Rpcinet is incorrectly handling MSG_TRUNC with SOCK_STREAM

SOCK_STREAM has special behavior with respect to MSG_TRUNC. Specifically,
the data isn't actually copied back out to userspace when MSG_TRUNC is
provided on a SOCK_STREAM.

According to tcp(7): "Since version 2.4, Linux supports the use of
MSG_TRUNC in the flags argument of recv(2) (and recvmsg(2)). This flag
causes the received bytes of data to be discarded, rather than passed
back in a caller-supplied buffer."

PiperOrigin-RevId: 200134860
Change-Id: I70f17a5f60ffe7794c3f0cfafd131c069202e90d

1 files changed, 14 insertions, 6 deletions

diff --git a/pkg/sentry/socket/rpcinet/socket.go b/pkg/sentry/socket/rpcinet/socket.go
index ffe947500..6f1a4fe01 100644
--- a/pkg/sentry/socket/rpcinet/socket.go
+++ b/pkg/sentry/socket/rpcinet/socket.go
@@ -465,9 +465,13 @@ func (s *socketOperations) RecvMsg(t *kernel.Task, dst usermem.IOSequence, flags
 
 	res, err := rpcRecvMsg(t, req)
 	if err == nil {
-		n, e := dst.CopyOut(t, res.Data)
-		if e == nil && n != len(res.Data) {
-			panic("CopyOut failed to copy full buffer")
+		var e error
+		var n int
+		if len(res.Data) > 0 {
+			n, e = dst.CopyOut(t, res.Data)
+			if e == nil && n != len(res.Data) {
+				panic("CopyOut failed to copy full buffer")
+			}
 		}
 		return int(res.Length), res.Address.GetAddress(), res.Address.GetLength(), socket.ControlMessages{}, syserr.FromError(e)
 	}
@@ -484,9 +488,13 @@ func (s *socketOperations) RecvMsg(t *kernel.Task, dst usermem.IOSequence, flags
 	for {
 		res, err := rpcRecvMsg(t, req)
 		if err == nil {
-			n, e := dst.CopyOut(t, res.Data)
-			if e == nil && n != len(res.Data) {
-				panic("CopyOut failed to copy full buffer")
+			var e error
+			var n int
+			if len(res.Data) > 0 {
+				n, e = dst.CopyOut(t, res.Data)
+				if e == nil && n != len(res.Data) {
+					panic("CopyOut failed to copy full buffer")
+				}
 			}
 			return int(res.Length), res.Address.GetAddress(), res.Address.GetLength(), socket.ControlMessages{}, syserr.FromError(e)
 		}