Add support for the MSG_TRUNC msghdr flag.

The MSG_TRUNC flag is set in the msghdr when a message is truncated.

Fixes google/gvisor#200

PiperOrigin-RevId: 244440486
Change-Id: I03c7d5e7f5935c0c6b8d69b012db1780ac5b8456

7 files changed, 83 insertions, 57 deletions

diff --git a/pkg/sentry/socket/epsocket/epsocket.go b/pkg/sentry/socket/epsocket/epsocket.go
index f370b803b..23138d874 100644
--- a/pkg/sentry/socket/epsocket/epsocket.go
+++ b/pkg/sentry/socket/epsocket/epsocket.go
@@ -376,7 +376,7 @@ func (s *SocketOperations) Read(ctx context.Context, _ *fs.File, dst usermem.IOS
 	if dst.NumBytes() == 0 {
 		return 0, nil
 	}
-	n, _, _, _, err := s.nonBlockingRead(ctx, dst, false, false, false)
+	n, _, _, _, _, err := s.nonBlockingRead(ctx, dst, false, false, false)
 	if err == syserr.ErrWouldBlock {
 		return int64(n), syserror.ErrWouldBlock
 	}
@@ -1696,7 +1696,7 @@ func (s *SocketOperations) coalescingRead(ctx context.Context, dst usermem.IOSeq
 // nonBlockingRead issues a non-blocking read.
 //
 // TODO: Support timestamps for stream sockets.
-func (s *SocketOperations) nonBlockingRead(ctx context.Context, dst usermem.IOSequence, peek, trunc, senderRequested bool) (int, interface{}, uint32, socket.ControlMessages, *syserr.Error) {
+func (s *SocketOperations) nonBlockingRead(ctx context.Context, dst usermem.IOSequence, peek, trunc, senderRequested bool) (int, int, interface{}, uint32, socket.ControlMessages, *syserr.Error) {
 	isPacket := s.isPacketBased()
 
 	// Fast path for regular reads from stream (e.g., TCP) endpoints. Note
@@ -1712,14 +1712,14 @@ func (s *SocketOperations) nonBlockingRead(ctx context.Context, dst usermem.IOSe
 		s.readMu.Lock()
 		n, err := s.coalescingRead(ctx, dst, trunc)
 		s.readMu.Unlock()
-		return n, nil, 0, socket.ControlMessages{}, err
+		return n, 0, nil, 0, socket.ControlMessages{}, err
 	}
 
 	s.readMu.Lock()
 	defer s.readMu.Unlock()
 
 	if err := s.fetchReadView(); err != nil {
-		return 0, nil, 0, socket.ControlMessages{}, err
+		return 0, 0, nil, 0, socket.ControlMessages{}, err
 	}
 
 	if !isPacket && peek && trunc {
@@ -1727,14 +1727,14 @@ func (s *SocketOperations) nonBlockingRead(ctx context.Context, dst usermem.IOSe
 		// amount that could be read.
 		var rql tcpip.ReceiveQueueSizeOption
 		if err := s.Endpoint.GetSockOpt(&rql); err != nil {
-			return 0, nil, 0, socket.ControlMessages{}, syserr.TranslateNetstackError(err)
+			return 0, 0, nil, 0, socket.ControlMessages{}, syserr.TranslateNetstackError(err)
 		}
 		available := len(s.readView) + int(rql)
 		bufLen := int(dst.NumBytes())
 		if available < bufLen {
-			return available, nil, 0, socket.ControlMessages{}, nil
+			return available, 0, nil, 0, socket.ControlMessages{}, nil
 		}
-		return bufLen, nil, 0, socket.ControlMessages{}, nil
+		return bufLen, 0, nil, 0, socket.ControlMessages{}, nil
 	}
 
 	n, err := dst.CopyOut(ctx, s.readView)
@@ -1751,11 +1751,11 @@ func (s *SocketOperations) nonBlockingRead(ctx context.Context, dst usermem.IOSe
 	if peek {
 		if l := len(s.readView); trunc && l > n {
 			// isPacket must be true.
-			return l, addr, addrLen, s.controlMessages(), syserr.FromError(err)
+			return l, linux.MSG_TRUNC, addr, addrLen, s.controlMessages(), syserr.FromError(err)
 		}
 
 		if isPacket || err != nil {
-			return int(n), addr, addrLen, s.controlMessages(), syserr.FromError(err)
+			return n, 0, addr, addrLen, s.controlMessages(), syserr.FromError(err)
 		}
 
