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FIP-0030: Introducing the Filecoin Virtual Machine (FVM)

Simple Summary
Abstract
Change Motivation
FVM specification
Design Rationale
Backwards Compatibility
Test Cases
Security Considerations
Incentive Considerations
Product Considerations
Implementation
Copyright

Simple Summary

User-programmability is the ability to deploy and run arbitrary user-provided code on some infrastructure, in this case a blockchain network.

Today, the Filecoin blockchain lacks on-chain user-programmability. This FIP is part of a series that aims to introduce this capability.

A change of this magnitude impacts many components. Concretely:

The chain state layer, to store and load user-provided code.
The execution layer, to compile/interpret user-provided code, run state transitions involving such code, and verify the results.
The gas model, to secure the network and modulate chain capacity demand/supply dynamics in the face of user-programmable code.
The preexisting baseline protocol, to facilitate programmability against system functionality.

This FIP focuses on (1) and (2). The remaining points are out of scope of this FIP, and will be addressed by subsequent proposals.

Abstract

We specify the Filecoin Virtual Machine (FVM), an IPLD-ready Wasm-based execution layer capable of running arbitrary user-provided code. The FVM replaces the existing non-programmable execution layer, now termed Legacy VM.

The logic of native actors (the equivalent of smart contracts in Filecoin) is deployed as Wasm bytecode. Foreign actors targeting non-Filecoin runtimes such as the Ethereum Virtual Machine are also supported through runtime emulation.

We provide a reference architecture for the FVM, and explain its various components: (a) Machine, (b) Call Manager, (c) Invocation Container, (d) Kernel, (e) Syscalls, and (f) Externs. We define the contract that actor code must adhere to, the catalogue of available syscalls, the mechanics of IPLD state manipulation, and further technical details

This FIP introduces the technical foundations of the FVM. It is accompanied by two subsequent FIPs building on it: FIP-0031 and FIP-0032. Respectively, these propose: an atomic switch from the legacy VM to a non-programmable version of the FVM, and an iteration on the gas model to account for the execution footprint of the new VM runtime.

Change Motivation

The desire for user-programmability in Filecoin is pronounced and well-recognized. It is the key to unlock enormous latent potential, as it enables permissionless community-driven innovation on the Filecoin network.

Some of the envisioned benefits include:

The proliferation of innovative, trustless solutions in user space.
- The ability to program behaviors on top of the existing Filecoin primitives confers new degrees of freedom to developers and increases the potential for innovation.
- Some examples include: decentralized compute, Data DAOs, Layer 2 networks, cross-chain bridges, alternative storage markets, programmable storage, collateral loan markets, and more.
The migration of functionality from system space to user space, thus resulting in a lower dependence on built-in actor evolution, which is largely bottlenecked on the Core Devs and the FIP process.
The emergence of an ecosystem of composable and stackable building blocks and primitives exposed through standardised interfaces, enabling the construction of richer, composite, interconnected solutions.
Unification of built-in actors across implementations to speed up protocol development.
Ability to conduct protocol upgrades that modify built-in actor logic through on-chain governance processes.
Potential for more areas of the Filecoin protocol to migrate to Wasm space (e.g. block validation, fork choice rule, etc.), it becomes possible to deploy protocol changes as Wasm modules to all clients, conditional upon on-chain voting.

FVM specification

This section provides a fundamental definition of the Filecoin Virtual Machine (FVM). It is intended to act as a baseline for future FIPs to enhance and evolve, as new capabilities, innovations and mechanisms are worked out.

Writing style

The FVM is a system under development, and some aspects are still in flux. For this reason, this spec combines two styles to provide a well-rounded picture:

Normative style, for concrete behaviors, rules, interfaces and types that we propose to introduce imminently. Conveyed by present tense.
Directional style: lays out the thinking for upcoming behaviors, rules, interfaces, and types, without mandating concrete technical choices yet. Conveyed by future tense and/or speculative language.

Reference implementation

filecoin-project/ref-fvm is a reference implementation to serve as a companion to this specification.

