lotus/documentation/en/block-validation.md

Incoming block validations

This document reviews the code flow that takes place inside the full node after receiving a new block from the GossipSub /fil/blocks topic and traces all of its protocol-related validation logic. We do not include validation logic inside the VM, the analysis stops at (*VM).Invoke(). The V: tag explicitly signals validations throughout the text.

modules.HandleIncomingBlocks()

We subscribe to the /fil/blocks PubSub topic to receive external blocks from peers in the network and register a block validator that operates at the PubSub (libp2p stack) level, validating each PubSub message containing a Filecoin block header.

V: PubSub message is a valid CBOR BlockMsg.

V: Total messages in block are under BlockMessageLimit.

V: Aggregate message CIDs, encapsulated in the MsgMeta structure, serialize to the Messages CID in the block header (ValidateMsgMeta()).

V: Miner Address in block header is present and corresponds to a public-key address in the current chain state.

V: Block signature (BlockSig) is present and belongs to the public-key address retrieved for the miner (CheckBlockSignature()).

sub.HandleIncomingBlocks()

Assemble a FullBlock from the received block header retrieving its Filecoin messages.

V: Block messages CIDs can be retrieved from the network and decode into valid CBOR Message/SignedMessage.

(*Syncer).InformNewHead()

Assemble a FullTipSet populated with the single block received earlier.

V: ValidateMsgMeta() (already done in the topic validator).

V: Block's ParentWeight is greater than the one from the (first block of the) heaviest tipset.

(*Syncer).Sync()

(*Syncer).collectHeaders(): we retrieve all tipsets from the received block down to our chain. Validation now is expanded to every block inside these tipsets.

V: Beacon entires are ordered by their round number.

V: Tipset Parents CIDs match the fetched parent tipset through block sync. (This check is not enforced correctly at the moment, see issue.)

(*Syncer).ValidateBlock()

This function contains most of the validation logic grouped in separate closures that run asynchronously, this list does not reflect validation order then.

V: Block Timestamp:

• Is not bigger than current time plus AllowableClockDrift.
• Is not smaller than previous block's Timestamp plus BlockDelay (including null blocks).

Messages

We check all the messages contained in one block at a time ((*Syncer).checkBlockMessages()).

V: The block's BLSAggregate matches the aggregate of BLS messages digests and public keys (extracted from the messages From).

V: Each secp256k1 message Signature is signed with the public key extracted from the message From.

V: Aggregate message CIDs, encapsulated in the MsgMeta structure, serialize to the Messages CID in block header (similar to ValidateMsgMeta() call).

V: For each message, in ValidForBlockInclusion():

• Message fields Version, To, From, Value, GasPrice, and GasLimit are correctly defined.
• Message GasLimit is under the message minimum gas cost (derived from chain height and message length).

V: Actor associated with message From exists and is an account actor, its Nonce matches the message Nonce.

Miner

V: Miner address is registered in the Claims HAMT of the Power actor.

Compute parent tipset state

V: Block's ParentStateRoot CID matches the state CID computed from the parent tipset.

V: Block's ParentMessageReceipts CID matches receipts CID computed from the parent tipset.

Winner

Draw randomness for current epoch with minimum ticket from previous tipset, using ElectionProofProduction domain separation tag. V: ElectionProof.VRFProof is computed correctly by checking BLS signature using miner's key. V: Miner is not slashed in StoragePowerActor. V: Check if ticket is a winning ticket:

h := blake2b(VRFProof)
lhs := AsInt(h) * totalNetworkPower
rhs := minerPower * 2^256
if lhs < rhs { return "Winner" } else { return "Not a winner" }

Block signature

V: CheckBlockSignature() (same signature validation as the one applied to the incoming block).

Beacon values check

V: Validate that all BeaconEntries are valid. Check that every one of them is a signature of a message: previousSignature || round signed using drand's public key. V: All entries between MaxBeaconRoundForEpoch down to prevEntry (from previous tipset) are included.

Verify VRF Ticket chain

Draw randomness for current epoch with minimum ticket from previous tipset, using TicketProduction domain separation tag. V: VerifyVRF using drawn randomness and miner public key.

Winning PoSt proof

Draw randomness for current epoch with WinningPoSt domain separation tag. Get list of sectors challanged in this epoch for this miner, based on the randomness drawn.

V: Use filecoin proofs system to verify that miner prooved access to sealed versions of these sectors.

(*StateManager).TipSetState()

Called throughout the validation process for the parent of each tipset being validated. The checks here then do not apply to the received new head itself that started the validation process.

(*StateManager).computeTipSetState()

V: Every block in the tipset should belong to different a miner.

(*StateManager).ApplyBlocks()

We create a new VM with the tipset's ParentStateRoot (this is then the parent state of the parent of the tipset currently being validated) on which to apply all messages from all blocks in the tipset. For each message independently we apply the validations listed next.

(*VM).ApplyMessage()

V: Basic gas and value checks in checkMessage():

• Message GasLimit is bigger than zero.
• Message GasPrice and Value are set.

V: Message storage gas cost is under the message's GasLimit.

V: Message's Nonce matches nonce in actor retrieved from message's From.

V: Message's maximum gas cost (derived from its GasLimit, GasPrice, and Value) is under the balance of the actor retrieved from message's From.

(*VM).send()

V: Message's transfer Value is under the balance in actor retrieved from message's From.