 		// We need to peek beyond the first message.
@@ -1773,7 +1773,7 @@ func (s *SocketOperations) nonBlockingRead(ctx context.Context, dst usermem.IOSe
 			// We got some data, so no need to return an error.
 			err = nil
 		}
-		return int(n), nil, 0, s.controlMessages(), syserr.FromError(err)
+		return n, 0, nil, 0, s.controlMessages(), syserr.FromError(err)
 	}
 
 	var msgLen int
@@ -1785,11 +1785,16 @@ func (s *SocketOperations) nonBlockingRead(ctx context.Context, dst usermem.IOSe
 		s.readView.TrimFront(int(n))
 	}
 
+	var flags int
+	if msgLen > int(n) {
+		flags |= linux.MSG_TRUNC
+	}
+
 	if trunc {
-		return msgLen, addr, addrLen, s.controlMessages(), syserr.FromError(err)
+		n = msgLen
 	}
 
-	return int(n), addr, addrLen, s.controlMessages(), syserr.FromError(err)
+	return n, flags, addr, addrLen, s.controlMessages(), syserr.FromError(err)
 }
 
 func (s *SocketOperations) controlMessages() socket.ControlMessages {
@@ -1810,7 +1815,7 @@ func (s *SocketOperations) updateTimestamp() {
 
 // RecvMsg implements the linux syscall recvmsg(2) for sockets backed by
 // tcpip.Endpoint.
-func (s *SocketOperations) RecvMsg(t *kernel.Task, dst usermem.IOSequence, flags int, haveDeadline bool, deadline ktime.Time, senderRequested bool, controlDataLen uint64) (n int, senderAddr interface{}, senderAddrLen uint32, controlMessages socket.ControlMessages, err *syserr.Error) {
+func (s *SocketOperations) RecvMsg(t *kernel.Task, dst usermem.IOSequence, flags int, haveDeadline bool, deadline ktime.Time, senderRequested bool, controlDataLen uint64) (n int, msgFlags int, senderAddr interface{}, senderAddrLen uint32, controlMessages socket.ControlMessages, err *syserr.Error) {
 	trunc := flags&linux.MSG_TRUNC != 0
 	peek := flags&linux.MSG_PEEK != 0
 	dontWait := flags&linux.MSG_DONTWAIT != 0
@@ -1819,16 +1824,16 @@ func (s *SocketOperations) RecvMsg(t *kernel.Task, dst usermem.IOSequence, flags
 		// Stream sockets ignore the sender address.
 		senderRequested = false
 	}
-	n, senderAddr, senderAddrLen, controlMessages, err = s.nonBlockingRead(t, dst, peek, trunc, senderRequested)
+	n, msgFlags, senderAddr, senderAddrLen, controlMessages, err = s.nonBlockingRead(t, dst, peek, trunc, senderRequested)
 
 	if s.isPacketBased() && err == syserr.ErrClosedForReceive && flags&linux.MSG_DONTWAIT != 0 {
 		// In this situation we should return EAGAIN.
-		return 0, nil, 0, socket.ControlMessages{}, syserr.ErrTryAgain
+		return 0, 0, nil, 0, socket.ControlMessages{}, syserr.ErrTryAgain
 	}
 
 	if err != nil && (err != syserr.ErrWouldBlock || dontWait) {
 		// Read failed and we should not retry.
-		return 0, nil, 0, socket.ControlMessages{}, err
+		return 0, 0, nil, 0, socket.ControlMessages{}, err
 	}
 
