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/PlayerConnection.java b/src/main/java/net/minecraft/server/network/PlayerConnection.java
index b7b431a649c6bc3066c267783007eee4384d27bd..c0c9af81a933135e940587c10155a24cd399b922 100644
--- a/src/main/java/net/minecraft/server/network/PlayerConnection.java
+++ b/src/main/java/net/minecraft/server/network/PlayerConnection.java
@@ -256,6 +256,7 @@ public class PlayerConnection implements PacketListenerPlayIn {
     private int E;
     private int receivedMovePackets;
     private int processedMovePackets;
+    private static final int MAX_SIGN_LINE_LENGTH = Integer.getInteger("Paper.maxSignLength", 80);
     private static final long KEEPALIVE_LIMIT = Long.getLong("paper.playerconnection.keepalive", 30) * 1000; // Paper - provide property to set keepalive limit
 
     public PlayerConnection(MinecraftServer minecraftserver, NetworkManager networkmanager, EntityPlayer entityplayer) {
@@ -2862,7 +2863,17 @@ public class PlayerConnection implements PacketListenerPlayIn {
             List<net.kyori.adventure.text.Component> lines = new java.util.ArrayList<>();
 
             for (int i = 0; i < list.size(); ++i) {
-                lines.add(net.kyori.adventure.text.Component.text(SharedConstants.filterAllowedChatCharacters(list.get(i)))); // Paper - Replaced with anvil color stripping method to stop exploits that allow colored signs to be created.
+                // Paper start - cap line length - modified clients can send longer data than normal
+                String currentLine = list.get(i);
+                if (MAX_SIGN_LINE_LENGTH > 0 && currentLine.length() > MAX_SIGN_LINE_LENGTH) {
+                    // This handles multibyte characters as 1
+                    int offset = currentLine.codePoints().limit(MAX_SIGN_LINE_LENGTH).map(Character::charCount).sum();
+                    if (offset < currentLine.length()) {
+                        list.set(i, currentLine = currentLine.substring(0, offset));
+                    }
+                }
+                // Paper end
+                lines.add(net.kyori.adventure.text.Component.text(SharedConstants.filterAllowedChatCharacters(currentLine))); // Paper - Replaced with anvil color stripping method to stop exploits that allow colored signs to be created.
             }
             SignChangeEvent event = new SignChangeEvent(org.bukkit.craftbukkit.block.CraftBlock.at(worldserver, blockposition), this.getPlayer(), lines);
             this.server.getPluginManager().callEvent(event);