From 0000000000000000000000000000000000000000 Mon Sep 17 00:00:00 2001
From: Aikar <aikar@aikar.co>
Date: Wed, 27 Feb 2019 22:18:40 -0500
Subject: [PATCH] Limit Client Sign length more

modified clients can send more data from the client
to the server and it would get stored on the sign as sent.

Mojang has a limit of 384 which is much higher than reasonable.

the client can barely render around 16 characters as-is, but formatting
codes can get it to be more than 16 actual length.

Set a limit of 80 which should give an average of 16 characters 2
sets of legacy formatting codes which should be plenty for all uses.

This does not strip any existing data from the NBT as plugins
may use this for storing data out of the rendered area.

it only impacts data sent from the client.

Set -DPaper.maxSignLength=XX to change limit or -1 to disable

diff --git a/src/main/java/net/minecraft/server/network/ServerGamePacketListenerImpl.java b/src/main/java/net/minecraft/server/network/ServerGamePacketListenerImpl.java
index 833fb2f340065a72903620f974e6ac71cc86f3f9..00fa6126ca51b6ffdb46b59f07725ffdcca9f20a 100644
--- a/src/main/java/net/minecraft/server/network/ServerGamePacketListenerImpl.java
+++ b/src/main/java/net/minecraft/server/network/ServerGamePacketListenerImpl.java
@@ -291,6 +291,7 @@ public class ServerGamePacketListenerImpl extends ServerCommonPacketListenerImpl
     private final MessageSignatureCache messageSignatureCache = MessageSignatureCache.createDefault();
     private final FutureChain chatMessageChain;
     private boolean waitingForSwitchToConfig;
+    private static final int MAX_SIGN_LINE_LENGTH = Integer.getInteger("Paper.maxSignLength", 80); // Paper
 
     public ServerGamePacketListenerImpl(MinecraftServer server, Connection connection, ServerPlayer player, CommonListenerCookie clientData) {
         super(server, connection, clientData, player); // CraftBukkit
@@ -3066,7 +3067,19 @@ public class ServerGamePacketListenerImpl extends ServerCommonPacketListenerImpl
 
     @Override
     public void handleSignUpdate(ServerboundSignUpdatePacket packet) {
-        List<String> list = (List) Stream.of(packet.getLines()).map(ChatFormatting::stripFormatting).collect(Collectors.toList());
+        // Paper start - cap line length - modified clients can send longer data than normal
+        String[] lines = packet.getLines();
+        for (int i = 0; i < lines.length; ++i) {
+            if (MAX_SIGN_LINE_LENGTH > 0 && lines[i].length() > MAX_SIGN_LINE_LENGTH) {
+                // This handles multibyte characters as 1
+                int offset = lines[i].codePoints().limit(MAX_SIGN_LINE_LENGTH).map(Character::charCount).sum();
+                if (offset < lines[i].length()) {
+                    lines[i] = lines[i].substring(0, offset); // this will break any filtering, but filtering is NYI as of 1.17
+                }
+            }
+        }
+        List<String> list = (List) Stream.of(lines).map(ChatFormatting::stripFormatting).collect(Collectors.toList());
+        // Paper end
 
         this.filterTextPacket(list).thenAcceptAsync((list1) -> {
             this.updateSignText(packet, list1);