Context and VM interface

In the Filecoin blockchain, the virtual machine (VM) is the component in the execution layer responsible for applying transactions over a state tree. These transactions are encapsulated in messages. There are two kinds of messages:

Explicit messages are sent by secp256k1 or BLS account actors, appear on-chain when executed, and carry a signature to prove their authenticity.
Implicit messages are generated by the system, do not appear on chain, and they carry no signature. Examples include cron ticks and reward cycles.

A VM is instantiated with these input parameters:

/// These code samples are simplified with respect to the actual implementation in ref-fvm.
let machine = Machine::new(
    epoch,             // chain epoch at which to execute messages
    base_fee,          // base fee currently in effect
    base_circ_supply,  // base circulating supply
    nv,                // network version
    state_root,        // root CID of the initial state tree
    blockstore,        // blockstore to load and save state objects during execution
)

Messages are then applied to the machine. Every application generates a return object containing at least a message receipt, a miner penalty, and a miner tip. Implementations may return extra data such as diagnostics info, backtraces, or gas traces.

let ApplyRet { receipt, penalty, tip } := machine.apply_message(msg1);
let ApplyRet { receipt, penalty, tip } := machine.apply_message(msg2);
let ApplyRet { receipt, penalty, tip } := machine.apply_message(msg3);

The message receipt is recorded on chain. It holds the exit code of the transaction, the gas used, and (optionally) return data only if successful.

The penalty and the tip are values used by the cryptoeconomic rules of the protocol to calculate the block reward for the block producer.

Once the caller has finished applying all messages, it flushes the machine. This computes the final state root. Depending on implementation, this operation may also commit newly written blocks to the blockstore.

let final_state_root = machine.flush();

With the introduction of the FVM, this VM API is largerly preserved, although implementations may opt to deviate from it.

Technology

The proposed FVM execution runtime is WebAssembly (Wasm) with 32-bit memories. 64-bit memories are still under development in the Wasm memory64 extension.

Wasm with 32-bit memories supports 64-bit integer types and arithmetic, it just can't address memory beyond 4GiB. This address space is deemed sufficient for blockchain state transition use cases, but may be limiting for future and more generalised applications of the FVM.

The FVM requires Wasm multi-value support. It currently forbids Wasm SIMD instructions, floating number values and operations, and Wasm threads to prevent Nondeterministic behavior. However, these constraints may vary in the future.

Responsibilities

The FVM applies messages to the state tree. It is not responsible for advancing the chain itself. Concretely, it is mainly responsible for:

Loading on-chain Wasm modules and compiling them to machine code.
Performing preflight checks on the supplied messages.
Instantiating the object stack described in the core technical design to apply messages.
Routing calls across actors, via the Call Manager abstraction.
Processing syscalls from actors, and delivering the results.
Interfacing with the outer environment (Filecoin client) when necessary.
Managing IPLD state read and write operations.

Core technical design

This diagram serves as a reference for the technical design of the FVM.

Machine

The Machine is an instantiation of the FVM on a concrete epoch, over a specific state root, ready to take in messages for execution as explained above.

Call Manager

The Call Manager coordinates the call stack involved in processing a given message. This entails setting up and destroying Invocation Containers, tracking gas usage, and halting execution when the gas limit is exceeded.

Invocation Container

The Invocation Container (IC) is the environment/sandbox that runs actor code within the context of a single invocation. Reentrancy or recursion on the same actor will spin off another IC.

The IC is an instance of a Wasm engine fulfilling the FVM-side of the contract. The contract consists of:

Syscalls available as importable host-provided functions.
Shared types to represent data across the FVM boundary.
Actor-owned memory to exchange data in syscalls.
Transparent gas accounting and automatic execution halting.
(Future) Dynamic linkage of popular Wasm modules "blessed" by the community through some governance process, in order to reduce actor bytecode size (e.g. FVM SDKs, IPLD codecs, etc).

Because this contract may change over time, user-deployed actors may specify an IC version at InitActor#InstallActor time. The strategy for deprecating IC versions over time has not been yet determined.

Syscalls

Syscalls are functions made available to the actor as Wasm imports, allowing it to interact with the outer environment, and to call complex operations that are more performant to execute in client land instead of Wasm (e.g. cryptographic primitives).

