
+		// before the timer is disabled. Throttling while the timer is disabled
+		// doesn't matter, as nothing is running or reading cpuClock anyways.
+		//
+		// S/R also adds complication, as there are two cases. Recall that
+		// monotonicClock will jump forward on restore.
+		//
+		// 1. If the ticker is enabled during save, then on Restore Notify is
+		// called with many expirations, covering the time jump, but cpuClock
+		// is only incremented by 1.
+		//
+		// 2. If the ticker is disabled during save, then after Restore the
+		// first wakeup will call this function and cpuClock will be
+		// incremented by the number of expirations across the S/R.
+		//
+		// These cause very different value of cpuClock. But again, since
+		// nothing was running while the ticker was disabled, those differences
+		// don't matter.
+		setting, exp := k.cpuClockTickerSetting.At(k.monotonicClock.Now())
+		if exp > 0 {
+			atomic.AddUint64(&k.cpuClock, exp)
+		}
+
+		// Now that cpuClock is updated it is safe to allow other tasks to
+		// transition to running.
+		atomic.StoreInt64(&k.runningTasks, 1)
+
+		// N.B. we must unlock before calling Swap to maintain lock ordering.
+		//
+		// cpuClockTickerDisabled need not wait until after Swap to become
+		// true. It is sufficient that the timer *will* be enabled.
+		k.cpuClockTickerDisabled = false
+		k.runningTasksMu.Unlock()
+
+		// This won't call Notify (unless it's been ClockTick since setting.At
+		// above). This means we skip the thread group work in Notify. However,
+		// since nothing was running while we were disabled, none of the timers
+		// could have expired.
+		k.cpuClockTicker.Swap(setting)
+
+		return
+	}
+}
+
+func (k *Kernel) decRunningTasks() {
+	tasks := atomic.AddInt64(&k.runningTasks, -1)
+	if tasks < 0 {
+		panic(fmt.Sprintf("Invalid running count %d", tasks))
+	}
+
+	// Nothing to do. The next CPU clock tick will disable the timer if
+	// there is still nothing running. This provides approximately one tick
+	// of slack in which we can switch back and forth between idle and
+	// active without an expensive transition.
+}
+
 // WaitExited blocks until all tasks in k have exited.
 func (k *Kernel) WaitExited() {
 	k.tasks.liveGoroutines.Wait()
diff --git a/pkg/sentry/kernel/memevent/BUILD b/pkg/sentry/kernel/memevent/BUILD
index ebcfaa619..d7a7d1169 100644
--- a/pkg/sentry/kernel/memevent/BUILD
+++ b/pkg/sentry/kernel/memevent/BUILD
@@ -1,5 +1,6 @@
 load("//tools/go_stateify:defs.bzl", "go_library")
 load("@io_bazel_rules_go//proto:def.bzl", "go_proto_library")
+load("@rules_cc//cc:defs.bzl", "cc_proto_library")
 
 package(licenses = ["notice"])
 
@@ -24,6 +25,12 @@ proto_library(
     visibility = ["//visibility:public"],
 )
 
+cc_proto_library(
+    name = "memory_events_cc_proto",
+    visibility = ["//visibility:public"],
+    deps = [":memory_events_proto"],
+)
+
 go_proto_library(
     name = "memory_events_go_proto",
     importpath = "gvisor.dev/gvisor/pkg/sentry/kernel/memevent/memory_events_go_proto",
diff --git a/pkg/sentry/kernel/pipe/BUILD b/pkg/sentry/kernel/pipe/BUILD
index 4d15cca85..2ce8952e2 100644
--- a/pkg/sentry/kernel/pipe/BUILD
+++ b/pkg/sentry/kernel/pipe/BUILD
@@ -1,7 +1,9 @@
+load("@io_bazel_rules_go//go:def.bzl", "go_test")
+
 package(licenses = ["notice"])
 
 load("//tools/go_generics:defs.bzl", "go_template_instance")
-load("//tools/go_stateify:defs.bzl", "go_library", "go_test")
+load("//tools/go_stateify:defs.bzl", "go_library")
 
 go_template_instance(
     name = "buffer_list",
diff --git a/pkg/sentry/kernel/pipe/buffer.go b/pkg/sentry/kernel/pipe/buffer.go
index 69ef2a720..95bee2d37 100644
--- a/pkg/sentry/kernel/pipe/buffer.go
+++ b/pkg/sentry/kernel/pipe/buffer.go
@@ -15,6 +15,7 @@
 package pipe
 
 import (
+	"io"
 	"sync"
 
 	"gvisor.dev/gvisor/pkg/sentry/safemem"
@@ -67,6 +68,17 @@ func (b *buffer) WriteFromBlocks(srcs safemem.BlockSeq) (uint64, error) {
 	return n, err
 }
 