 	if err == nil && (dontWait || !waitAll || s.isPacketBased() || int64(n) >= dst.NumBytes()) {
@@ -1847,7 +1852,7 @@ func (s *SocketOperations) RecvMsg(t *kernel.Task, dst usermem.IOSequence, flags
 
 	for {
 		var rn int
-		rn, senderAddr, senderAddrLen, controlMessages, err = s.nonBlockingRead(t, dst, peek, trunc, senderRequested)
+		rn, msgFlags, senderAddr, senderAddrLen, controlMessages, err = s.nonBlockingRead(t, dst, peek, trunc, senderRequested)
 		n += rn
 		if err != nil && err != syserr.ErrWouldBlock {
 			// Always stop on errors other than would block as we generally
@@ -1866,12 +1871,12 @@ func (s *SocketOperations) RecvMsg(t *kernel.Task, dst usermem.IOSequence, flags
 
 		if err := t.BlockWithDeadline(ch, haveDeadline, deadline); err != nil {
 			if n > 0 {
-				return n, senderAddr, senderAddrLen, controlMessages, nil
+				return n, msgFlags, senderAddr, senderAddrLen, controlMessages, nil
 			}
 			if err == syserror.ETIMEDOUT {
-				return 0, nil, 0, socket.ControlMessages{}, syserr.ErrTryAgain
+				return 0, 0, nil, 0, socket.ControlMessages{}, syserr.ErrTryAgain
 			}
-			return 0, nil, 0, socket.ControlMessages{}, syserr.FromError(err)
+			return 0, 0, nil, 0, socket.ControlMessages{}, syserr.FromError(err)
 		}
 	}
 }
diff --git a/pkg/sentry/socket/hostinet/socket.go b/pkg/sentry/socket/hostinet/socket.go
index be63823d8..c4848b313 100644
--- a/pkg/sentry/socket/hostinet/socket.go
+++ b/pkg/sentry/socket/hostinet/socket.go
@@ -345,14 +345,14 @@ func (s *socketOperations) SetSockOpt(t *kernel.Task, level int, name int, opt [
 }
 
 // RecvMsg implements socket.Socket.RecvMsg.
-func (s *socketOperations) RecvMsg(t *kernel.Task, dst usermem.IOSequence, flags int, haveDeadline bool, deadline ktime.Time, senderRequested bool, controlDataLen uint64) (int, interface{}, uint32, socket.ControlMessages, *syserr.Error) {
+func (s *socketOperations) RecvMsg(t *kernel.Task, dst usermem.IOSequence, flags int, haveDeadline bool, deadline ktime.Time, senderRequested bool, controlDataLen uint64) (int, int, interface{}, uint32, socket.ControlMessages, *syserr.Error) {
 	// Whitelist flags.
 	//
 	// FIXME: We can't support MSG_ERRQUEUE because it uses ancillary
 	// messages that netstack/tcpip/transport/unix doesn't understand. Kill the
 	// Socket interface's dependence on netstack.
 	if flags&^(syscall.MSG_DONTWAIT|syscall.MSG_PEEK|syscall.MSG_TRUNC) != 0 {
-		return 0, nil, 0, socket.ControlMessages{}, syserr.ErrInvalidArgument
+		return 0, 0, nil, 0, socket.ControlMessages{}, syserr.ErrInvalidArgument
 	}
 
 	var senderAddr []byte
@@ -360,6 +360,8 @@ func (s *socketOperations) RecvMsg(t *kernel.Task, dst usermem.IOSequence, flags
 		senderAddr = make([]byte, sizeofSockaddr)
 	}
 
+	var msgFlags int
+
 	recvmsgToBlocks := safemem.ReaderFunc(func(dsts safemem.BlockSeq) (uint64, error) {
 		// Refuse to do anything if any part of dst.Addrs was unusable.
 		if uint64(dst.NumBytes()) != dsts.NumBytes() {
@@ -391,6 +393,7 @@ func (s *socketOperations) RecvMsg(t *kernel.Task, dst usermem.IOSequence, flags
 			return 0, err
 		}
 		senderAddr = senderAddr[:msg.Namelen]
+		msgFlags = int(msg.Flags)
 		return n, nil
 	})
 