Kernel

The Kernel is an object created by the Call Manager and injected into the Invocation Container. It processes syscalls, and stores invocation-scoped state.

Externs

Similar to syscalls being functions provided to the actor by the FVM, Externs are functions provided by the node to the FVM. Depending on the programming languages at play, the Externs layer may be only logical (if both the client and FVM are written in the same language), or may have a physical representation in the form of an FFI boundary between languages.

Actors

Actors are equivalent to smart contracts in other networks: they are pieces of logic that live on-chain and can be triggered via messages.

Actor types

We distinguish three types of actors:

Built-in actors.
Native actors.
Foreign actors.

Built-in actors

Built-in actors are protocol-defined actor types. Up until now, these actors shipped alongside the legacy VM and were tightly integrated with it. In the case of Lotus (reference Filecoin client), these actors lived in filecoin-project/specs-actors.

With the adoption of a universal, portable execution bytecode format, all Filecoin clients can rely on a single codebase compiled to Wasm bytecode. This speeds up protocol development, shortens protocol upgrade timelines, and reduces coordination overhead.

This codebase sits at filecoin-project/builtin-actors, and FIP-0031 proposes its standardisation in the network through a state migration.

This approach has a few practical implications:

The canonical actors become a common-good codebase for the Filecoin network, with core devs converging development effort around it.
Filecoin clients would embed the canonical Wasm bytecode into their binaries. The concrete mechanisms are implementation-dependent. The recommended approach is to embed the CAR bundle generated by the cited repo into the client's state blockstore.
Network upgrades would agree on the exact CodeCIDs of built-in actors to migrate to at the upgrade epoch.

Native user-defined actors

Native user-defined actors are actors written specifically for the FVM runtime.

Users can technically write native user-defined actors in any programming language that compiles to Wasm. However, language-specific overheads (e.g. runtime, garbage collection, stdlibs, etc.) may lead to oversized Wasm bytecode and execution overheads, translating to higher gas costs.

Furthermore, there will likely be hard limits enforced on the size of bytecode deployables by the time support for user-deployed actors is introduced.

There's a FVM Reference SDK written in Rust to accompany this FIP. As of today, it is used by the filecoin-project/builtin-actors codebase.

Generally speaking, we find that Rust is a reasonable choice to write native user-defined actors due to the lack of a runtime, the ability to elide the stdlib through no_std, and its mature Wasm support.

Exploration of other languages is something we encourage the community to pursue. Concretely, we believe that AssemblyScript, Swift, Kotlin, and TinyGo SDKs are worth exploring.

Foreign actors

Foreign actors are smart contracts or programs targeting non-FVM runtimes, running inside the Filecoin network through emulation.

The ability to run foreign actors is useful for cross-chain interoperability, and to extend Filecoin's reach and utility to other blockchains, allowing them to access the network's storage capabilities more seamlessly.

The first kind of foreign actor we aim to support are Ethereum smart contracts, through the deployment of EVM bytecode. This allows for straightforward and relatively risk-free porting of existing battle-tested Ethereum smart contracts to the Filecoin network.

Every foreign runtime is in turn an actor type, identified by a CodeCID. Foreign runtimes are installed just like any other actor type. Foreign actors instances are constructed through the InitActor, supplying the source deployable as a constructor.

In the case of the EVM foreign runtime, we are looking to standardise on either one of these interpreters:

SputnikVM, or
bluealloy/revm (a fork of the above)

Every foreign runtime will need to specify bindings to map runtime-specific aspects (e.g. addressing, crypto primitives, gas accounting, etc.) to Filecoin counterparts. A preliminary EVM <> FVM binding specification exists.

Future foreign runtime candidates include Agoric SES, Solana eBPF, and others. In all cases, compatibility should target the lowest level executable output in its source form, rather than dealing with high-level languages using alternative/custom toolchains. This reduces risk surface by increasing execution fidelity/parity, and makes it possible to reuse developer tooling available in the original ecosystem.

