I host a small, private Hexxit server. We were running into an issue with the 'moved too quickly' and 'moved wrongly' SMP checks interfering with the hookshot mods that are included in the pack. More specifically, players would get caught in a horrible tug-of-war between a hookshot pulling the player towards a location, and Minecraft repeatedly teleporting them back to their starting location.

 

Since cheating isn't an issue on this server, I fixed the problem by writing a small coremod that patches those checks out of the 'NetServerHandler.handleFlying' method. I haven't had any issues since doing that, but this seems like an intrusive way to fix a simple issue. I was wondering if anybody knows of a cleaner way to do this (preferably without the coremod).

 

If that isn't possible, then I would appreciate somebody having a look at the code for my IClassTransformer implementation below. I'm completely unfamiliar with modding Minecraft (although not with programming in general), so while the changes I have added seem to work, there is a high chance that I have done something terribly wrong:

package jmn.mods.mtqfix;

import java.util.logging.Level;
import java.util.logging.Logger;

import org.objectweb.asm.ClassReader;
import org.objectweb.asm.ClassWriter;
import org.objectweb.asm.tree.AbstractInsnNode;
import org.objectweb.asm.tree.ClassNode;
import org.objectweb.asm.tree.InsnList;
import org.objectweb.asm.tree.JumpInsnNode;
import org.objectweb.asm.tree.LabelNode;
import org.objectweb.asm.tree.LdcInsnNode;
import org.objectweb.asm.tree.MethodNode;
import org.objectweb.asm.Opcodes;

import cpw.mods.fml.common.FMLLog;
import cpw.mods.fml.relauncher.IClassTransformer;

public class AsmTransformer implements IClassTransformer {
private static Logger logger = FMLLog.getLogger();

@Override
public byte[] transform(String name, String transformedName, byte[] bytes) {
	if(transformedName.equals("net.minecraft.network.NetServerHandler")) {
		ClassReader reader = new ClassReader(bytes);
		ClassNode node = new ClassNode();
		reader.accept(node, 0);

		if(analyzeClass(node)) {
			ClassWriter writer = new ClassWriter(ClassWriter.COMPUTE_MAXS | ClassWriter.COMPUTE_FRAMES);
			node.accept(writer);
			bytes = writer.toByteArray();
		} else {
			logger.log(Level.SEVERE, "[MTQFix] Failed to patch methods");
		}
	}

	return bytes;
}

private boolean analyzeClass(ClassNode node) {
	for(MethodNode method : node.methods) {
		// Method NetServerHandler.handleFlying()
		if(method.name.equals("a") && method.desc.equals("(Lee;)V")) {
			return disableMovedTooQuickly(method) && disableMovedWrongly(method);
		}
	}

	logger.log(Level.SEVERE, "[MTQFix] Unabled to find method to patch");
	return false;
}

private static enum MovedWronglyState {
	FIND_LDC,
	FIND_DCMPL,
	FIND_IFLE,
	DONE
}

/** 
 * Before:
 * LDC2_W 0.0625
 * DCMPL
 * IFLE <label>
 * (code to execute if the value was greater)
 * 
 * After:
 * LDC2_W 0.0625
 * DCMPL
 * IFLE <label>
 * GOTO <same label>
 * (code that no longer ever gets executed)
 * 
 * I leave the IFLE in just so the DCMPL result gets popped off the stack
 */
private boolean disableMovedWrongly(MethodNode node) {
	MovedWronglyState state = MovedWronglyState.FIND_LDC;
	InsnList insnList = node.instructions;
	AbstractInsnNode insn = insnList.getFirst();
	AbstractInsnNode patchTarget = null;
	LabelNode gotoLabel = null;

	while(insn != null && state != MovedWronglyState.DONE) {
		switch(state) {
		case FIND_LDC:
			if(insn.getOpcode() == Opcodes.LDC) {
				Object value = ((LdcInsnNode)insn).cst;
				// 0.0625 is 1/16, which can be perfectly represented by a float (power of two denominator)
				if(value instanceof Double && ((Double)value).doubleValue() == 0.0625) {
					state = MovedWronglyState.FIND_DCMPL;
				}

			}
			break;

		case FIND_DCMPL:
			if(insn.getOpcode() == Opcodes.DCMPL) {
				state = MovedWronglyState.FIND_IFLE;
			} else {
				state = MovedWronglyState.FIND_LDC;
			}
			break;

		case FIND_IFLE:
			if(insn.getOpcode() == Opcodes.IFLE) {
				state = MovedWronglyState.DONE;
				gotoLabel = ((JumpInsnNode)insn).label;
				patchTarget = insn;
			} else {
				state = MovedWronglyState.FIND_LDC;
			}
			break;
		case DONE:
			break;
		default:
			break;
		}

		insn = insn.getNext();
	}

	if(state == MovedWronglyState.DONE) {
		applyGotoPatch(insnList, gotoLabel, patchTarget);
		return true;
	} else {
		return false;
	}

}

private static enum MovedTooQuicklyState {
	FIND_LDC,
	FIND_DCMPL,
	FIND_IFLE,
	DONE
}