@@ -417,7 +420,10 @@ func (s *socketOperations) RecvMsg(t *kernel.Task, dst usermem.IOSequence, flags
 		}
 	}
 
-	return int(n), senderAddr, uint32(len(senderAddr)), socket.ControlMessages{}, syserr.FromError(err)
+	// We don't allow control messages.
+	msgFlags &^= linux.MSG_CTRUNC
+
+	return int(n), msgFlags, senderAddr, uint32(len(senderAddr)), socket.ControlMessages{}, syserr.FromError(err)
 }
 
 // SendMsg implements socket.Socket.SendMsg.
diff --git a/pkg/sentry/socket/netlink/socket.go b/pkg/sentry/socket/netlink/socket.go
index 2503a67c5..0fe9b39b6 100644
--- a/pkg/sentry/socket/netlink/socket.go
+++ b/pkg/sentry/socket/netlink/socket.go
@@ -397,7 +397,7 @@ func (s *Socket) GetPeerName(t *kernel.Task) (interface{}, uint32, *syserr.Error
 }
 
 // RecvMsg implements socket.Socket.RecvMsg.
-func (s *Socket) RecvMsg(t *kernel.Task, dst usermem.IOSequence, flags int, haveDeadline bool, deadline ktime.Time, senderRequested bool, controlDataLen uint64) (int, interface{}, uint32, socket.ControlMessages, *syserr.Error) {
+func (s *Socket) RecvMsg(t *kernel.Task, dst usermem.IOSequence, flags int, haveDeadline bool, deadline ktime.Time, senderRequested bool, controlDataLen uint64) (int, int, interface{}, uint32, socket.ControlMessages, *syserr.Error) {
 	from := linux.SockAddrNetlink{
 		Family: linux.AF_NETLINK,
 		PortID: 0,
@@ -412,10 +412,14 @@ func (s *Socket) RecvMsg(t *kernel.Task, dst usermem.IOSequence, flags int, have
 	}
 
 	if n, err := dst.CopyOutFrom(t, &r); err != syserror.ErrWouldBlock || flags&linux.MSG_DONTWAIT != 0 {
+		var mflags int
+		if n < int64(r.MsgSize) {
+			mflags |= linux.MSG_TRUNC
+		}
 		if trunc {
 			n = int64(r.MsgSize)
 		}
-		return int(n), from, fromLen, socket.ControlMessages{}, syserr.FromError(err)
+		return int(n), mflags, from, fromLen, socket.ControlMessages{}, syserr.FromError(err)
 	}
 
 	// We'll have to block. Register for notification and keep trying to
@@ -426,17 +430,21 @@ func (s *Socket) RecvMsg(t *kernel.Task, dst usermem.IOSequence, flags int, have
 
 	for {
 		if n, err := dst.CopyOutFrom(t, &r); err != syserror.ErrWouldBlock {
+			var mflags int
+			if n < int64(r.MsgSize) {
+				mflags |= linux.MSG_TRUNC
+			}
 			if trunc {
 				n = int64(r.MsgSize)
 			}
-			return int(n), from, fromLen, socket.ControlMessages{}, syserr.FromError(err)
+			return int(n), mflags, from, fromLen, socket.ControlMessages{}, syserr.FromError(err)
 		}
 
 		if err := t.BlockWithDeadline(ch, haveDeadline, deadline); err != nil {
 			if err == syserror.ETIMEDOUT {
-				return 0, nil, 0, socket.ControlMessages{}, syserr.ErrTryAgain
+				return 0, 0, nil, 0, socket.ControlMessages{}, syserr.ErrTryAgain
 			}
-			return 0, nil, 0, socket.ControlMessages{}, syserr.FromError(err)
+			return 0, 0, nil, 0, socket.ControlMessages{}, syserr.FromError(err)
 		}
 	}
 }
diff --git a/pkg/sentry/socket/rpcinet/socket.go b/pkg/sentry/socket/rpcinet/socket.go
index 896b5b7ce..3418a6d75 100644
--- a/pkg/sentry/socket/rpcinet/socket.go
+++ b/pkg/sentry/socket/rpcinet/socket.go
@@ -673,7 +673,7 @@ func (s *socketOperations) extractControlMessages(payload *pb.RecvmsgResponse_Re
 }
 
