From de54d280eb9fe4604c05c8956adc9d9dce4a7309 Mon Sep 17 00:00:00 2001
From: Ken Brown <kbrown@cornell.edu>
Date: Tue, 6 Oct 2020 09:06:10 -0400
Subject: [PATCH] Cygwin: AF_UNIX: sendmsg: handle messages that are too long

Return with errno EMSGSIZE if AF_UNIX_PKT_DATA_APPEND fails before any
data has been added to the packet to be sent.
---
 winsup/cygwin/fhandler_socket_unix.cc | 10 +++++++++-
 1 file changed, 9 insertions(+), 1 deletion(-)

diff --git a/winsup/cygwin/fhandler_socket_unix.cc b/winsup/cygwin/fhandler_socket_unix.cc
index 30009500c..f6f1db82d 100644
--- a/winsup/cygwin/fhandler_socket_unix.cc
+++ b/winsup/cygwin/fhandler_socket_unix.cc
@@ -2053,7 +2053,15 @@ fhandler_socket_unix::sendmsg (const struct msghdr *msg, int flags)
       for (int i = 0; i < msg->msg_iovlen; ++i)
 	if (!AF_UNIX_PKT_DATA_APPEND (packet, msg->msg_iov[i].iov_base,
 				      msg->msg_iov[i].iov_len))
-	  break;
+	  {
+	    if (packet->data_len == 0)
+	      {
+		set_errno (EMSGSIZE);
+		__leave;
+	      }
+	    else
+	      break;
+	  }
       io_lock ();
       /* Handle MSG_DONTWAIT in blocking mode */
       if (!is_nonblocking () && (flags & MSG_DONTWAIT))