Avoid re-allocating to encrypt gossip messages when forwarding

When we forward gossip messages, we store them in a separate buffer
before we encrypt them (and commit to the order in which they'll
appear on the wire). Rather than storing that buffer encoded with
no headroom, requiring re-allocating to add the message length and
two MAC blocks, we here add the headroom prior to pushing it into
the gossip buffer, avoiding an allocation.

Author: TheBlueMatt

fuzz/src/peer_crypt.rs

@@ -7,7 +7,7 @@
 // You may not use this file except in accordance with one or both of these
 // licenses.
 
-use lightning::ln::peer_channel_encryptor::PeerChannelEncryptor;
+use lightning::ln::peer_channel_encryptor::{PeerChannelEncryptor, MessageBuf};
 use lightning::util::test_utils::TestNodeSigner;
 
 use bitcoin::secp256k1::{Secp256k1, PublicKey, SecretKey};
@@ -77,7 +77,7 @@ pub fn do_test(data: &[u8]) {
 	let mut buf = [0; 65536 + 16];
 	loop {
 		if get_slice!(1)[0] == 0 {
-			crypter.encrypt_buffer(get_slice!(slice_to_be16(get_slice!(2))));
+			crypter.encrypt_buffer(MessageBuf::from_encoded(&get_slice!(slice_to_be16(get_slice!(2)))));
 		} else {
 			let len = match crypter.decrypt_length_header(get_slice!(16+2)) {
 				Ok(len) => len,

lightning/src/ln/peer_channel_encryptor.rs

@@ -427,16 +427,21 @@ impl PeerChannelEncryptor {
 		Ok(self.their_node_id.unwrap().clone())
 	}
 
-	/// Encrypts the given pre-serialized message, returning the encrypted version.
-	/// panics if msg.len() > 65535 or Noise handshake has not finished.
-	pub fn encrypt_buffer(&mut self, msg: &[u8]) -> Vec<u8> {
-		if msg.len() > LN_MAX_MSG_LEN {
+	/// Builds sendable bytes for a message.
+	///
+	/// `msgbuf` must begin with 16 + 2 dummy/0 bytes, which will be filled with the encrypted
+	/// message length and its MAC. It should then be followed by the message bytes themselves
+	/// (including the two byte message type).
+	///
+	/// For effeciency, the [`Vec::capacity`] should be at least 16 bytes larger than the
+	/// [`Vec::length`], to avoid reallocating for the message MAC, which will be appended to the
+	/// vec.
+	fn encrypt_message_with_header_0s(&mut self, msgbuf: &mut Vec<u8>) {
+		let msg_len = msgbuf.len() - 16 - 2;
+		if msg_len > LN_MAX_MSG_LEN {
 			panic!("Attempted to encrypt message longer than 65535 bytes!");
 		}
 
-		let mut res = Vec::with_capacity(msg.len() + 16*2 + 2);
-		res.resize(msg.len() + 16*2 + 2, 0);
-
 		match self.noise_state {
 			NoiseState::Finished { ref mut sk, ref mut sn, ref mut sck, rk: _, rn: _, rck: _ } => {
 				if *sn >= 1000 {
@@ -446,16 +451,21 @@ impl PeerChannelEncryptor {
 					*sn = 0;
 				}
 
-				Self::encrypt_with_ad(&mut res[0..16+2], *sn, sk, &[0; 0], &(msg.len() as u16).to_be_bytes());
+				Self::encrypt_with_ad(&mut msgbuf[0..16+2], *sn, sk, &[0; 0], &(msg_len as u16).to_be_bytes());
 				*sn += 1;
 
-				Self::encrypt_with_ad(&mut res[16+2..], *sn, sk, &[0; 0], msg);
+				Self::encrypt_in_place_with_ad(msgbuf, 16+2, *sn, sk, &[0; 0]);
 				*sn += 1;
 			},
 			_ => panic!("Tried to encrypt a message prior to noise handshake completion"),
 		}
+	}
 
-		res
+	/// Encrypts the given pre-serialized message, returning the encrypted version.
+	/// panics if msg.len() > 65535 or Noise handshake has not finished.
+	pub fn encrypt_buffer(&mut self, mut msg: MessageBuf) -> Vec<u8> {
+		self.encrypt_message_with_header_0s(&mut msg.0);
+		msg.0
 	}
 