 // RecvMsg implements socket.Socket.RecvMsg.
-func (s *socketOperations) RecvMsg(t *kernel.Task, dst usermem.IOSequence, flags int, haveDeadline bool, deadline ktime.Time, senderRequested bool, controlDataLen uint64) (int, interface{}, uint32, socket.ControlMessages, *syserr.Error) {
+func (s *socketOperations) RecvMsg(t *kernel.Task, dst usermem.IOSequence, flags int, haveDeadline bool, deadline ktime.Time, senderRequested bool, controlDataLen uint64) (int, int, interface{}, uint32, socket.ControlMessages, *syserr.Error) {
 	req := &pb.SyscallRequest_Recvmsg{&pb.RecvmsgRequest{
 		Fd:         s.fd,
 		Length:     uint32(dst.NumBytes()),
@@ -694,10 +694,10 @@ func (s *socketOperations) RecvMsg(t *kernel.Task, dst usermem.IOSequence, flags
 			}
 		}
 		c := s.extractControlMessages(res)
-		return int(res.Length), res.Address.GetAddress(), res.Address.GetLength(), c, syserr.FromError(e)
+		return int(res.Length), 0, res.Address.GetAddress(), res.Address.GetLength(), c, syserr.FromError(e)
 	}
 	if err != syserr.ErrWouldBlock && err != syserr.ErrTryAgain || flags&linux.MSG_DONTWAIT != 0 {
-		return 0, nil, 0, socket.ControlMessages{}, err
+		return 0, 0, nil, 0, socket.ControlMessages{}, err
 	}
 
 	// We'll have to block. Register for notifications and keep trying to
@@ -718,23 +718,23 @@ func (s *socketOperations) RecvMsg(t *kernel.Task, dst usermem.IOSequence, flags
 				}
 			}
 			c := s.extractControlMessages(res)
-			return int(res.Length), res.Address.GetAddress(), res.Address.GetLength(), c, syserr.FromError(e)
+			return int(res.Length), 0, res.Address.GetAddress(), res.Address.GetLength(), c, syserr.FromError(e)
 		}
 		if err != syserr.ErrWouldBlock && err != syserr.ErrTryAgain {
-			return 0, nil, 0, socket.ControlMessages{}, err
+			return 0, 0, nil, 0, socket.ControlMessages{}, err
 		}
 
 		if s.isShutRdSet() {
 			// Blocking would have caused us to block indefinitely so we return 0,
 			// this is the same behavior as Linux.
-			return 0, nil, 0, socket.ControlMessages{}, nil
+			return 0, 0, nil, 0, socket.ControlMessages{}, nil
 		}
 
 		if err := t.BlockWithDeadline(ch, haveDeadline, deadline); err != nil {
 			if err == syserror.ETIMEDOUT {
-				return 0, nil, 0, socket.ControlMessages{}, syserr.ErrTryAgain
+				return 0, 0, nil, 0, socket.ControlMessages{}, syserr.ErrTryAgain
 			}
-			return 0, nil, 0, socket.ControlMessages{}, syserr.FromError(err)
+			return 0, 0, nil, 0, socket.ControlMessages{}, syserr.FromError(err)
 		}
 	}
 }
diff --git a/pkg/sentry/socket/socket.go b/pkg/sentry/socket/socket.go
index 5ab423f3c..62ba13782 100644
--- a/pkg/sentry/socket/socket.go
+++ b/pkg/sentry/socket/socket.go
@@ -88,7 +88,7 @@ type Socket interface {
 	// not necessarily the actual length of the address.
 	//
 	// If err != nil, the recv was not successful.
-	RecvMsg(t *kernel.Task, dst usermem.IOSequence, flags int, haveDeadline bool, deadline ktime.Time, senderRequested bool, controlDataLen uint64) (n int, senderAddr interface{}, senderAddrLen uint32, controlMessages ControlMessages, err *syserr.Error)
+	RecvMsg(t *kernel.Task, dst usermem.IOSequence, flags int, haveDeadline bool, deadline ktime.Time, senderRequested bool, controlDataLen uint64) (n int, msgFlags int, senderAddr interface{}, senderAddrLen uint32, controlMessages ControlMessages, err *syserr.Error)
 