+// WriteFromReader writes to the buffer from an io.Reader.
+func (b *buffer) WriteFromReader(r io.Reader, count int64) (int64, error) {
+	dst := b.data[b.write:]
+	if count < int64(len(dst)) {
+		dst = b.data[b.write:][:count]
+	}
+	n, err := r.Read(dst)
+	b.write += n
+	return int64(n), err
+}
+
 // ReadToBlocks implements safemem.Reader.ReadToBlocks.
 func (b *buffer) ReadToBlocks(dsts safemem.BlockSeq) (uint64, error) {
 	src := safemem.BlockSeqOf(safemem.BlockFromSafeSlice(b.data[b.read:b.write]))
@@ -75,6 +87,19 @@ func (b *buffer) ReadToBlocks(dsts safemem.BlockSeq) (uint64, error) {
 	return n, err
 }
 
+// ReadToWriter reads from the buffer into an io.Writer.
+func (b *buffer) ReadToWriter(w io.Writer, count int64, dup bool) (int64, error) {
+	src := b.data[b.read:b.write]
+	if count < int64(len(src)) {
+		src = b.data[b.read:][:count]
+	}
+	n, err := w.Write(src)
+	if !dup {
+		b.read += n
+	}
+	return int64(n), err
+}
+
 // bufferPool is a pool for buffers.
 var bufferPool = sync.Pool{
 	New: func() interface{} {
diff --git a/pkg/sentry/kernel/pipe/pipe.go b/pkg/sentry/kernel/pipe/pipe.go
index 247e2928e..93b50669f 100644
--- a/pkg/sentry/kernel/pipe/pipe.go
+++ b/pkg/sentry/kernel/pipe/pipe.go
@@ -23,7 +23,6 @@ import (
 
 	"gvisor.dev/gvisor/pkg/sentry/context"
 	"gvisor.dev/gvisor/pkg/sentry/fs"
-	"gvisor.dev/gvisor/pkg/sentry/usermem"
 	"gvisor.dev/gvisor/pkg/syserror"
 	"gvisor.dev/gvisor/pkg/waiter"
 )
@@ -173,13 +172,24 @@ func (p *Pipe) Open(ctx context.Context, d *fs.Dirent, flags fs.FileFlags) *fs.F
 	}
 }
 
+type readOps struct {
+	// left returns the bytes remaining.
+	left func() int64
+
+	// limit limits subsequence reads.
+	limit func(int64)
+
+	// read performs the actual read operation.
+	read func(*buffer) (int64, error)
+}
+
 // read reads data from the pipe into dst and returns the number of bytes
 // read, or returns ErrWouldBlock if the pipe is empty.
 //
 // Precondition: this pipe must have readers.
-func (p *Pipe) read(ctx context.Context, dst usermem.IOSequence) (int64, error) {
+func (p *Pipe) read(ctx context.Context, ops readOps) (int64, error) {
 	// Don't block for a zero-length read even if the pipe is empty.
-	if dst.NumBytes() == 0 {
+	if ops.left() == 0 {
 		return 0, nil
 	}
 
@@ -196,12 +206,12 @@ func (p *Pipe) read(ctx context.Context, dst usermem.IOSequence) (int64, error)
 	}
 
 	// Limit how much we consume.
-	if dst.NumBytes() > p.size {
-		dst = dst.TakeFirst64(p.size)
+	if ops.left() > p.size {
+		ops.limit(p.size)
 	}
 
 	done := int64(0)
-	for dst.NumBytes() > 0 {
+	for ops.left() > 0 {
 		// Pop the first buffer.
 		first := p.data.Front()
 		if first == nil {
@@ -209,10 +219,9 @@ func (p *Pipe) read(ctx context.Context, dst usermem.IOSequence) (int64, error)
 		}
 
 		// Copy user data.
-		n, err := dst.CopyOutFrom(ctx, first)
+		n, err := ops.read(first)
 		done += int64(n)
 		p.size -= n
-		dst = dst.DropFirst64(n)
 
 		// Empty buffer?
 		if first.Empty() {
@@ -230,12 +239,57 @@ func (p *Pipe) read(ctx context.Context, dst usermem.IOSequence) (int64, error)
 	return done, nil
 }
 