 	/// Encrypts the given message, returning the encrypted version.
@@ -468,29 +478,7 @@ impl PeerChannelEncryptor {
 		res.0.resize(16 + 2, 0);
 		wire::write(message, &mut res).expect("In-memory messages must never fail to serialize");
 
-		let msg_len = res.0.len() - 16 - 2;
-		if msg_len > LN_MAX_MSG_LEN {
-			panic!("Attempted to encrypt message longer than 65535 bytes!");
-		}
-
-		match self.noise_state {
-			NoiseState::Finished { ref mut sk, ref mut sn, ref mut sck, rk: _, rn: _, rck: _ } => {
-				if *sn >= 1000 {
-					let (new_sck, new_sk) = hkdf_extract_expand_twice(sck, sk);
-					*sck = new_sck;
-					*sk = new_sk;
-					*sn = 0;
-				}
-
-				Self::encrypt_with_ad(&mut res.0[0..16+2], *sn, sk, &[0; 0], &(msg_len as u16).to_be_bytes());
-				*sn += 1;
-
-				Self::encrypt_in_place_with_ad(&mut res.0, 16+2, *sn, sk, &[0; 0]);
-				*sn += 1;
-			},
-			_ => panic!("Tried to encrypt a message prior to noise handshake completion"),
-		}
-
+		self.encrypt_message_with_header_0s(&mut res.0);
 		res.0
 	}
 
@@ -557,9 +545,30 @@ impl PeerChannelEncryptor {
 	}
 }
 
+/// A buffer which stores an encoded message (including the two message-type bytes) with some
+/// padding to allow for future encryption/MACing.
+pub struct MessageBuf(Vec<u8>);
+impl MessageBuf {
+	/// Creates a new buffer from an encoded message (i.e. the two message-type bytes followed by
+	/// the message contents).
+	///
+	/// Panics if the message is longer than 2^16.
+	pub fn from_encoded(encoded_msg: &[u8]) -> Self {
+		if encoded_msg.len() > LN_MAX_MSG_LEN {
+			panic!("Attempted to encrypt message longer than 65535 bytes!");
+		}
+		// In addition to the message (continaing the two message type bytes), we also have to add
+		// the message length header (and its MAC) and the message MAC.
+		let mut res = Vec::with_capacity(encoded_msg.len() + 16*2 + 2);
+		res.resize(encoded_msg.len() + 16 + 2, 0);
+		res[16 + 2..].copy_from_slice(&encoded_msg);
+		Self(res)
+	}
+}
+
 #[cfg(test)]
 mod tests {
-	use super::LN_MAX_MSG_LEN;
+	use super::{MessageBuf, LN_MAX_MSG_LEN};
 
 	use bitcoin::secp256k1::{PublicKey, SecretKey};
 	use bitcoin::secp256k1::Secp256k1;
@@ -775,7 +784,7 @@ mod tests {
 
 		for i in 0..1005 {
 			let msg = [0x68, 0x65, 0x6c, 0x6c, 0x6f];
-			let mut res = outbound_peer.encrypt_buffer(&msg);
+			let mut res = outbound_peer.encrypt_buffer(MessageBuf::from_encoded(&msg));
 			assert_eq!(res.len(), 5 + 2*16 + 2);
 
 			let len_header = res[0..2+16].to_vec();
@@ -811,7 +820,7 @@ mod tests {
 	fn max_message_len_encryption() {
 		let mut outbound_peer = get_outbound_peer_for_initiator_test_vectors();
 		let msg = [4u8; LN_MAX_MSG_LEN + 1];
-		outbound_peer.encrypt_buffer(&msg);
+		outbound_peer.encrypt_buffer(MessageBuf::from_encoded(&msg));
 	}
 