Actor bytecode representation

Wasm bytecode payloads will be represented as an IPLD DAG, and will be stored in the state blockstore, owned by the node implementation.

The concrete DAG layout, multihash function, and CID codec are yet to be defined.

Code will be linked to from the state tree through the existing code CID field in the ActorState object:

pub struct ActorState {
    /// Link to code for the actor.
    pub code: Cid,
    /// Link to the state of the actor.
    pub state: Cid,
    /// Current sequential nonce of the actor.
    pub nonce: u64,
    /// Filecoin tokens available to the actor.
    pub balance: TokenAmount,
}

Actor deployment

The current InitActor (f00) will be extended with a (InstallActor) method taking a single input parameter: the actor's Wasm bytecode. As mentioned above, DAG representation format is to be defined. The actor's Wasm bytecode must specify the version of the Invocation Container the bytecode requires, inside a reserved exported global.

The state object of the InitActor will be extended with an ActorRegistry HAMT of CID => ActorSpec, where ActorSpec is:

/// A simple actor specification/metadata structure, to be extended with
/// additional metadata in the future.
struct ActorSpec {
  /// The version of the invocation container.
  version: int, 
}

The logic of InitActor#LoadActor is as follows.

Validate the Wasm bytecode.
- Perform syntax validation and structural validation, as per standard.
- No floating point instructions.
Multihash the bytecode and calculate the code CID.
Check if the code CID is already registered in the ActorRegistry.
If yes, charge gas for the above operations and return the existing CID.
Insert an entry in the ActorRegistry, binding the bytecode CID with the actor spec.
Return the CID, and charge the cost of the above operations, and potentially a price for rent/storage.

At this point, the actor can be instantiated many times through the standard InitActor#Exec method. Parameters provided will be passed through to the actor's constructor (currently identified by method_number=1). This includes byte streams corresponding to deployables for foreign actors (e.g. EVM bytecode).

Actor message dispatch

With the exception of simple value transfers, messages trigger some actor logic on the recipient actor. (In the future, value transfers may also trigger arbitrary logic.)

Today, Filecoin chain messages carry a method number. The current VM matches on recipient actor type and method number to dispatch to the appropriate handler. This approach is external method dispatch.

With the FVM, actors now expose a single entrypoint in the form of a Wasm-exported function, and conduct internal method dispatch.

The signature of an actor entrypoint is:

pub fn invoke(params_block_id: u32) -> u32 { }

The single uint32 argument corresponds to the block ID of the encoded input parameters struct, or 0 for none.
The single uint32 return value corresponds to the block ID of the return data struct, or 0 for none.

More information on block IDs can be found in IPLD memory model

This design decouples actors from their runtime, and is more aligned with the actor model, where actors interact through inboxes. It also awards maximal degrees of freedom and sophistication to support techniques such as structural pattern matching, efficient pass-through proxying, interception, and more.

Because internal dispatch logic will rapidly become boilerplate, FVM SDKs should offer dispatch sugar.

Backwards compatibility

The method number field on messages remains unchanged to preserve backwards compatibility. It may be deprecated and eventually removed later. Built-in actors continue relying on method number, but now perform internal dispatch.

Syscalls

This section specifies all the syscalls that the IC must make available to the actor.

Currently, the FVM charges no gas for import linkage. In the future, the FVM may charge a static or a dynamic cost for linking the concrete functions or namespaces imported by the module.

Interface type conventions

off and len in argument names refers to "memory offset" and "buffer length". Together, they define the bounds of a slice of memory.
Syscall arguments prefixed with o_ represent output parameters.
The Result return type carries a syscall error number and the specified payload type. The latter only appears when the syscall succeeded (implied by error number 0). These error numbers are ephemeral, and have no meaning beyond the FVM syscall boundary. Refer to the FVM Error Conditions document for more info.