+// dup duplicates all data from this pipe into the given writer.
+//
+// There is no blocking behavior implemented here. The writer may propagate
+// some blocking error. All the writes must be complete writes.
+func (p *Pipe) dup(ctx context.Context, ops readOps) (int64, error) {
+	p.mu.Lock()
+	defer p.mu.Unlock()
+
+	// Is the pipe empty?
+	if p.size == 0 {
+		if !p.HasWriters() {
+			// See above.
+			return 0, nil
+		}
+		return 0, syserror.ErrWouldBlock
+	}
+
+	// Limit how much we consume.
+	if ops.left() > p.size {
+		ops.limit(p.size)
+	}
+
+	done := int64(0)
+	for buf := p.data.Front(); buf != nil; buf = buf.Next() {
+		n, err := ops.read(buf)
+		done += n
+		if err != nil {
+			return done, err
+		}
+	}
+
+	return done, nil
+}
+
+type writeOps struct {
+	// left returns the bytes remaining.
+	left func() int64
+
+	// limit should limit subsequent writes.
+	limit func(int64)
+
+	// write should write to the provided buffer.
+	write func(*buffer) (int64, error)
+}
+
 // write writes data from sv into the pipe and returns the number of bytes
 // written. If no bytes are written because the pipe is full (or has less than
 // atomicIOBytes free capacity), write returns ErrWouldBlock.
 //
 // Precondition: this pipe must have writers.
-func (p *Pipe) write(ctx context.Context, src usermem.IOSequence) (int64, error) {
+func (p *Pipe) write(ctx context.Context, ops writeOps) (int64, error) {
 	p.mu.Lock()
 	defer p.mu.Unlock()
 
@@ -246,17 +300,16 @@ func (p *Pipe) write(ctx context.Context, src usermem.IOSequence) (int64, error)
 
 	// POSIX requires that a write smaller than atomicIOBytes (PIPE_BUF) be
 	// atomic, but requires no atomicity for writes larger than this.
-	wanted := src.NumBytes()
+	wanted := ops.left()
 	if avail := p.max - p.size; wanted > avail {
 		if wanted <= p.atomicIOBytes {
 			return 0, syserror.ErrWouldBlock
 		}
-		// Limit to the available capacity.
-		src = src.TakeFirst64(avail)
+		ops.limit(avail)
 	}
 
 	done := int64(0)
-	for src.NumBytes() > 0 {
+	for ops.left() > 0 {
 		// Need a new buffer?
 		last := p.data.Back()
 		if last == nil || last.Full() {
@@ -266,10 +319,9 @@ func (p *Pipe) write(ctx context.Context, src usermem.IOSequence) (int64, error)
 		}
 
 		// Copy user data.
-		n, err := src.CopyInTo(ctx, last)
+		n, err := ops.write(last)
 		done += int64(n)
 		p.size += n
-		src = src.DropFirst64(n)
 
 		// Handle errors.
 		if err != nil {
diff --git a/pkg/sentry/kernel/pipe/reader_writer.go b/pkg/sentry/kernel/pipe/reader_writer.go
index f69dbf27b..7c307f013 100644
--- a/pkg/sentry/kernel/pipe/reader_writer.go
+++ b/pkg/sentry/kernel/pipe/reader_writer.go
@@ -15,6 +15,7 @@
 package pipe
 
 import (
+	"io"
 	"math"
 	"syscall"
 
@@ -55,7 +56,45 @@ func (rw *ReaderWriter) Release() {
 
 // Read implements fs.FileOperations.Read.
 func (rw *ReaderWriter) Read(ctx context.Context, _ *fs.File, dst usermem.IOSequence, _ int64) (int64, error) {
-	n, err := rw.Pipe.read(ctx, dst)
+	n, err := rw.Pipe.read(ctx, readOps{
+		left: func() int64 {
+			return dst.NumBytes()
+		},
+		limit: func(l int64) {
+			dst = dst.TakeFirst64(l)
+		},
+		read: func(buf *buffer) (int64, error) {
+			n, err := dst.CopyOutFrom(ctx, buf)
+			dst = dst.DropFirst64(n)
+			return n, err
+		},
+	})
+	if n > 0 {
+		rw.Pipe.Notify(waiter.EventOut)
+	}
+	return n, err
+}
+
+// WriteTo implements fs.FileOperations.WriteTo.
+func (rw *ReaderWriter) WriteTo(ctx context.Context, _ *fs.File, w io.Writer, count int64, dup bool) (int64, error) {
+	ops := readOps{
+		left: func() int64 {
+			return count
+		},
+		limit: func(l int64) {
+			count = l
+		},
+		read: func(buf *buffer) (int64, error) {
+			n, err := buf.ReadToWriter(w, count, dup)
+			count -= n
+			return n, err
+		},
+	}
+	if dup {
+		// There is no notification for dup operations.
+		return rw.Pipe.dup(ctx, ops)
+	}
+	n, err := rw.Pipe.read(ctx, ops)
 	if n > 0 {
 		rw.Pipe.Notify(waiter.EventOut)
 	}
@@ -64,7 +103,40 @@ func (rw *ReaderWriter) Read(ctx context.Context, _ *fs.File, dst usermem.IOSequ
 