 	#[test]

lightning/src/ln/peer_handler.rs

@@ -27,7 +27,7 @@ use crate::ln::msgs::{ChannelMessageHandler, LightningError, SocketAddress, Onio
 #[cfg(not(c_bindings))]
 use crate::ln::channelmanager::{SimpleArcChannelManager, SimpleRefChannelManager};
 use crate::util::ser::{VecWriter, Writeable, Writer};
-use crate::ln::peer_channel_encryptor::{PeerChannelEncryptor, NextNoiseStep, MSG_BUF_ALLOC_SIZE};
+use crate::ln::peer_channel_encryptor::{PeerChannelEncryptor, NextNoiseStep, MessageBuf, MSG_BUF_ALLOC_SIZE};
 use crate::ln::wire;
 use crate::ln::wire::{Encode, Type};
 #[cfg(not(c_bindings))]
@@ -495,7 +495,7 @@ struct Peer {
 	/// prioritize channel messages over them.
 	///
 	/// Note that these messages are *not* encrypted/MAC'd, and are only serialized.
-	gossip_broadcast_buffer: VecDeque<Vec<u8>>,
+	gossip_broadcast_buffer: VecDeque<MessageBuf>,
 	awaiting_write_event: bool,
 
 	pending_read_buffer: Vec<u8>,
@@ -1102,7 +1102,7 @@ impl<Descriptor: SocketDescriptor, CM: Deref, RM: Deref, OM: Deref, L: Deref, CM
 			}
 			if peer.should_buffer_gossip_broadcast() {
 				if let Some(msg) = peer.gossip_broadcast_buffer.pop_front() {
-					peer.pending_outbound_buffer.push_back(peer.channel_encryptor.encrypt_buffer(&msg[..]));
+					peer.pending_outbound_buffer.push_back(peer.channel_encryptor.encrypt_buffer(msg));
 				}
 			}
 			if peer.should_buffer_gossip_backfill() {
@@ -1251,7 +1251,7 @@ impl<Descriptor: SocketDescriptor, CM: Deref, RM: Deref, OM: Deref, L: Deref, CM
 	}
 
 	/// Append a message to a peer's pending outbound/write gossip broadcast buffer
-	fn enqueue_encoded_gossip_broadcast(&self, peer: &mut Peer, encoded_message: Vec<u8>) {
+	fn enqueue_encoded_gossip_broadcast(&self, peer: &mut Peer, encoded_message: MessageBuf) {
 		peer.msgs_sent_since_pong += 1;
 		debug_assert!(peer.gossip_broadcast_buffer.len() <= OUTBOUND_BUFFER_LIMIT_DROP_GOSSIP);
 		peer.gossip_broadcast_buffer.push_back(encoded_message);
@@ -1800,7 +1800,7 @@ impl<Descriptor: SocketDescriptor, CM: Deref, RM: Deref, OM: Deref, L: Deref, CM
 					if except_node.is_some() && peer.their_node_id.as_ref().map(|(pk, _)| pk) == except_node {
 						continue;
 					}
-					self.enqueue_encoded_gossip_broadcast(&mut *peer, encoded_msg.clone());
+					self.enqueue_encoded_gossip_broadcast(&mut *peer, MessageBuf::from_encoded(&encoded_msg));
 				}
 			},
 			wire::Message::NodeAnnouncement(ref msg) => {
@@ -1827,7 +1827,7 @@ impl<Descriptor: SocketDescriptor, CM: Deref, RM: Deref, OM: Deref, L: Deref, CM
 					if except_node.is_some() && peer.their_node_id.as_ref().map(|(pk, _)| pk) == except_node {
 						continue;
 					}
-					self.enqueue_encoded_gossip_broadcast(&mut *peer, encoded_msg.clone());
+					self.enqueue_encoded_gossip_broadcast(&mut *peer, MessageBuf::from_encoded(&encoded_msg));
 				}
 			},
 			wire::Message::ChannelUpdate(ref msg) => {
@@ -1849,7 +1849,7 @@ impl<Descriptor: SocketDescriptor, CM: Deref, RM: Deref, OM: Deref, L: Deref, CM
 					if except_node.is_some() && peer.their_node_id.as_ref().map(|(pk, _)| pk) == except_node {
 						continue;
 					}
-					self.enqueue_encoded_gossip_broadcast(&mut *peer, encoded_msg.clone());
+					self.enqueue_encoded_gossip_broadcast(&mut *peer, MessageBuf::from_encoded(&encoded_msg));
 				}
 			},
 			_ => debug_assert!(false, "We shouldn't attempt to forward anything but gossip messages"),