Namespace: `actor`

#[repr(C)]
pub struct ResolveAddress {
    pub resolved: i32,
    pub value: u64,
}

/// Resolves the ID address of an actor.
pub fn resolve_address(
  addr_off: *const u8,
  addr_len: u32,
) -> Result<ResolveAddress>;

/// Gets the CodeCID of an actor by address.
pub fn get_actor_code_cid(
    addr_off: *const u8,
    addr_len: u32,
    o_cid_off: *mut u8,
    o_cid_len: u32,
) -> Result<i32>;

/// Generates a new actor address for an actor deployed
/// by the calling actor.
pub fn new_actor_address(o_addr_off: *mut u8, o_addr_len: u32) -> Result<u32>;

/// Creates a new actor of the specified type in the state tree, under
/// the provided address.
///
/// NOTE: This syscall with these input parameters is provisional.
/// It is unsafe in an untrusted environment.
pub fn create_actor(actor_id: u64, typ_off: *const u8) -> Result<()>;

#[repr(i32)]
pub enum Type {
    System = 1,
    Init = 2,
    Cron = 3,
    Account = 4,
    Power = 5,
    Miner = 6,
    Market = 7,
    PaymentChannel = 8,
    Multisig = 9,
    Reward = 10,
    VerifiedRegistry = 11,
}

/// Returns whether the supplied code_cid belongs to a known built-in actor type.
fn resolve_builtin_actor_type(&self, code_cid: &Cid) -> Option<actor::builtin::Type>;

/// Returns the CodeCID for the supplied built-in actor type.
fn get_code_cid_for_type(&self, typ: actor::builtin::Type) -> Result<Cid>;

Namespace: `crypto`

/// Verifies that a signature is valid for an address and plaintext.
pub fn verify_signature(
    sig_off: *const u8,
    sig_len: u32,
    addr_off: *const u8,
    addr_len: u32,
    plaintext_off: *const u8,
    plaintext_len: u32,
) -> Result<i32>;

/// Hashes input data using blake2b with 256 bit output.
///
/// The output buffer must be sized to a minimum of 32 bytes.
pub fn hash_blake2b(
    data_off: *const u8,
    data_len: u32,
) -> Result<[u8; 32]>;

/// Computes an unsealed sector CID (CommD) from its constituent piece CIDs
/// (CommPs) and sizes.
///
/// Writes the CID in the provided output buffer, and returns the length of
/// the written CID.
pub fn compute_unsealed_sector_cid(
    proof_type: i64,
    pieces_off: *const u8,
    pieces_len: u32,
    o_cid_off: *mut u8,
    o_cid_len: u32,
) -> Result<u32>;

/// Verifies a sector seal proof.
///
/// It takes the buffer offset and length containing a CBOR-encoded
/// SealVerifyInfo struct, to serve as input. A return payload of 0
/// means verification success; any other value means verification
/// failure.
pub fn verify_seal(info_off: *const u8, info_len: u32) -> Result<i32>;

/// Verifies a window proof of spacetime.
///
/// It takes the buffer offset and length containing a CBOR-encoded
/// WindowPoStVerifyInfo struct, to serve as input. A return payload
/// of 0 means verification success; any other value means verification
/// failure.
pub fn verify_post(info_off: *const u8, info_len: u32) -> Result<i32>;

/// Verifies that two block headers provide proof of a consensus fault.
///
/// A syscall success implies that the logic ran successfully, but it
/// does not imply that a fault was detected. The caller must inspect the
/// return payload. If the fault field equals 0, a consensus fault was
/// recognized at the specified epoch, incurred by the specified actor.
/// Otherwise, no consensus fault was recognized, and the remaining fields
/// must be ignored.
pub fn verify_consensus_fault(
    h1_off: *const u8,
    h1_len: u32,
    h2_off: *const u8,
    h2_len: u32,
    extra_off: *const u8,
    extra_len: u32,
) -> Result<VerifyConsensusFault>;

/// Verifies an aggregated batch of sector seal proofs.
///
/// It takes the buffer offset and length containing a CBOR-encoded
/// AggregateSealVerifyProofAndInfos struct, to serve as input.
/// A return payload of 0 means verification success; any other value
/// means verification failure.
pub fn verify_aggregate_seals(agg_off: *const u8, agg_len: u32) -> Result<i32>;