 // Write implements fs.FileOperations.Write.
 func (rw *ReaderWriter) Write(ctx context.Context, _ *fs.File, src usermem.IOSequence, _ int64) (int64, error) {
-	n, err := rw.Pipe.write(ctx, src)
+	n, err := rw.Pipe.write(ctx, writeOps{
+		left: func() int64 {
+			return src.NumBytes()
+		},
+		limit: func(l int64) {
+			src = src.TakeFirst64(l)
+		},
+		write: func(buf *buffer) (int64, error) {
+			n, err := src.CopyInTo(ctx, buf)
+			src = src.DropFirst64(n)
+			return n, err
+		},
+	})
+	if n > 0 {
+		rw.Pipe.Notify(waiter.EventIn)
+	}
+	return n, err
+}
+
+// ReadFrom implements fs.FileOperations.WriteTo.
+func (rw *ReaderWriter) ReadFrom(ctx context.Context, _ *fs.File, r io.Reader, count int64) (int64, error) {
+	n, err := rw.Pipe.write(ctx, writeOps{
+		left: func() int64 {
+			return count
+		},
+		limit: func(l int64) {
+			count = l
+		},
+		write: func(buf *buffer) (int64, error) {
+			n, err := buf.WriteFromReader(r, count)
+			count -= n
+			return n, err
+		},
+	})
 	if n > 0 {
 		rw.Pipe.Notify(waiter.EventIn)
 	}
diff --git a/pkg/sentry/kernel/posixtimer.go b/pkg/sentry/kernel/posixtimer.go
index c5d095af7..2e861a5a8 100644
--- a/pkg/sentry/kernel/posixtimer.go
+++ b/pkg/sentry/kernel/posixtimer.go
@@ -117,9 +117,9 @@ func (it *IntervalTimer) signalRejectedLocked() {
 }
 
 // Notify implements ktime.TimerListener.Notify.
-func (it *IntervalTimer) Notify(exp uint64) {
+func (it *IntervalTimer) Notify(exp uint64, setting ktime.Setting) (ktime.Setting, bool) {
 	if it.target == nil {
-		return
+		return ktime.Setting{}, false
 	}
 
 	it.target.tg.pidns.owner.mu.RLock()
@@ -129,7 +129,7 @@ func (it *IntervalTimer) Notify(exp uint64) {
 
 	if it.sigpending {
 		it.overrunCur += exp
-		return
+		return ktime.Setting{}, false
 	}
 
 	// sigpending must be set before sendSignalTimerLocked() so that it can be
@@ -148,6 +148,8 @@ func (it *IntervalTimer) Notify(exp uint64) {
 	if err := it.target.sendSignalTimerLocked(si, it.group, it); err != nil {
 		it.signalRejectedLocked()
 	}
+
+	return ktime.Setting{}, false
 }
 
 // Destroy implements ktime.TimerListener.Destroy. Users of Timer should call
diff --git a/pkg/sentry/kernel/sched/BUILD b/pkg/sentry/kernel/sched/BUILD
index 1725b8562..98ea7a0d8 100644
--- a/pkg/sentry/kernel/sched/BUILD
+++ b/pkg/sentry/kernel/sched/BUILD
@@ -1,4 +1,5 @@
-load("//tools/go_stateify:defs.bzl", "go_library", "go_test")
+load("//tools/go_stateify:defs.bzl", "go_library")
+load("@io_bazel_rules_go//go:def.bzl", "go_test")
 
 package(licenses = ["notice"])
 
diff --git a/pkg/sentry/kernel/semaphore/BUILD b/pkg/sentry/kernel/semaphore/BUILD
index 36edf10f3..80e5e5da3 100644
--- a/pkg/sentry/kernel/semaphore/BUILD
+++ b/pkg/sentry/kernel/semaphore/BUILD
@@ -1,7 +1,9 @@
+load("@io_bazel_rules_go//go:def.bzl", "go_test")
+
 package(licenses = ["notice"])
 
 load("//tools/go_generics:defs.bzl", "go_template_instance")
-load("//tools/go_stateify:defs.bzl", "go_library", "go_test")
+load("//tools/go_stateify:defs.bzl", "go_library")
 
