Assert that the transmit frame returned by driver has right length.

We checked for frames too short before, but frames too long are
troublesome too, since e.g. TCP and UDP do not carry an explicit
payload length in their headers.

src/iface/ethernet.rs

@@ -368,11 +368,12 @@ impl<'a, 'b, 'c, DeviceT: Device + 'a> Interface<'a, 'b, 'c, DeviceT> {
                         Some(hardware_addr) => hardware_addr
                     };
 
-                let frame_len = EthernetFrame::<&[u8]>::buffer_len($ip_repr.buffer_len() +
-                                                                   $ip_repr.payload_len);
-                $tx_buffer = self.device.transmit(timestamp, frame_len)?;
-                $frame = EthernetFrame::new_checked(&mut $tx_buffer)
-                                       .expect("transmit frame too small");
+                let tx_len = EthernetFrame::<&[u8]>::buffer_len($ip_repr.buffer_len() +
+                                                                $ip_repr.payload_len);
+                $tx_buffer = self.device.transmit(timestamp, tx_len)?;
+                debug_assert!($tx_buffer.as_ref().len() == tx_len);
+
+                $frame = EthernetFrame::new(&mut $tx_buffer);
                 $frame.set_src_addr(self.hardware_addr);
                 $frame.set_dst_addr(dst_hardware_addr);
                 $frame.set_ethertype(EthernetProtocol::Ipv4);
@@ -387,8 +388,9 @@ impl<'a, 'b, 'c, DeviceT: Device + 'a> Interface<'a, 'b, 'c, DeviceT> {
             Response::Arp(repr) => {
                 let tx_len = EthernetFrame::<&[u8]>::buffer_len(repr.buffer_len());
                 let mut tx_buffer = self.device.transmit(timestamp, tx_len)?;
-                let mut frame = EthernetFrame::new_checked(&mut tx_buffer)
-                                              .expect("transmit frame too small");
+                debug_assert!(tx_buffer.as_ref().len() == tx_len);
+
+                let mut frame = EthernetFrame::new(&mut tx_buffer);
                 frame.set_src_addr(self.hardware_addr);
                 frame.set_dst_addr(match repr {
                     ArpRepr::EthernetIpv4 { target_hardware_addr, .. } => target_hardware_addr,
@@ -449,8 +451,9 @@ impl<'a, 'b, 'c, DeviceT: Device + 'a> Interface<'a, 'b, 'c, DeviceT> {
                         let tx_len = EthernetFrame::<&[u8]>::buffer_len(repr.buffer_len() +
                                                                         payload.buffer_len());
                         let mut tx_buffer = device.transmit(timestamp, tx_len)?;
-                        let mut frame = EthernetFrame::new_checked(&mut tx_buffer)
-                                                      .expect("transmit frame too small");
+                        debug_assert!(tx_buffer.as_ref().len() == tx_len);
+
+                        let mut frame = EthernetFrame::new(&mut tx_buffer);
                         frame.set_src_addr(src_hardware_addr);
                         frame.set_dst_addr(dst_hardware_addr);
                         frame.set_ethertype(EthernetProtocol::Ipv4);
@@ -480,8 +483,9 @@ impl<'a, 'b, 'c, DeviceT: Device + 'a> Interface<'a, 'b, 'c, DeviceT> {
 
                         let tx_len = EthernetFrame::<&[u8]>::buffer_len(payload.buffer_len());
                         let mut tx_buffer = device.transmit(timestamp, tx_len)?;
-                        let mut frame = EthernetFrame::new_checked(&mut tx_buffer)
-                                                      .expect("transmit frame too small");
+                        debug_assert!(tx_buffer.as_ref().len() == tx_len);
+
+                        let mut frame = EthernetFrame::new(&mut tx_buffer);
                         frame.set_src_addr(src_hardware_addr);
                         frame.set_dst_addr(EthernetAddress([0xff; 6]));
                         frame.set_ethertype(EthernetProtocol::Arp);