PVF Pre-checking Overview
Motivation
Parachains' validation function is described by a wasm module that we refer to as a PVF. Since a PVF is a wasm module the typical way of executing it is to compile it to machine code.
Typically an optimizing compiler consists of algorithms that are able to optimize the resulting machine code heavily. However, while those algorithms perform quite well for a typical wasm code produced by standard toolchains (e.g. rustc/LLVM), those algorithms can be abused to consume a lot of resources. Moreover, since those algorithms are rather complex there is a lot of room for a bug that can crash the compiler.
If compilation of a Parachain Validation Function (PVF) takes too long or uses too much memory, this can leave a node in limbo as to whether a candidate of that parachain is valid or not.
The amount of time that a PVF takes to compile is a subjective resource limit and as such PVFs may be maliciously crafted so that there is e.g. a 50/50 split of validators which can and cannot compile and execute the PVF.
This has the following implications:
- In backing, inclusion may be slow due to backing groups being unable to execute the block
- In approval checking, there may be many no-shows, leading to slow finality
- In disputes, neither side may reach supermajority. Nobody will get slashed and the chain will not be reverted or finalized.
As a result of this issue we need a fairly hard guarantee that the PVFs of registered parachains/threads can be compiled within a reasonable amount of time.
Solution
The problem is solved by having a pre-checking process.
Pre-checking
Pre-checking mostly consists of attempting to prepare (compile) the PVF WASM blob. We use more strict limits (e.g. timeouts) here compared to regular preparation for execution. This way errors during preparation later are likely unrelated to the PVF itself, as it already passed pre-checking. We can treat such errors as local node issues.
We also have an additional step where we attempt to instantiate the WASM runtime without running it. This is unrelated to preparation so we don't time it, but it does help us catch more issues.
Protocol
Pre-checking is run when a new validation code is included in the chain. A new PVF can be added in two cases:
- A new parachain is registered.
- An existing parachain signalled an upgrade of its validation code.
Before any of those operations finish, the PVF pre-checking vote is initiated. The PVF pre-checking vote is identified by the PVF code hash that is being voted on. If there is already PVF pre-checking process running, then no new PVF pre-checking vote will be started. Instead, the operation just subscribes to the existing vote.
The pre-checking vote can be concluded either by obtaining a threshold of votes for a decision, or if it expires. The threshold to accept is a supermajority of 2/3 of validators. We reject once a supermajority is no longer possible.
Each validator checks the list of PVFs available for voting. The vote is binary, i.e. accept or reject a given PVF. As soon as the threshold of votes are collected for one of the sides of the vote, the voting is concluded in that direction and the effects of the voting are enacted.
Only validators from the active set can participate in the vote. The set of active validators can change each session. That's why we reset the votes each session. A voting that observed a certain number of sessions will be rejected.
The effects of the PVF accepting depend on the operations requested it:
- All onboardings subscribed to the approved PVF pre-checking process will get scheduled and after passing 2 session boundaries they will be onboarded.
- All upgrades subscribed to the approved PVF pre-checking process will get scheduled very similarly to the existing process. Upgrades with pre-checking are really the same process that is just delayed by the time required for pre-checking voting. In case of instant approval the mechanism is exactly the same.
In case PVF pre-checking process was concluded with rejection, then all the operations that are subscribed to the rejected PVF pre-checking process will be processed as follows. That is, onboarding or upgrading will be cancelled.
The logic described above is implemented by the paras module.
Subsystem
On the node-side, there is a PVF pre-checking subsystem that scans the chain for new PVFs via using runtime APIs. Upon finding a new PVF, the subsystem will initiate a PVF pre-checking request and wait for the result. Whenever the result is obtained, the subsystem will use the runtime API to submit a vote for the PVF. The vote is an unsigned transaction. The vote will be distributed via the gossip similarly to a normal transaction. Eventually a block producer will include the vote into the block where it will be handled by the runtime.
Summary
Parachains' validation function is described by a wasm module that we refer to as a PVF.
In order to make the PVF usable for candidate validation it has to be registered on-chain.
As part of the registration process, it has to go through pre-checking. Pre-checking is a game of attempting preparation and additional checks, and reporting the results back on-chain.
We define preparation as a process that: validates the consistency of the wasm binary (aka prevalidation) and the compilation of the wasm module into machine code (referred to as an artifact).
Besides pre-checking, preparation can also be triggered by execution, since a compiled artifact is needed for the execution. If an artifact already exists, execution will skip preparation. If it does do preparation, execution uses a more lenient timeout than preparation, to avoid the situation where honest validators fail on valid, pre-checked PVFs.