GCS_MAVLink: deny setting MIS_TOTAL parameter

Closes #11413

GCS_MAVLink: conform to parameter protocol requirements while ignoring sets

GCS_MAVLink: use internal-use-only param bit

libraries/GCS_MAVLink/GCS.h

@@ -116,6 +116,10 @@ public:
     virtual uint8_t sysid_my_gcs() const = 0;
     virtual bool sysid_enforce() const { return false; }
 
+    void send_parameter_value(const char *param_name,
+                              ap_var_type param_type,
+                              float param_value);
+
     // NOTE! The streams enum below and the
     // set of AP_Int16 stream rates _must_ be
     // kept in the same order

libraries/GCS_MAVLink/GCS_Param.cpp

@@ -268,12 +268,24 @@ void GCS_MAVLINK::handle_param_set(const mavlink_message_t &msg)
     key[AP_MAX_NAME_SIZE] = 0;
 
     // find existing param so we can get the old value
-    vp = AP_Param::find(key, &var_type);
+    uint16_t parameter_flags = 0;
+    vp = AP_Param::find(key, &var_type, &parameter_flags);
     if (vp == nullptr) {
         return;
     }
+
     float old_value = vp->cast_to_float(var_type);
 
+    if (parameter_flags & AP_PARAM_FLAG_INTERNAL_USE_ONLY) {
+        gcs().send_text(MAV_SEVERITY_WARNING, "Param write denied (%s)", key);
+        // echo back the incorrect value so that we fulfull the
+        // parameter state machine requirements:
+        send_parameter_value(key, var_type, packet.param_value);
+        // and then announce what the correct value is:
+        send_parameter_value(key, var_type, old_value);
+        return;
+    }
+
     // set the value
     vp->set_float(packet.param_value, var_type);
 
@@ -295,6 +307,20 @@ void GCS_MAVLINK::handle_param_set(const mavlink_message_t &msg)
     }
 }
 
+void GCS_MAVLINK::send_parameter_value(const char *param_name, ap_var_type param_type, float param_value)
+{
+    if (!HAVE_PAYLOAD_SPACE(chan, PARAM_VALUE)) {
+        return;
+    }
+    mavlink_msg_param_value_send(
+        chan,
+        param_name,
+        param_value,
+        mav_param_type(param_type),
+        AP_Param::count_parameters(),
+        -1);
+}
+
 /*
   send a parameter value message to all active MAVLink connections
  */