 	// SendMsg implements the sendmsg(2) linux syscall. SendMsg does not take
 	// ownership of the ControlMessage on error.
diff --git a/pkg/sentry/socket/unix/unix.go b/pkg/sentry/socket/unix/unix.go
index 92411c901..01efd24d3 100644
--- a/pkg/sentry/socket/unix/unix.go
+++ b/pkg/sentry/socket/unix/unix.go
@@ -477,7 +477,7 @@ func (s *SocketOperations) Read(ctx context.Context, _ *fs.File, dst usermem.IOS
 
 // RecvMsg implements the linux syscall recvmsg(2) for sockets backed by
 // a transport.Endpoint.
-func (s *SocketOperations) RecvMsg(t *kernel.Task, dst usermem.IOSequence, flags int, haveDeadline bool, deadline ktime.Time, senderRequested bool, controlDataLen uint64) (n int, senderAddr interface{}, senderAddrLen uint32, controlMessages socket.ControlMessages, err *syserr.Error) {
+func (s *SocketOperations) RecvMsg(t *kernel.Task, dst usermem.IOSequence, flags int, haveDeadline bool, deadline ktime.Time, senderRequested bool, controlDataLen uint64) (n int, msgFlags int, senderAddr interface{}, senderAddrLen uint32, controlMessages socket.ControlMessages, err *syserr.Error) {
 	trunc := flags&linux.MSG_TRUNC != 0
 	peek := flags&linux.MSG_PEEK != 0
 	dontWait := flags&linux.MSG_DONTWAIT != 0
@@ -515,11 +515,17 @@ func (s *SocketOperations) RecvMsg(t *kernel.Task, dst usermem.IOSequence, flags
 		if r.From != nil {
 			from, fromLen = epsocket.ConvertAddress(linux.AF_UNIX, *r.From)
 		}
-		if trunc {
-			n = int64(r.MsgSize)
-		}
+
 		if err != nil || dontWait || !waitAll || s.isPacket || n >= dst.NumBytes() {
-			return int(n), from, fromLen, socket.ControlMessages{Unix: r.Control}, syserr.FromError(err)
+			if s.isPacket && n < int64(r.MsgSize) {
+				msgFlags |= linux.MSG_TRUNC
+			}
+
+			if trunc {
+				n = int64(r.MsgSize)
+			}
+
+			return int(n), msgFlags, from, fromLen, socket.ControlMessages{Unix: r.Control}, syserr.FromError(err)
 		}
 
 		// Don't overwrite any data we received.
@@ -541,14 +547,19 @@ func (s *SocketOperations) RecvMsg(t *kernel.Task, dst usermem.IOSequence, flags
 				from, fromLen = epsocket.ConvertAddress(linux.AF_UNIX, *r.From)
 			}
 			if trunc {
-				n = int64(r.MsgSize)
+				// n and r.MsgSize are the same for streams.
+				total += int64(r.MsgSize)
+			} else {
+				total += n
 			}
-			total += n
 			if err != nil || !waitAll || s.isPacket || n >= dst.NumBytes() {
 				if total > 0 {
 					err = nil
 				}
-				return int(total), from, fromLen, socket.ControlMessages{Unix: r.Control}, syserr.FromError(err)
+				if s.isPacket && n < int64(r.MsgSize) {
+					msgFlags |= linux.MSG_TRUNC
+				}
+				return int(total), msgFlags, from, fromLen, socket.ControlMessages{Unix: r.Control}, syserr.FromError(err)
 			}
 