 go_template_instance(
     name = "waiter_list",
diff --git a/pkg/sentry/kernel/sessions.go b/pkg/sentry/kernel/sessions.go
index e5f297478..047b5214d 100644
--- a/pkg/sentry/kernel/sessions.go
+++ b/pkg/sentry/kernel/sessions.go
@@ -328,8 +328,14 @@ func (tg *ThreadGroup) createSession() error {
 			childTG.processGroup.incRefWithParent(pg)
 			childTG.processGroup.decRefWithParent(oldParentPG)
 		})
-		tg.processGroup.decRefWithParent(oldParentPG)
+		// If tg.processGroup is an orphan, decRefWithParent will lock
+		// the signal mutex of each thread group in tg.processGroup.
+		// However, tg's signal mutex may already be locked at this
+		// point. We change tg's process group before calling
+		// decRefWithParent to avoid locking tg's signal mutex twice.
+		oldPG := tg.processGroup
 		tg.processGroup = pg
+		oldPG.decRefWithParent(oldParentPG)
 	} else {
 		// The current process group may be nil only in the case of an
 		// unparented thread group (i.e. the init process). This would
diff --git a/pkg/sentry/kernel/signalfd/BUILD b/pkg/sentry/kernel/signalfd/BUILD
new file mode 100644
index 000000000..50b69d154
--- /dev/null
+++ b/pkg/sentry/kernel/signalfd/BUILD
@@ -0,0 +1,22 @@
+package(licenses = ["notice"])
+
+load("//tools/go_stateify:defs.bzl", "go_library")
+
+go_library(
+    name = "signalfd",
+    srcs = ["signalfd.go"],
+    importpath = "gvisor.dev/gvisor/pkg/sentry/kernel/signalfd",
+    visibility = ["//pkg/sentry:internal"],
+    deps = [
+        "//pkg/abi/linux",
+        "//pkg/binary",
+        "//pkg/sentry/context",
+        "//pkg/sentry/fs",
+        "//pkg/sentry/fs/anon",
+        "//pkg/sentry/fs/fsutil",
+        "//pkg/sentry/kernel",
+        "//pkg/sentry/usermem",
+        "//pkg/syserror",
+        "//pkg/waiter",
+    ],
+)
diff --git a/pkg/sentry/kernel/signalfd/signalfd.go b/pkg/sentry/kernel/signalfd/signalfd.go
new file mode 100644
index 000000000..06fd5ec88
--- /dev/null
+++ b/pkg/sentry/kernel/signalfd/signalfd.go
@@ -0,0 +1,137 @@
+// 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 signalfd provides an implementation of signal file descriptors.
+package signalfd
+
+import (
+	"sync"
+
+	"gvisor.dev/gvisor/pkg/abi/linux"
+	"gvisor.dev/gvisor/pkg/binary"
+	"gvisor.dev/gvisor/pkg/sentry/context"
+	"gvisor.dev/gvisor/pkg/sentry/fs"
+	"gvisor.dev/gvisor/pkg/sentry/fs/anon"
+	"gvisor.dev/gvisor/pkg/sentry/fs/fsutil"
+	"gvisor.dev/gvisor/pkg/sentry/kernel"
+	"gvisor.dev/gvisor/pkg/sentry/usermem"
+	"gvisor.dev/gvisor/pkg/syserror"
+	"gvisor.dev/gvisor/pkg/waiter"
+)
+
+// SignalOperations represent a file with signalfd semantics.
+//
+// +stateify savable
+type SignalOperations struct {
+	fsutil.FileNoopRelease          `state:"nosave"`
+	fsutil.FilePipeSeek             `state:"nosave"`
+	fsutil.FileNotDirReaddir        `state:"nosave"`
+	fsutil.FileNoIoctl              `state:"nosave"`
+	fsutil.FileNoFsync              `state:"nosave"`
+	fsutil.FileNoMMap               `state:"nosave"`
+	fsutil.FileNoSplice             `state:"nosave"`
+	fsutil.FileNoWrite              `state:"nosave"`
+	fsutil.FileNoopFlush            `state:"nosave"`
+	fsutil.FileUseInodeUnstableAttr `state:"nosave"`
+
+	// target is the original task target.
+	//
+	// The semantics here are a bit broken. Linux will always use current
+	// for all reads, regardless of where the signalfd originated. We can't
+	// do exactly that because we need to plumb the context through
+	// EventRegister in order to support proper blocking behavior. This
+	// will undoubtedly become very complicated quickly.
+	target *kernel.Task
+
+	// mu protects below.
+	mu sync.Mutex `state:"nosave"`
+
+	// mask is the signal mask. Protected by mu.
+	mask linux.SignalSet
+}
+
+// New creates a new signalfd object with the supplied mask.
+func New(ctx context.Context, mask linux.SignalSet) (*fs.File, error) {
+	t := kernel.TaskFromContext(ctx)
+	if t == nil {
+		// No task context? Not valid.
+		return nil, syserror.EINVAL
+	}
+	// name matches fs/signalfd.c:signalfd4.
+	dirent := fs.NewDirent(ctx, anon.NewInode(ctx), "anon_inode:[signalfd]")
+	return fs.NewFile(ctx, dirent, fs.FileFlags{Read: true, Write: true}, &SignalOperations{
+		target: t,
+		mask:   mask,
+	}), nil
+}
+
+// Release implements fs.FileOperations.Release.
+func (s *SignalOperations) Release() {}
+
+// Mask returns the signal mask.
+func (s *SignalOperations) Mask() linux.SignalSet {
+	s.mu.Lock()
+	mask := s.mask
+	s.mu.Unlock()
+	return mask
+}
+
+// SetMask sets the signal mask.
+func (s *SignalOperations) SetMask(mask linux.SignalSet) {
+	s.mu.Lock()
+	s.mask = mask
+	s.mu.Unlock()
+}
+
+// Read implements fs.FileOperations.Read.
+func (s *SignalOperations) Read(ctx context.Context, _ *fs.File, dst usermem.IOSequence, _ int64) (int64, error) {
+	// Attempt to dequeue relevant signals.
+	info, err := s.target.Sigtimedwait(s.Mask(), 0)
+	if err != nil {
+		// There must be no signal available.
+		return 0, syserror.ErrWouldBlock
+	}
+
+	// Copy out the signal info using the specified format.
+	var buf [128]byte
+	binary.Marshal(buf[:0], usermem.ByteOrder, &linux.SignalfdSiginfo{
+		Signo:   uint32(info.Signo),
+		Errno:   info.Errno,
+		Code:    info.Code,
+		PID:     uint32(info.Pid()),
+		UID:     uint32(info.Uid()),
+		Status:  info.Status(),
+		Overrun: uint32(info.Overrun()),
+		Addr:    info.Addr(),
+	})
+	n, err := dst.CopyOut(ctx, buf[:])
+	return int64(n), err
+}
+
+// Readiness implements waiter.Waitable.Readiness.
+func (s *SignalOperations) Readiness(mask waiter.EventMask) waiter.EventMask {
+	return mask & waiter.EventIn
+}
+
+// EventRegister implements waiter.Waitable.EventRegister.
+func (s *SignalOperations) EventRegister(entry *waiter.Entry, _ waiter.EventMask) {
+	// Register for the signal set; ignore the passed events.
+	s.target.SignalRegister(entry, waiter.EventMask(s.Mask()))
+}
+
+// EventUnregister implements waiter.Waitable.EventUnregister.
+func (s *SignalOperations) EventUnregister(entry *waiter.Entry) {
+	// Unregister the original entry.
+	s.target.SignalUnregister(entry)
+}
diff --git a/pkg/sentry/kernel/task.go b/pkg/sentry/kernel/task.go
index e91f82bb3..c82ef5486 100644
--- a/pkg/sentry/kernel/task.go
+++ b/pkg/sentry/kernel/task.go
@@ -35,6 +35,7 @@ import (
 	"gvisor.dev/gvisor/pkg/sentry/uniqueid"
 	"gvisor.dev/gvisor/pkg/sentry/usage"
 	"gvisor.dev/gvisor/pkg/sentry/usermem"
+	"gvisor.dev/gvisor/pkg/waiter"
 	"gvisor.dev/gvisor/third_party/gvsync"
 )
 
