Add task work mechanism.

Like task_work in Linux, this allows us to register callbacks to be executed
before returning to userspace. This is needed for kcov support, which requires
coverage information to be up-to-date whenever we are in user mode. We will
provide coverage data through the kcov interface to enable coverage-directed
fuzzing in syzkaller.

One difference from Linux is that task work cannot queue work before the
transition to userspace that it precedes; queued work will be picked up before
the next transition.

PiperOrigin-RevId: 322889984

1 files changed, 38 insertions, 0 deletions

diff --git a/pkg/sentry/kernel/task_work.go b/pkg/sentry/kernel/task_work.go
new file mode 100644
index 000000000..dda5a433a
--- /dev/null
+++ b/pkg/sentry/kernel/task_work.go
@@ -0,0 +1,38 @@
+// Copyright 2020 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 kernel
+
+import "sync/atomic"
+
+// TaskWorker is a deferred task.
+//
+// This must be savable.
+type TaskWorker interface {
+	// TaskWork will be executed prior to returning to user space. Note that
+	// TaskWork may call RegisterWork again, but this will not be executed until
+	// the next return to user space, unlike in Linux. This effectively allows
+	// registration of indefinite user return hooks, but not by default.
+	TaskWork(t *Task)
+}
+
+// RegisterWork can be used to register additional task work that will be
+// performed prior to returning to user space. See TaskWorker.TaskWork for
+// semantics regarding registration.
+func (t *Task) RegisterWork(work TaskWorker) {
+	t.taskWorkMu.Lock()
+	defer t.taskWorkMu.Unlock()
+	atomic.AddInt32(&t.taskWorkCount, 1)
+	t.taskWork = append(t.taskWork, work)
+}