Physics change between geyser 2.4.4-SNAPSHOT 2.5.0-SNAPSHOT causing anti-cheat issues

[SergioK29]

Describe the bug

As we all know, despite mojang removing the obfuscation maps, there do still exist cheats for bedrock edition,
therefore as java server owners we must check bedrock players for movement cheats.

I have my own anticheat which checks the physics of bedrock players and it has worked mostly without flaw since 2022 but a recent change in geyser has changed the physics of bedrock players, making it farther from java movement.

I know geyser is the cause as on either bedrock version (1.21.43 / 1.21.44), it works perfectly fine on 2.4.4-SNAPSHOT but flags on 2.5.0-SNAPSHOT, a pattern I have noticed is the difference primarily happens when sprint jumping on slabs or right after sprint-jumping up a block.

To Reproduce

Measure physics differences for the same bedrock client between geyser versions 2.4.4-SNAPSHOT and 2.5.0-SNAPSHOT, often happens while sprint jumping on slabs or sprint jumping up blocks

Expected behaviour

Physics should not have changed, physics of bedrock itself hasn't since at-least 2022.

Screenshots / Videos

No response

Server Version and Plugins

latest paper 1.21.1

This server is running Paper version 1.21.1-DEV-master@d348cb8 (2024-10-30T15:37:43Z) (Implementing API version 1.21.1-R0.1-SNAPSHOT)
You are 138 version(s) behind
Download the new version at: https://papermc.io/downloads/paper
Previous version: 1.21.1-DEV-14a48cd (MC: 1.21.1)

Geyser Dump

https://pastebin.com/2tPsW21u (issue occurs on both test environment and production, 2 very different machines, so likely not an issue caused by plugins/architecture)

Geyser Version

2.5.0-SNAPSHOT 6983095/g

Minecraft: Bedrock Edition Device/Version

Issue with versions 1.21.43 and 1.21.44

Additional Context

I need help finding version 2.4.4-SNAPSHOT in order to drill down which exact commit hash changed physics, I have a jar of it but on the geyser github the version jumps from 2.4.3-SNAPSHOT to 2.5.0-SNAPSHOT

[onebeastchris]

This is likely due to the switch in Geyser from Bedrock's client-auth to server-auth movement. This was forced upon us as Bedrock is due to remove client auth movement this fall.
The Java client performs a lot of movement checks client-side, which Geyser currently doesn't perfectly (if at all) replicate, hence somewhat relying on the Bedrock client. Implementing exact movement is technically possible - with e.g. server auth with rewind even more seo - but is very time-intensive and not currently a priority.

That said, if there are specific examples around "different physics" that you would like to share, we'd consider looking into them. However, the things mentioned were relatively vague - we'd need a bit more (replication steps, mainly; and a better description than just "different physics").

[SergioK29]

its confirmed working on d61ad7b, but when I try to branch from the commit after that, d61ad7b disappears from the git history, wtf? 😕

[Tim203]

It got, most likely, merged with the 1.21.2/3 branch after that.

[SergioK29]

It got, most likely, merged with the 1.21.2/3 branch after that.

how can I roll git back to 4d15c7f without d61ad7b (the commit right before it) disappearing? I tried branch from commit but it disappears, so weird.

[SergioK29]

anywhere I can just browse the jars for each commit hash? would make life alot easier.

[onebeastchris]

This is the latest 2.4.2 build - https://download.geysermc.org/v2/projects/geyser/versions/2.4.4/builds/705/downloads/spigot
That's the one without any of the 1.21.2/3 changes, which introduced server auth movement.

[SergioK29]

This is the latest 2.4.2 build - https://download.geysermc.org/v2/projects/geyser/versions/2.4.4/builds/705/downloads/spigot That's the one without any of the 1.21.2/3 changes, which introduced server auth movement.

Can confirm that this build has no issues whatsoever. This build will work in the short term but leaves me in kind of a tricky spot as I will need to update eventually. is there any way to disable server auth movement or make it so the movement is more accurate?

[Camotoy]

Even if we had that option - and it would be very difficult, as the code required for client versus server movement is radically different - Bedrock will be removing the client option soon, so we don't have a choice but to move.

However, PRs are most certainly open, as this would be something we're interested in fixing on our end. Here is where our movement code is triggered: https://github.com/GeyserMC/Geyser/blob/master/core/src/main/java/org/geysermc/geyser/translator/protocol/bedrock/entity/player/input/BedrockMovePlayer.java
If it's something you would wish to explore, don't hesitate to reach out; you can find us in #dev-general on our Discord server.

[SergioK29]

More testing of what exactly has changed from a server-side perspective.

Before geyser server auth movement (2.4.2):
https://streamable.com/a55egl

After geyser server auth movement (broken) (latest 881b8e9):
https://streamable.com/uup699

2 immediately noticeable problems,
onGround: true lags behind by a tick whenever the player lands on the ground as compared to previous geyser
onGround is always false when on slabs, when on previous geyser it was correctly set to true

[Kasi07]

I am not 100% sure but I thing this could also cause this bug: #5127

[SergioK29]

I am not 100% sure but I thing this could also cause this bug: #5127

I am almost certain that is the cause, fall damage accumulates for as many ticks as onGround=false

[Camotoy]

Please try with the next build once it builds.

[SergioK29]

Please try with the next build once it builds.

Very awesome, this seems to fix the ground behaviour and also my anti-cheat issues

[SergioK29]

It seems the physics is still broken for certain versions of bedrock (Version: 1.21.0.26 for example), is there some kind of version check somewhere?

[Camotoy]

Yes: 1b4d257#diff-eced209b21f9d6d71ffc4a3f5ee74fecf92301875c5719b411ac6dba78d7b3a5R92-R94

I don't intend to fix the issue, since the versions will be dropped entirely eventually.

[onebeastchris]

For now, you could use this extension - https://github.com/onebeastchris/BlockVersion - to block any version older than 1.21.4x from joining :p Most Bedrock platforms automatically update anyways

[SergioK29]

Yes: 1b4d257#diff-eced209b21f9d6d71ffc4a3f5ee74fecf92301875c5719b411ac6dba78d7b3a5R92-R94

I don't intend to fix the issue, since the versions will be dropped entirely eventually.

How many months is the version supported for?

[Camotoy]

Support will be dropped for everything before 1.21.40 when the winter drop update is official - we don't want to map all those old blocks to older versions.