@@ -133,6 +134,13 @@ type Task struct {
 	// signalStack is exclusive to the task goroutine.
 	signalStack arch.SignalStack
 
+	// signalQueue is a set of registered waiters for signal-related events.
+	//
+	// signalQueue is protected by the signalMutex. Note that the task does
+	// not implement all queue methods, specifically the readiness checks.
+	// The task only broadcast a notification on signal delivery.
+	signalQueue waiter.Queue `state:"zerovalue"`
+
 	// If groupStopPending is true, the task should participate in a group
 	// stop in the interrupt path.
 	//
diff --git a/pkg/sentry/kernel/task_identity.go b/pkg/sentry/kernel/task_identity.go
index 78ff14b20..ce3e6ef28 100644
--- a/pkg/sentry/kernel/task_identity.go
+++ b/pkg/sentry/kernel/task_identity.go
@@ -465,8 +465,8 @@ func (t *Task) SetKeepCaps(k bool) {
 // disables the features we don't support anyway, is always set. This
 // drastically simplifies this function.
 //
-// - We don't implement AT_SECURE, because no_new_privs always being set means
-// that the conditions that require AT_SECURE never arise. (Compare Linux's
+// - We don't set AT_SECURE = 1, because no_new_privs always being set means
+// that the conditions that require AT_SECURE = 1 never arise. (Compare Linux's
 // security/commoncap.c:cap_bprm_set_creds() and cap_bprm_secureexec().)
 //
 // - We don't check for CAP_SYS_ADMIN in prctl(PR_SET_SECCOMP), since
diff --git a/pkg/sentry/kernel/task_sched.go b/pkg/sentry/kernel/task_sched.go
index e76c069b0..8b148db35 100644
--- a/pkg/sentry/kernel/task_sched.go
+++ b/pkg/sentry/kernel/task_sched.go
@@ -126,12 +126,22 @@ func (t *Task) accountTaskGoroutineEnter(state TaskGoroutineState) {
 	t.gosched.Timestamp = now
 	t.gosched.State = state
 	t.goschedSeq.EndWrite()
+
+	if state != TaskGoroutineRunningApp {
+		// Task is blocking/stopping.
+		t.k.decRunningTasks()
+	}
 }
 
