[CIFS] Do not send SMBEcho requests on new sockets until SMBNegotiate

In order to determine whether an SMBEcho request can be sent
we need to know that the socket is established (server tcpStatus == CifsGood)
AND that an SMB NegotiateProtocol has been sent (server maxBuf != 0).
Without the second check we can send an Echo request during reconnection
before the server can accept it.

CC: JG <jg@cms.ac>
Reviewed-by: Jeff Layton <jlayton@redhat.com>
Signed-off-by: Steve French <sfrench@us.ibm.com>

fs/cifs/cifsglob.h

@@ -188,6 +188,8 @@ struct TCP_Server_Info {
 	/* multiplexed reads or writes */
 	unsigned int maxBuf;	/* maxBuf specifies the maximum */
 	/* message size the server can send or receive for non-raw SMBs */
+	/* maxBuf is returned by SMB NegotiateProtocol so maxBuf is only 0 */
+	/* when socket is setup (and during reconnect) before NegProt sent */
 	unsigned int max_rw;	/* maxRw specifies the maximum */
 	/* message size the server can send or receive for */
 	/* SMB_COM_WRITE_RAW or SMB_COM_READ_RAW. */

fs/cifs/connect.c

@@ -341,7 +341,7 @@ cifs_echo_request(struct work_struct *work)
 	 * We cannot send an echo until the NEGOTIATE_PROTOCOL request is done.
 	 * Also, no need to ping if we got a response recently
 	 */
-	if (server->tcpStatus != CifsGood ||
+	if ((server->tcpStatus != CifsGood) || (server->maxBuf == 0) ||
 	    time_before(jiffies, server->lstrp + SMB_ECHO_INTERVAL - HZ))
 		goto requeue_echo;