Plane: added SYSID_ENFORCE parameter

this allows safer sharing of telemetry links between aircraft

ArduPlane/GCS_Mavlink.cpp

@@ -2432,3 +2432,17 @@ void Plane::gcs_retry_deferred(void)
     gcs_send_message(MSG_RETRY_DEFERRED);
     GCS_MAVLINK::service_statustext();
 }
+
+/*
+  return true if we will accept this packet. Used to implement SYSID_ENFORCE
+ */
+bool GCS_MAVLINK_Plane::accept_packet(const mavlink_status_t &status, mavlink_message_t &msg)
+{
+    if (!plane.g2.sysid_enforce) {
+        return true;
+    }
+    if (msg.msgid == MAVLINK_MSG_ID_RADIO || msg.msgid == MAVLINK_MSG_ID_RADIO_STATUS) {
+        return true;
+    }
+    return (msg.sysid == plane.g.sysid_my_gcs);
+}

ArduPlane/GCS_Mavlink.h

@@ -13,6 +13,8 @@ protected:
 
     uint32_t telem_delay() const override;
 
+    bool accept_packet(const mavlink_status_t &status, mavlink_message_t &msg) override;
+    
 private:
 
     void handleMessage(mavlink_message_t * msg) override;

ArduPlane/Parameters.cpp

@@ -1382,6 +1382,13 @@ const AP_Param::GroupInfo ParametersG2::var_info[] = {
     // @Group: SERVO_
     // @Path: ../libraries/RC_Channel/SRV_Channel.cpp
     AP_SUBGROUPINFO(servo_channels, "SERVO", 3, ParametersG2, SRV_Channels),
+
+    // @Group: SYSID_ENFORCE
+    // @DisplayName: GCS sysid enforcement
+    // @Description: This controls whether packets from other than the expected GCS system ID will be accepted
+    // @Values: 0:NotEnforced,1:Enforced
+    // @User: Advanced
+    AP_GROUPINFO("SYSID_ENFORCE", 4, ParametersG2, sysid_enforce, 0),
     
     AP_GROUPEND
 };

ArduPlane/Parameters.h

@@ -588,6 +588,9 @@ public:
 
     // control over servo output ranges
     SRV_Channels servo_channels;
+
+    // whether to enforce acceptance of packets only from sysid_my_gcs
+    AP_Int8 sysid_enforce;
 };
 
 extern const AP_Param::Info var_info[];