[PATCH 5.7 77/79] mptcp: fix bogus sendmsg() return code under pressure

[Greg Kroah-Hartman]

From: Paolo Abeni <pabeni@xxxxxxxxxx>

[ Upstream commit 8555c6bfd5fddb1cf363d3cd157d70a1bb27f718 ]

In case of memory pressure, mptcp_sendmsg() may call
sk_stream_wait_memory() after succesfully xmitting some
bytes. If the latter fails we currently return to the
user-space the error code, ignoring the succeful xmit.

Address the issue always checking for the xmitted bytes
before mptcp_sendmsg() completes.

Fixes: f296234c98a8 ("mptcp: Add handling of incoming MP_JOIN requests")
Reviewed-by: Matthieu Baerts <matthieu.baerts@xxxxxxxxxxxx>
Signed-off-by: Paolo Abeni <pabeni@xxxxxxxxxx>
Signed-off-by: David S. Miller <davem@xxxxxxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
---
 net/mptcp/protocol.c |    3 +--
 1 file changed, 1 insertion(+), 2 deletions(-)

--- a/net/mptcp/protocol.c
+++ b/net/mptcp/protocol.c
@@ -802,7 +802,6 @@ fallback:
 
 	mptcp_set_timeout(sk, ssk);
 	if (copied) {
-		ret = copied;
 		tcp_push(ssk, msg->msg_flags, mss_now, tcp_sk(ssk)->nonagle,
 			 size_goal);
 
@@ -815,7 +814,7 @@ fallback:
 	release_sock(ssk);
 out:
 	release_sock(sk);
-	return ret;
+	return copied ? : ret;
 }
 
 static void mptcp_wait_data(struct sock *sk, long *timeo)