 // Preconditions: The caller must be running on the task goroutine, and leaving
 // a state indicated by a previous call to
 // t.accountTaskGoroutineEnter(state).
 func (t *Task) accountTaskGoroutineLeave(state TaskGoroutineState) {
+	if state != TaskGoroutineRunningApp {
+		// Task is unblocking/continuing.
+		t.k.incRunningTasks()
+	}
+
 	now := t.k.CPUClockNow()
 	if t.gosched.State != state {
 		panic(fmt.Sprintf("Task goroutine switching from state %v (expected %v) to %v", t.gosched.State, state, TaskGoroutineRunningSys))
@@ -330,7 +340,7 @@ func newKernelCPUClockTicker(k *Kernel) *kernelCPUClockTicker {
 }
 
 // Notify implements ktime.TimerListener.Notify.
-func (ticker *kernelCPUClockTicker) Notify(exp uint64) {
+func (ticker *kernelCPUClockTicker) Notify(exp uint64, setting ktime.Setting) (ktime.Setting, bool) {
 	// Only increment cpuClock by 1 regardless of the number of expirations.
 	// This approximately compensates for cases where thread throttling or bad
 	// Go runtime scheduling prevents the kernelCPUClockTicker goroutine, and
@@ -426,6 +436,27 @@ func (ticker *kernelCPUClockTicker) Notify(exp uint64) {
 		tgs[i] = nil
 	}
 	ticker.tgs = tgs[:0]
+
+	// If nothing is running, we can disable the timer.
+	tasks := atomic.LoadInt64(&ticker.k.runningTasks)
+	if tasks == 0 {
+		ticker.k.runningTasksMu.Lock()
+		defer ticker.k.runningTasksMu.Unlock()
+		tasks := atomic.LoadInt64(&ticker.k.runningTasks)
+		if tasks != 0 {
+			// Raced with a 0 -> 1 transition.
+			return setting, false
+		}
+
+		// Stop the timer. We must cache the current setting so the
+		// kernel can access it without violating the lock order.
+		ticker.k.cpuClockTickerSetting = setting
+		ticker.k.cpuClockTickerDisabled = true
+		setting.Enabled = false
+		return setting, true
+	}
+
+	return setting, false
 }
 
 // Destroy implements ktime.TimerListener.Destroy.
diff --git a/pkg/sentry/kernel/task_signals.go b/pkg/sentry/kernel/task_signals.go
index 266959a07..39cd1340d 100644
--- a/pkg/sentry/kernel/task_signals.go
+++ b/pkg/sentry/kernel/task_signals.go
@@ -28,6 +28,7 @@ import (
 	ucspb "gvisor.dev/gvisor/pkg/sentry/kernel/uncaught_signal_go_proto"
 	"gvisor.dev/gvisor/pkg/sentry/usermem"
 	"gvisor.dev/gvisor/pkg/syserror"
+	"gvisor.dev/gvisor/pkg/waiter"
 )
 