/// Batch verifies many sector seal proofs within a single syscall.
///
/// It takes the buffer offset and length containing a CBOR-encoded
/// list of SealVerifyInfo structs, to serve as input.
///
/// The return payload is an array of results in the same order as the
/// input sector seal proofs. A value 0 in index i means that proof[i] was
/// verified successfully. Non-zero values denote verification failure.
pub fn batch_verify_seals(batch_off: *const u8, batch_len: u32, o_result_off: *const u8) -> Result<()>;

#[repr(C)]
pub struct VerifyConsensusFault {
    pub fault: u32,
    pub epoch: ChainEpoch,
    pub actor: ActorID,
}

Namespace: `gas`

/// Charge the specified amount of gas for the supplied operation name,
/// encoded as an UTF-8 string.
pub fn charge(name_off: *const u8, name_len: u32, amount: u64) -> Result<()>;

Namespace: `ipld`

/// Opens a block from the "reachable" set, returning an ID for the block, its codec, and its
/// size in bytes.
///
/// - The reachable set is extended to include the direct children of loaded blocks until the
///   end of the invocation.
pub fn block_open(cid: *const u8) -> Result<IpldOpen>;

/// Creates a new block, returning the block's ID. The block's children must be in the reachable
/// set. The new block isn't added to the reachable set until the CID is computed.
pub fn block_create(codec: u64, data: *const u8, len: u32) -> Result<u32>;

/// Reads the identified block into o_buf, starting at offset, reading _at most_ len bytes.
/// Returns the number of bytes read.
pub fn block_read(id: u32, offset: u32, o_buf_off: *mut u8, o_buf_len: u32) -> Result<u32>;

/// Returns the codec and size of the specified block.
pub fn block_stat(id: u32) -> Result<IpldStat>;

/// Computes the given block's CID, returning the actual size of the CID.
///
/// If the CID is longer than cid_max_len, no data is written and the actual size is returned.
///
/// The returned CID is added to the reachable set.
pub fn block_link(
    id: u32,
    hash_fun: u64,
    hash_len: u32,
    o_cid_off: *mut u8,
    o_cid_len: u32,
) -> Result<u32>;

#[repr(C)]
pub struct IpldOpen {
    pub id: u32,
    pub codec: u64,
    pub size: u32,
}

#[repr(C)]
pub struct IpldStat {
    pub codec: u64,
    pub size: u32,
}

Namespace: `message`

/// Returns the caller's actor ID.
pub fn caller() -> Result<u64>;

/// Returns the receiver's actor ID (i.e. ourselves).
pub fn receiver() -> Result<u64>;

/// Returns the method number from the message.
pub fn method_number() -> Result<u64>;

/// Returns the value that was received, as little-Endian
/// tuple of u64 values to be concatenated in a u128.
pub fn value_received() -> Result<TokenAmount>;

Namespace: `network`

/// Gets the current epoch.
pub fn curr_epoch() -> Result<u64>;

/// Gets the network version.
pub fn version() -> Result<u32>;

/// Gets the base fee for the epoch as little-Endian
/// tuple of u64 values to be concatenated in a u128.
pub fn base_fee() -> Result<TokenAmount>;

/// Gets the circulating supply as little-Endian
/// tuple of u64 values to be concatenated in a u128.
/// Note that how this value is calculated is expected to change in nv15
pub fn total_fil_circ_supply() -> Result<TokenAmount>;

Namespace: `rand`

/// Gets 32 bytes of randomness from the ticket chain.
/// The supplied output buffer must have at least 32 bytes of capacity.
/// If this syscall succeeds, exactly 32 bytes will be written starting at the
/// supplied offset.
pub fn get_chain_randomness(
    dst: i64,
    round: i64,
    o_entropy_off: *const u8,
    o_entropy_len: u32,
) -> Result<[u8; 32]>;

/// Gets 32 bytes of randomness from the beacon system (currently Drand).
/// The supplied output buffer must have at least 32 bytes of capacity.
/// If this syscall succeeds, exactly 32 bytes will be written starting at the
/// supplied offset.
pub fn get_beacon_randomness(
    dst: i64,
    round: i64,
    o_entropy_off: *const u8,
    o_entropy_len: u32,
) -> Result<[u8; 32]>;

