mptcp: sendmsg: reset iter on error redux

This fix wasn't correct: When this function is invoked from the
retransmission worker, the iterator contains garbage and resetting
it causes a crash.

As the work queue should not be performance critical also zero the
msghdr struct.

Fixes: 3575938 "(mptcp: sendmsg: reset iter on error)"
Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: David S. Miller <davem@davemloft.net>

Author: Florian Westphal

net/mptcp/protocol.c

@@ -740,7 +740,8 @@ static int mptcp_sendmsg_frag(struct sock *sk, struct sock *ssk,
 	ret = do_tcp_sendpages(ssk, page, offset, psize,
 			       msg->msg_flags | MSG_SENDPAGE_NOTLAST | MSG_DONTWAIT);
 	if (ret <= 0) {
-		iov_iter_revert(&msg->msg_iter, psize);
+		if (!retransmission)
+			iov_iter_revert(&msg->msg_iter, psize);
 		return ret;
 	}
 
@@ -1392,7 +1393,9 @@ static void mptcp_worker(struct work_struct *work)
 	struct mptcp_data_frag *dfrag;
 	u64 orig_write_seq;
 	size_t copied = 0;
-	struct msghdr msg;
+	struct msghdr msg = {
+		.msg_flags = MSG_DONTWAIT,
+	};
 	long timeo = 0;
 
 	lock_sock(sk);
@@ -1425,7 +1428,6 @@ static void mptcp_worker(struct work_struct *work)
 
 	lock_sock(ssk);
 
-	msg.msg_flags = MSG_DONTWAIT;
 	orig_len = dfrag->data_len;
 	orig_offset = dfrag->offset;
 	orig_write_seq = dfrag->data_seq;