 // SignalAction is an internal signal action.
@@ -497,6 +498,9 @@ func (tg *ThreadGroup) applySignalSideEffectsLocked(sig linux.Signal) {
 //
 // Preconditions: The signal mutex must be locked.
 func (t *Task) canReceiveSignalLocked(sig linux.Signal) bool {
+	// Notify that the signal is queued.
+	t.signalQueue.Notify(waiter.EventMask(linux.MakeSignalSet(sig)))
+
 	// - Do not choose tasks that are blocking the signal.
 	if linux.SignalSetOf(sig)&t.signalMask != 0 {
 		return false
@@ -1108,3 +1112,17 @@ func (*runInterruptAfterSignalDeliveryStop) execute(t *Task) taskRunState {
 	t.tg.signalHandlers.mu.Unlock()
 	return t.deliverSignal(info, act)
 }
+
+// SignalRegister registers a waiter for pending signals.
+func (t *Task) SignalRegister(e *waiter.Entry, mask waiter.EventMask) {
+	t.tg.signalHandlers.mu.Lock()
+	t.signalQueue.EventRegister(e, mask)
+	t.tg.signalHandlers.mu.Unlock()
+}
+
+// SignalUnregister unregisters a waiter for pending signals.
+func (t *Task) SignalUnregister(e *waiter.Entry) {
+	t.tg.signalHandlers.mu.Lock()
+	t.signalQueue.EventUnregister(e)
+	t.tg.signalHandlers.mu.Unlock()
+}
diff --git a/pkg/sentry/kernel/thread_group.go b/pkg/sentry/kernel/thread_group.go
index 0eef24bfb..72568d296 100644
--- a/pkg/sentry/kernel/thread_group.go
+++ b/pkg/sentry/kernel/thread_group.go
@@ -511,8 +511,9 @@ type itimerRealListener struct {
 }
 
 // Notify implements ktime.TimerListener.Notify.
-func (l *itimerRealListener) Notify(exp uint64) {
+func (l *itimerRealListener) Notify(exp uint64, setting ktime.Setting) (ktime.Setting, bool) {
 	l.tg.SendSignal(SignalInfoPriv(linux.SIGALRM))
+	return ktime.Setting{}, false
 }
 
 // Destroy implements ktime.TimerListener.Destroy.
diff --git a/pkg/sentry/kernel/time/time.go b/pkg/sentry/kernel/time/time.go
index aa6c75d25..107394183 100644
--- a/pkg/sentry/kernel/time/time.go
+++ b/pkg/sentry/kernel/time/time.go
@@ -280,13 +280,16 @@ func (ClockEventsQueue) Readiness(mask waiter.EventMask) waiter.EventMask {
 // A TimerListener receives expirations from a Timer.
 type TimerListener interface {
 	// Notify is called when its associated Timer expires. exp is the number of
-	// expirations.
+	// expirations. setting is the next timer Setting.
 	//
 	// Notify is called with the associated Timer's mutex locked, so Notify
 	// must not take any locks that precede Timer.mu in lock order.
 	//
+	// If Notify returns true, the timer will use the returned setting
+	// rather than the passed one.
+	//
 	// Preconditions: exp > 0.
-	Notify(exp uint64)
+	Notify(exp uint64, setting Setting) (newSetting Setting, update bool)
 
 	// Destroy is called when the timer is destroyed.
 	Destroy()
@@ -533,7 +536,9 @@ func (t *Timer) Tick() {
 	s, exp := t.setting.At(now)
 	t.setting = s
 	if exp > 0 {
-		t.listener.Notify(exp)
+		if newS, ok := t.listener.Notify(exp, t.setting); ok {
+			t.setting = newS
+		}
 	}
 	t.resetKickerLocked(now)
 }
@@ -588,7 +593,9 @@ func (t *Timer) Get() (Time, Setting) {
 	s, exp := t.setting.At(now)
 	t.setting = s
 	if exp > 0 {
-		t.listener.Notify(exp)
+		if newS, ok := t.listener.Notify(exp, t.setting); ok {
+			t.setting = newS
+		}
 	}
 	t.resetKickerLocked(now)
 	return now, s
@@ -620,7 +627,9 @@ func (t *Timer) SwapAnd(s Setting, f func()) (Time, Setting) {
 	}
 	oldS, oldExp := t.setting.At(now)
 	if oldExp > 0 {
-		t.listener.Notify(oldExp)
+		t.listener.Notify(oldExp, oldS)
+		// N.B. The returned Setting doesn't matter because we're about
+		// to overwrite.
 	}
 	if f != nil {
 		f()
@@ -628,7 +637,9 @@ func (t *Timer) SwapAnd(s Setting, f func()) (Time, Setting) {
 	newS, newExp := s.At(now)
 	t.setting = newS
 	if newExp > 0 {
-		t.listener.Notify(newExp)
+		if newS, ok := t.listener.Notify(newExp, t.setting); ok {
+			t.setting = newS
+		}
 	}
 	t.resetKickerLocked(now)
 	return now, oldS
@@ -683,11 +694,13 @@ func NewChannelNotifier() (TimerListener, <-chan struct{}) {
 }
 
 // Notify implements ktime.TimerListener.Notify.
-func (c *ChannelNotifier) Notify(uint64) {
+func (c *ChannelNotifier) Notify(uint64, Setting) (Setting, bool) {
 	select {
 	case c.tchan <- struct{}{}:
 	default:
 	}
+
+	return Setting{}, false
 }
 
 // Destroy implements ktime.TimerListener.Destroy and will close the channel.