bgp: validate message length in unit tests

In normal operation, `Message::get_message_len()` is used to
ensure the input buffer contains a complete BGP message before
decoding. Apply the same validation in unit tests to avoid passing
malformed data to the decoding function, which assumes the message
length is valid.

Additionally, simplify the logic for determining the presence of
the NLRI field to improve readability.

Fixes #44

Signed-off-by: Renato Westphal <renato@opensourcerouting.org>

holo-bgp/src/packet/message.rs

@@ -781,8 +781,7 @@ impl UpdateMsg {
         // Path Attributes.
         if attr_len != 0 {
             let mut buf_attr = buf.copy_to_bytes(attr_len as usize);
-            let nlri_present =
-                (msg_len - Self::MIN_LEN - wdraw_len - attr_len) > 0;
+            let nlri_present = buf.remaining() > 0;
             attrs = Attrs::decode(
                 &mut buf_attr,
                 cxt,

holo-bgp/tests/packet/mod.rs

@@ -35,6 +35,8 @@ fn test_decode_msg(bytes: &[u8], msg_expected: &Message) {
         capabilities: [NegotiatedCapability::FourOctetAsNumber].into(),
     };
 
-    let msg_actual = Message::decode(&bytes, &cxt).unwrap();
+    let msg_size = Message::get_message_len(&bytes)
+        .expect("Buffer doesn't contain a full BGP message");
+    let msg_actual = Message::decode(&bytes[0..msg_size], &cxt).unwrap();
     assert_eq!(*msg_expected, msg_actual);
 }

holo-bgp/tests/packet/update.rs

@@ -185,7 +185,19 @@ fn test_decode_malformed_updates() {
             0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
             0x00,
         ],
+        // Malformed message length.
+        vec![
+            0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff, 0xff,
+            0xff, 0xff, 0xff, 0xff, 0xff, 0x00, 0x1b, 0x02, 0x00, 0x00, 0x00,
+            0x2d, 0x00, 0x00, 0x00, 0xf5, 0x00, 0x00, 0x00, 0x00, 0x14, 0x9f,
+            0x49, 0x00, 0x00, 0xff, 0x00, 0x00, 0x00, 0x00, 0x14, 0x9f, 0x49,
+            0x00, 0x00, 0x00, 0x00, 0x00, 0x2d, 0x00, 0x00, 0x00, 0xf5, 0x00,
+            0x00, 0x00, 0x00, 0x14, 0x9f, 0x49, 0x00, 0x00, 0xff, 0x00, 0x00,
+            0x00, 0x00, 0x14, 0x9f, 0x49, 0x00, 0x00,
+        ],
     ] {
-        let _ = Message::decode(bytes, &cxt);
+        if let Some(msg_size) = Message::get_message_len(bytes) {
+            let _ = Message::decode(&bytes[0..msg_size], &cxt);
+        }
     }
 }