Fix Ipv4Packet::payload{,_mut}() returning overly long buffers.

These functions only skipped the header, but what they should have
done is cut the packet at its total length and skip the header.
Otherwise, if the buffer the IP packet is constructed from contains
padding, such as Ethernet packets, the payload would be too long.

src/wire/ipv4.rs

@@ -248,9 +248,9 @@ impl<'a, T: AsRef<[u8]> + ?Sized> Packet<&'a T> {
     /// Return a pointer to the payload.
     #[inline]
     pub fn payload(&self) -> &'a [u8] {
-        let range = self.header_len() as usize;
+        let range = self.header_len() as usize..self.total_len() as usize;
         let data = self.buffer.as_ref();
-        &data[range..]
+        &data[range]
     }
 }
 
@@ -381,7 +381,7 @@ impl<'a, T: AsRef<[u8]> + AsMut<[u8]> + ?Sized> Packet<&'a mut T> {
     /// Return a mutable pointer to the payload.
     #[inline]
     pub fn payload_mut(&mut self) -> &mut [u8] {
-        let range = self.header_len() as usize..;
+        let range = self.header_len() as usize..self.total_len() as usize;
         let data = self.buffer.as_mut();
         &mut data[range]
     }
@@ -611,6 +611,18 @@ mod test {
         assert_eq!(&packet.into_inner()[..], &PACKET_BYTES[..]);
     }
 
+    #[test]
+    fn test_overlong() {
+        let mut bytes = vec![];
+        bytes.extend(&PACKET_BYTES[..]);
+        bytes.push(0);
+
+        assert_eq!(Packet::new(&bytes).payload().len(),
+                   PAYLOAD_BYTES.len());
+        assert_eq!(Packet::new(&mut bytes).payload_mut().len(),
+                   PAYLOAD_BYTES.len());
+    }
+
     static REPR_PACKET_BYTES: [u8; 24] =
         [0x45, 0x00, 0x00, 0x18,
          0x00, 0x00, 0x40, 0x00,