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

1 files changed, 21 insertions, 10 deletions

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)
 		}
 	}
 }