 			// Don't overwrite any data we received.
@@ -560,9 +571,9 @@ func (s *SocketOperations) RecvMsg(t *kernel.Task, dst usermem.IOSequence, flags
 				err = nil
 			}
 			if err == syserror.ETIMEDOUT {
-				return int(total), nil, 0, socket.ControlMessages{}, syserr.ErrTryAgain
+				return int(total), msgFlags, nil, 0, socket.ControlMessages{}, syserr.ErrTryAgain
 			}
-			return int(total), nil, 0, socket.ControlMessages{}, syserr.FromError(err)
+			return int(total), msgFlags, nil, 0, socket.ControlMessages{}, syserr.FromError(err)
 		}
 	}
 }
diff --git a/pkg/sentry/syscalls/linux/sys_socket.go b/pkg/sentry/syscalls/linux/sys_socket.go
index 49e6f4aeb..30ccc3f66 100644
--- a/pkg/sentry/syscalls/linux/sys_socket.go
+++ b/pkg/sentry/syscalls/linux/sys_socket.go
@@ -742,17 +742,15 @@ func recvSingleMsg(t *kernel.Task, s socket.Socket, msgPtr usermem.Addr, flags i
 
 	// Fast path when no control message nor name buffers are provided.
 	if msg.ControlLen == 0 && msg.NameLen == 0 {
-		n, _, _, cms, err := s.RecvMsg(t, dst, int(flags), haveDeadline, deadline, false, 0)
+		n, mflags, _, _, cms, err := s.RecvMsg(t, dst, int(flags), haveDeadline, deadline, false, 0)
 		if err != nil {
 			return 0, syserror.ConvertIntr(err.ToError(), kernel.ERESTARTSYS)
 		}
 		cms.Unix.Release()
 
-		if msg.Flags != 0 {
+		if int(msg.Flags) != mflags {
 			// Copy out the flags to the caller.
-			//
-			// TODO: Plumb through actual flags.
-			if _, err := t.CopyOut(msgPtr+flagsOffset, int32(0)); err != nil {
+			if _, err := t.CopyOut(msgPtr+flagsOffset, int32(mflags)); err != nil {
 				return 0, err
 			}
 		}
@@ -763,7 +761,7 @@ func recvSingleMsg(t *kernel.Task, s socket.Socket, msgPtr usermem.Addr, flags i
 	if msg.ControlLen > maxControlLen {
 		return 0, syscall.ENOBUFS
 	}
-	n, sender, senderLen, cms, e := s.RecvMsg(t, dst, int(flags), haveDeadline, deadline, msg.NameLen != 0, msg.ControlLen)
+	n, mflags, sender, senderLen, cms, e := s.RecvMsg(t, dst, int(flags), haveDeadline, deadline, msg.NameLen != 0, msg.ControlLen)
 	if e != nil {
 		return 0, syserror.ConvertIntr(e.ToError(), kernel.ERESTARTSYS)
 	}
@@ -802,9 +800,7 @@ func recvSingleMsg(t *kernel.Task, s socket.Socket, msgPtr usermem.Addr, flags i
 	}
 
 	// Copy out the flags to the caller.
-	//
-	// TODO: Plumb through actual flags.
-	if _, err := t.CopyOut(msgPtr+flagsOffset, int32(0)); err != nil {
+	if _, err := t.CopyOut(msgPtr+flagsOffset, int32(mflags)); err != nil {
 		return 0, err
 	}
 
@@ -856,7 +852,7 @@ func recvFrom(t *kernel.Task, fd kdefs.FD, bufPtr usermem.Addr, bufLen uint64, f
 		flags |= linux.MSG_DONTWAIT
 	}
 
-	n, sender, senderLen, cm, e := s.RecvMsg(t, dst, int(flags), haveDeadline, deadline, nameLenPtr != 0, 0)
+	n, _, sender, senderLen, cm, e := s.RecvMsg(t, dst, int(flags), haveDeadline, deadline, nameLenPtr != 0, 0)
 	cm.Unix.Release()
 	if e != nil {
 		return 0, syserror.ConvertIntr(e.ToError(), kernel.ERESTARTSYS)