1 files changed, 33 insertions, 20 deletions

diff --git a/pkg/sentry/kernel/msgqueue/msgqueue.go b/pkg/sentry/kernel/msgqueue/msgqueue.go
index c111297d7..fab396d7c 100644
--- a/pkg/sentry/kernel/msgqueue/msgqueue.go
+++ b/pkg/sentry/kernel/msgqueue/msgqueue.go
@@ -208,11 +208,13 @@ func (r *Registry) FindByID(id ipc.ID) (*Queue, error) {
 
 // Send appends a message to the message queue, and returns an error if sending
 // fails. See msgsnd(2).
-func (q *Queue) Send(ctx context.Context, m Message, b Blocker, wait bool, pid int32) (err error) {
+func (q *Queue) Send(ctx context.Context, m Message, b Blocker, wait bool, pid int32) error {
 	// Try to perform a non-blocking send using queue.append. If EWOULDBLOCK
 	// is returned, start the blocking procedure. Otherwise, return normally.
 	creds := auth.CredentialsFromContext(ctx)
-	if err := q.append(ctx, m, creds, pid); err != linuxerr.EWOULDBLOCK {
+
+	// Fast path: first attempt a non-blocking push.
+	if err := q.push(ctx, m, creds, pid); err != linuxerr.EWOULDBLOCK {
 		return err
 	}
 
@@ -220,25 +222,30 @@ func (q *Queue) Send(ctx context.Context, m Message, b Blocker, wait bool, pid i
 		return linuxerr.EAGAIN
 	}
 
+	// Slow path: at this point, the queue was found to be full, and we were
+	// asked to block.
+
 	e, ch := waiter.NewChannelEntry(nil)
 	q.senders.EventRegister(&e, waiter.EventOut)
+	defer q.senders.EventUnregister(&e)
 
+	// Note: we need to check again before blocking the first time since space
+	// may have become available.
 	for {
-		if err = q.append(ctx, m, creds, pid); err != linuxerr.EWOULDBLOCK {
-			break
+		if err := q.push(ctx, m, creds, pid); err != linuxerr.EWOULDBLOCK {
+			return err
+		}
+		if err := b.Block(ch); err != nil {
+			return err
 		}
-		b.Block(ch)
 	}
-
-	q.senders.EventUnregister(&e)
-	return err
 }
 
-// append appends a message to the queue's message list and notifies waiting
+// push appends a message to the queue's message list and notifies waiting
 // receivers that a message has been inserted. It returns an error if adding
 // the message would cause the queue to exceed its maximum capacity, which can
 // be used as a signal to block the task. Other errors should be returned as is.
-func (q *Queue) append(ctx context.Context, m Message, creds *auth.Credentials, pid int32) error {
+func (q *Queue) push(ctx context.Context, m Message, creds *auth.Credentials, pid int32) error {
 	if m.Type <= 0 {
 		return linuxerr.EINVAL
 	}
@@ -295,15 +302,14 @@ func (q *Queue) append(ctx context.Context, m Message, creds *auth.Credentials,
 }
 
 // Receive removes a message from the queue and returns it. See msgrcv(2).
-func (q *Queue) Receive(ctx context.Context, b Blocker, mType int64, maxSize int64, wait, truncate, except bool, pid int32) (msg *Message, err error) {
+func (q *Queue) Receive(ctx context.Context, b Blocker, mType int64, maxSize int64, wait, truncate, except bool, pid int32) (*Message, error) {
 	if maxSize < 0 || maxSize > maxMessageBytes {
 		return nil, linuxerr.EINVAL
 	}
 	max := uint64(maxSize)
-
-	// Try to perform a non-blocking receive using queue.pop. If EWOULDBLOCK
-	// is returned, start the blocking procedure. Otherwise, return normally.
 	creds := auth.CredentialsFromContext(ctx)
+
+	// Fast path: first attempt a non-blocking pop.
 	if msg, err := q.pop(ctx, creds, mType, max, truncate, except, pid); err != linuxerr.EWOULDBLOCK {
 		return msg, err
 	}
@@ -312,24 +318,30 @@ func (q *Queue) Receive(ctx context.Context, b Blocker, mType int64, maxSize int
 		return nil, linuxerr.ENOMSG
 	}
 
+	// Slow path: at this point, the queue was found to be empty, and we were
+	// asked to block.
+
 	e, ch := waiter.NewChannelEntry(nil)
 	q.receivers.EventRegister(&e, waiter.EventIn)
+	defer q.receivers.EventUnregister(&e)
 
+	// Note: we need to check again before blocking the first time since a
+	// message may have become available.
 	for {
-		if msg, err = q.pop(ctx, creds, mType, max, truncate, except, pid); err != linuxerr.EWOULDBLOCK {
-			break
+		if msg, err := q.pop(ctx, creds, mType, max, truncate, except, pid); err != linuxerr.EWOULDBLOCK {
+			return msg, err
+		}
+		if err := b.Block(ch); err != nil {
+			return nil, err
 		}
-		b.Block(ch)
 	}
-	q.receivers.EventUnregister(&e)
-	return msg, err
 }
 
 // pop pops the first message from the queue that matches the given type. It
 // returns an error for all the cases specified in msgrcv(2). If the queue is
 // empty or no message of the specified type is available, a EWOULDBLOCK error
 // is returned, which can then be used as a signal to block the process or fail.
-func (q *Queue) pop(ctx context.Context, creds *auth.Credentials, mType int64, maxSize uint64, truncate, except bool, pid int32) (msg *Message, _ error) {
+func (q *Queue) pop(ctx context.Context, creds *auth.Credentials, mType int64, maxSize uint64, truncate, except bool, pid int32) (*Message, error) {
 	q.mu.Lock()
 	defer q.mu.Unlock()
 
@@ -350,6 +362,7 @@ func (q *Queue) pop(ctx context.Context, creds *auth.Credentials, mType int64, m
 	}
 
 	// Get a message from the queue.
+	var msg *Message
 	switch {
 	case mType == 0:
 		msg = q.messages.Front()