2 files changed, 34 insertions, 2 deletions
diff --git a/pkg/sentry/socket/unix/io.go b/pkg/sentry/socket/unix/io.go
index 2ec1a662d..2447f24ef 100644
--- a/pkg/sentry/socket/unix/io.go
+++ b/pkg/sentry/socket/unix/io.go
@@ -83,6 +83,19 @@ type EndpointReader struct {
 	ControlTrunc bool
 }
 
+// Truncate calls RecvMsg on the endpoint without writing to a destination.
+func (r *EndpointReader) Truncate() error {
+	// Ignore bytes read since it will always be zero.
+	_, ms, c, ct, err := r.Endpoint.RecvMsg(r.Ctx, [][]byte{}, r.Creds, r.NumRights, r.Peek, r.From)
+	r.Control = c
+	r.ControlTrunc = ct
+	r.MsgSize = ms
+	if err != nil {
+		return err.ToError()
+	}
+	return nil
+}
+
 // ReadToBlocks implements safemem.Reader.ReadToBlocks.
 func (r *EndpointReader) ReadToBlocks(dsts safemem.BlockSeq) (uint64, error) {
 	return safemem.FromVecReaderFunc{func(bufs [][]byte) (int64, error) {
diff --git a/pkg/sentry/socket/unix/unix.go b/pkg/sentry/socket/unix/unix.go
index 885758054..91effe89a 100644
--- a/pkg/sentry/socket/unix/unix.go
+++ b/pkg/sentry/socket/unix/unix.go
@@ -544,8 +544,27 @@ func (s *SocketOperations) RecvMsg(t *kernel.Task, dst usermem.IOSequence, flags
 	if senderRequested {
 		r.From = &tcpip.FullAddress{}
 	}
+
+	doRead := func() (int64, error) {
+		return dst.CopyOutFrom(t, &r)
+	}
+
+	// If MSG_TRUNC is set with a zero byte destination then we still need
+	// to read the message and discard it, or in the case where MSG_PEEK is
+	// set, leave it be. In both cases the full message length must be
+	// returned.
+	if trunc && dst.Addrs.NumBytes() == 0 {
+		doRead = func() (int64, error) {
+			err := r.Truncate()
+			// Always return zero for bytes read since the destination size is
+			// zero.
+			return 0, err
+		}
+
+	}
+
 	var total int64
-	if n, err := dst.CopyOutFrom(t, &r); err != syserror.ErrWouldBlock || dontWait {
+	if n, err := doRead(); err != syserror.ErrWouldBlock || dontWait {
 		var from linux.SockAddr
 		var fromLen uint32
 		if r.From != nil && len([]byte(r.From.Addr)) != 0 {
@@ -580,7 +599,7 @@ func (s *SocketOperations) RecvMsg(t *kernel.Task, dst usermem.IOSequence, flags
 	defer s.EventUnregister(&e)
 
 	for {
-		if n, err := dst.CopyOutFrom(t, &r); err != syserror.ErrWouldBlock {
+		if n, err := doRead(); err != syserror.ErrWouldBlock {
 			var from linux.SockAddr
 			var fromLen uint32
 			if r.From != nil {