Namespace: `send`

/// Sends a message to another actor, and returns the exit code and block ID of the return
/// result.
pub fn send(
    recipient_off: *const u8,
    recipient_len: u32,
    method: u64,
    params: u32,
    value_hi: u64,
    value_lo: u64,
) -> Result<Send>;

#[repr(C)]
pub struct Send {
    pub exit_code: u32,
    pub return_id: BlockId,
}

Namespace: `self`

/// Gets the current state root for the calling actor.
///
/// If the CID doesn't fit in the specified maximum length (and/or the length is 0), this
/// function returns the required size and does not update the cid buffer.
pub fn root(o_cid_off: *mut u8, o_cid_len: u32) -> Result<u32>;

/// Sets the root CID for the calling actor. The new root must be in the reachable set.
pub fn set_root(cid: *const u8) -> Result<()>;

/// Gets the current balance for the calling actor.
pub fn current_balance() -> Result<TokenAmount>;

/// Destroys the calling actor, sending its current balance
/// to the supplied address, which cannot be itself.
pub fn self_destruct(addr_off: *const u8, addr_len: u32) -> Result<()>;

Namespace: `vm`

/// Abort execution with the given code and message. The code is recorded in the receipt, the
/// message is for debugging only.
pub fn abort(code: u32, message: *const u8, message_len: u32) -> !;

Externs

/// Consensus related methods.
pub trait Consensus {
    /// Verify a consensus fault.
    fn verify_consensus_fault(
        &self,
        h1: &[u8],
        h2: &[u8],
        extra: &[u8],
    ) -> Result<Option<ConsensusFault>>;
}

/// Randomness provider trait
pub trait Rand {
    /// Gets 32 bytes of randomness for ChainRand paramaterized by the DomainSeparationTag,
    /// ChainEpoch, Entropy from the ticket chain.
    fn get_chain_randomness(
        &self,
        pers: DomainSeparationTag,
        round: ChainEpoch,
        entropy: &[u8],
    ) -> Result<[u8; 32]>;

    /// Gets 32 bytes of randomness for ChainRand paramaterized by the DomainSeparationTag,
    /// ChainEpoch, Entropy from the latest beacon entry.
    fn get_beacon_randomness(
        &self,
        pers: DomainSeparationTag,
        round: ChainEpoch,
        entropy: &[u8],
    ) -> Result<[u8; 32]>;
}

IPLD memory model

IPLD is the data model of Filecoin. Actors can be construed as logic that receives an input IPLD graph, performs computation, and returns a new IPLD graph (which is persisted by saving the root CID of the new graph in the actor's state).

All state is stored and retrieved as IPLD. The state store lives on the node side, and it is exposed to the FVM via an Extern. This is important as the VM itself needs to understand the IPLD so we can do garbage collection, etc.

The FVM must guarantee that actors can only access data that is in their state tree. This is done through the maintenance of an "accessible set" inside the kernel.

Because Filecoin IPLD state objects are highly atomized and linked, accessing and mutating entries in objects like HAMTs and AMT results in multiple sequential state IO operations, each of which traverses the Extern boundary in a non-parallelizable way. If the Extern is traversed through FFI, the cost of operating on ADLs may be non-negligible.

Block handles

IPLD blocks are dynamically-sized blobs of data. When loading a block by CID, the size is unknown, so the actor cannot allocate memory upfront. Moreover, host-side memory is inaccessible to Wasm code, so it's not possible for the Kernel to return a pointer to host-side memory for Wasm code to read.

For this reason, the low-level IPLD syscall API revolves around the concept of "block handles". Block handles are references to blocks, inspired by Unix file descriptors, and identified by a block ID (u32).

Parameters to actors and return data returned from actor calls are modelled as IPLD blocks too. But since they are not referenced from state or elsewhere, they aren't referenced by CID but merely by block IDs when invoking the Wasm actor entrypoint (see Actor message dispatch).