/** 
 * Before:
 * LDC2_W 100.0
 * DCMPL
 * IFLE <label>
 * (code to execute if the value was greater)
 * 
 * After:
 * LDC2_W 100.0
 * DCMPL
 * IFLE <label>
 * GOTO <same label>
 * (code that no longer ever gets executed)
 * 
 * As above, I leave the IFLE in so the DCMPL result gets popped off the stack
 */
private boolean disableMovedTooQuickly(MethodNode node) {
	MovedTooQuicklyState state = MovedTooQuicklyState.FIND_LDC;
	InsnList insnList = node.instructions;
	AbstractInsnNode insn = insnList.getFirst();
	AbstractInsnNode patchTarget = null;
	LabelNode gotoLabel = null;

	while(insn != null && state != MovedTooQuicklyState.DONE) {
		switch(state) {
		case FIND_LDC:
			if(insn.getOpcode() == Opcodes.LDC) {
				Object value = ((LdcInsnNode)insn).cst;
				if(value instanceof Double && ((Double)value).doubleValue() == 100.0) {
					state = MovedTooQuicklyState.FIND_DCMPL;
				}
			}
			break;

		case FIND_DCMPL:
			if(insn.getOpcode() == Opcodes.DCMPL) {
				state = MovedTooQuicklyState.FIND_IFLE;
			} else {
				state = MovedTooQuicklyState.FIND_LDC;
			}
			break;

		case FIND_IFLE:
			if(insn.getOpcode() == Opcodes.IFLE) {
				state = MovedTooQuicklyState.DONE;
				gotoLabel = ((JumpInsnNode)insn).label;
				patchTarget = insn;
			} else {
				state = MovedTooQuicklyState.FIND_LDC;
			}
			break;
		case DONE:
			break;
		default:
			break;
		}

		insn = insn.getNext();
	}

	if(state == MovedTooQuicklyState.DONE) {
		applyGotoPatch(insnList, gotoLabel, patchTarget);
		return true;
	} else {
		return false;
	}

}

private void applyGotoPatch(InsnList instructions, LabelNode label, AbstractInsnNode target) {
	instructions.insert(target, generatePatch(label));
}

private InsnList generatePatch(LabelNode label) {
	InsnList list = new InsnList();
	list.add(new JumpInsnNode(Opcodes.GOTO, label));
	return list;
}
}

 

I have tried to write this code so that it won't break if somebody patches the same method by adding or removing instructions, unless they patch the same area I am working on here.

Hi there guy. I'm not sure if that's the safest way to do it, but if it's working for you. Could you post a copy of the mod. I'm having the same problem and I have no idea how to code a mod like that.

Hi there guy. I'm not sure if that's the safest way to do it, but if it's working for you. Could you post a copy of the mod. I'm having the same problem and I have no idea how to code a mod like that.

I might actually make my mod download this mod into the core-mods folder in the pre-init method just to remove that problem with one of my mods.

I'll upload the mod, but I really have to caution against using it. I have very limited previous Minecraft modding knowledge, so there's no telling what I've broken. Even if I haven't broken anything, this will still allow cheaters to teleport around your world at whim.

 

With that in mind, both the jar and the source code are available here. Just stick the mtqfix.jar file in the coremods directory of your server (clients do not need it). The mod will be listed as 'disabled' in the mods list; this is normal.

 

To give credit where due, I picked most of this up by looking at the ChickenChunksCore source code (in addition to the official documentation for ASM4, and a tutorial on using access transformers).

 

e:

I might actually make my mod download this mod into the core-mods folder in the pre-init method just to remove that problem with one of my mods.

Coremods get loaded before Minecraft itself gets loaded. As best I was able to tell, Forge allows for bytecode patching by using a custom classloader. By the time your preInit method gets called, it is far too late to do anything of that sort.

 

At any rate, if you are writing your own mod, then I guarantee you that there will be ways to fix this without resorting to cludgy bytecode hacks. On the simple end of things, you could try clamping your velocity to a value marginally under 100.

 

e:e:

vvv

AFAICT, the issue with having the server do it is managing player velocity. There are no problems if you set the velocity to 0, but then the client will receive a visibly jarring position update each time you update the position. If you set the velocity appropriately, then you are going to run into the 'moved too quickly' issue as soon as the client sends a position update based on that velocity. Or at least, that's my understanding.

Though it not much help if your using a mod by another coder, having the server move the player gets around this issue.

 

when I first attempted moving the player I did it client side and kept getting the moved wrongly error.

Now server side I'm using entityPlayer.setPositionAndUpdate(placeX + .5, placeY, placeZ + .5); to move the player.

 

I wanted to upgrade this to 1.11.2 but I couldn't find the complete source code. Does anyone have it?

Sorry for the  re-bump/post, but I'm having this problem on a singleplayer world, as well as a multiplayer LAN world;  and when I placed this into my mods folder, MCForge listed it as disabled, and the errors in the latest.log (Player loved too quickly) stopped appearing, but it seems that the issue of the game kicking you back still persists, any way that this can be fixed? Gets kinda annoying when I'm doing stuff on a suspended platform...