Syscall mechanics

When accessing a block by CID through ipld::block_open, the Kernel returns a block ID, the size, and the codec.
To read the block data, ipld::block_read takes a block ID and a pointer to a Wasm-side buffer.
To write a block, ipld::block_create takes a buffer and a codec, and returns a block ID.
If the Wasm actor needs to compute the CID for a block, it calls the ipld::block_link syscall passing in the block ID.
An ipld::block_stat syscall is available to enquire the size and codec of the block by ID. This is useful to allocate a buffer for call parameters.

Module caching

Wasm execution performance is a determining factor in the ability to maintain blockchain epoch time (30 seconds).

We recommend implementations to compile Wasm bytecode into machine code for faster execution, instead of relying on interpretation. This incurs in an upfront CPU-bound compilation cost, in exchange for ongoing operating costs.

Actively or passively-managed module caching could be explored, but such approaches may introduce vulnerabilities. Instead, we may factor in the compilation compute cost and the perpetual storage cost, as deployment-time gas.

Any approach will need to account for eventual recompilation costs on events like Wasm Engine or compiler upgrades.

Gas charging

The current gas model relies on actor code being static and known ahead of time. Gas is currently charged at the syscall level and not at the instruction level.

With the ability to run arbitrary code on the network, the gas model must account for actual execution costs at the instruction level, in addition to syscall-level charges.

An initial gas remodelling proposal is submitted in FIP-0032.

Specific actor coupling

Due to how certain parts of the Filecoin protocol work, implicit coupling that exists between the VM and specific actor types will be carried over. Concretely:

Account actor: the FVM must create new account actors when processing sends to pubkey addresses.
Init actor: the FVM must load and query the init actor state when resolving addresses during message processing, and when the actor::resolve_address syscall is called. Additionally, a new init actor entry must be added when implicitly creating an account, as per above.
Power actor and Storage Market actor: state is queried when handling the network::total_fil_circ_supply syscall.
Burnt funds account and Reserve account: balances are queried when handling the network::total_fil_circ_supply syscall.

Design Rationale

Concerning the choice of runtime, various technologies were evaluated, including EVM, JavaScript/SES, LLVM IR, etc. Wasm was chosen for these reasons:

It has a strong and promising future in the blockchain space; with room for cross-chain collaborations and interest groups
It has a growing ecosystem of tooling and solutions (e.g. runtimes, package managers, language shims)
It is portable across architectures and operating systems
has deterministic execution as long as non-deterministic features aren't used (of which we use none)
It is usually compiled to machine code, but it can also be interpreted
Bytecode specifies actual execution as opposed to high-level constructions or intermediate representations; this makes it suitable for stable content-addressing
It is sandboxed and secure
Allows for a choice of programming language for actors

Backwards Compatibility

This FIP introduces a re-architecture of the execution layer, but it doesn't propose a specific protocol change. Existing built-in actors will continue to operate normally. Subsequent associated FIPs will explore the backwards compatibility aspects of their respective protocol changesets (e.g. atomic switch to the FVM, gas model changes).

Test Cases

FVM implementations can subject themselves to the FVM-specific test vectors hosted at: filecoin-project/fvm-test-vectors.

Security Considerations

This specification entails a complete rearchitecture of the execution layer of the Filecoin blockchain, and the transition to adopting a new actors codebase. Such low-level and deep changes comes with non-negligible risks, the extent of which will be analyzed as network upgrades with concrete scopes are proposed.

Incentive Considerations

This specification FIP does not affect incentives in itself. However, the ability to deploy arbitrary on-chain programs may impact chain utilization, gas expenditures and burn, and the flow of tokens as a result of the use cases that developers deploy on top of the FVM.

Product Considerations

New product possibilities and horizons are brought to life by on-chain user-programmability on the Filecoin network. As described in the Change Motivation section, we expect the FVM to accelerate the pace of innovation in the Filecoin network. Furthermore, as system functionality moves out to user space, product requirements depending on protocol changes may be implemented entirely in user space.

The FVM also creates new product requirements around developer tooling (e.g. sandboxes, test kits, etc.) and testnets.

Implementation

A reference implementation of the FVM is provided in the filecoin-project/ref-fvm repository.

Copyright

Copyright and related rights waived via CC0.

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