EIP-4788: Beacon root in EVM

Got a link?

https://eips.ethereum.org/EIPS/eip-2935

the idea is that everything is under one object, namely the execution state, rather than the execution state and some additional context (like it is today for block hash)

the former

the intention is to add a header field – I wrote it this way so it is independent of any other header fields as even in the last few weeks the header fields for EIP-4844 have been changing

it would be unexpected (and so we should assume this was not the intent) to do a completely new thing where we encode the header and then tack this extra data on

we decided on ACDE 163 to move this precompile to the low address range and I’ll be updating the EIP today

I see, thank you! That also seemingly explains why -2 wasn’t used… -2 was taken by this other proposed stateful precompile. :slight_smile: (And I can imagine that -1 wasn’t used for other reasons.) Although going by below sounds like these are being moved to low addresses regardless.

I do have to say I don’t really understand the distinction being drawn, though. One way or another, you need this list to be stored somewhere. I don’t understand why it makes a difference whether it is stored somewhere available to an opcode, or stored somewhere avialable to a precompile.

Like, you say “right now you need the execution state and some additional context”, but that statement depends on defining “execution state” in a particular way, right? I would have just said that, because this list is accessible, that makes it part of the execution state. Evidently “execution state” has some technical meaning here that excludes this sort of thing, but even granting that, why does it make a difference? Why is the line that is being drawn a useful line to draw, such that doing it the precompile way is easier than doing it the opcode way? To me it just seems like they both require storing this information – which I would have called state information, though I suppose it doesn’t fall under what is technically being called state – somewhere, and it’s not clear why opcode vs precompile makes a difference…

I’d say the opcode vs precompile question is a bit different than the state design question.

Evidently “execution state” has some technical meaning here that excludes this sort of thing

when we say “execution state”, we mean the thing committed to by the state root in each block header

but assuming the beacon roots live somewhere, there is an access question – do we have an opcode or just frame it as a precompile so we can just leverage the CALL infra? every opcode we add does introduce new semantics to the EVM virtual machine, whereas just CALLing a precompile is something done all the time so its less of a ask to just go via CALL

Should this precompile go OOG if the input length is not 32 bytes?

not sure – what do other precompiles do here?

Blake2F throws, ecrecover pads input with zeros to get the expected 129 bytes length. It should be noted though what should happen if < 32 bytes are given as input: do we left- or right-pad the storage key with zeros in order to retrieve it? (And if there are more than 32 bytes, just take the first 32 bytes)

I want to raise a point regarding gas, but I do not want to make the EIP implementation more complex:

What if someone sends a transaction with one of these storage slots warm? We could lower the gas (but yes, this would make the implementation more complex)

i’m not too worried about this, and someone could prototype a verification of say a validator balance to ensure it doesn’t feel too expensive

one option if it was a problem is either (1) update precompile in future fork, (2) have a “caching” contract that proxies to the precompile so you get the warm/cold gas distinction

more than 32, just take the first 32

less should throw

I added one option here: Update EIP-4788: Bound precompile storage by ralexstokes · Pull Request #7178 · ethereum/EIPs · GitHub

lmk if see anything we should change

I mean, new precompiles add more semantics too! Both need to be specified. I’m just saying it’s a little confusing because doesn’t match the roles of opcodes and precompiles as they’ve existed so far. Getting information about the environment has been the province of opcodes, precompiles are for complicated computations. Well, except for 0x4… but that oversight is being rectified with the addition of MCOPY!

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Why don’t you deploy the real bytecode at the given address instead of trying to describe what it suppose to do in pseudocode? And come up with some arbitrary gas cost of this?

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This strongly resonates with me. I also wonder.

Some historical context, this approach was once suggested for blockhash, with EIP-210: Blockhash refactoring

@axic already posted that link, however, I think that was early in the discussion, when the beacon root EIP also included an opcode? Later on, the opcode was dropped, and the EIP is now in a situation where it could be replaced by:

  • A contract X
  • A system-call to X, invoking it with the latest beaconroot+number at the start of block processing.

The contract X would store new values if the sender is system-address, otherwise respond with the value requested (as per the pseudo-code in the current version of the EIP).
The address of X could either be pre-determined, like a precompile, or we could just create it with CREATE2 in advance – before the hardfork, and then just ‘bless it’ at cancun by beginning the population of values (optional: the ‘blessing’ could also include auto-inclusion into the prepopulated access-list).

In that case, the core of the EIP would basically be:

At the start of processing any execution block where block.timestamp >= FORK_TIMESTAMP (i.e. before processing any transactions), make a transaction to the HISTORY_STORAGE_ADDRESS, with calldata BEACONROOT, sender SYSTEM_SENDER and gas 100_000.

The internals of what the contract does would not be consensus-critical (any more than other contract internals)

Optimism also utilizes a system-transaction to expose the block commitment of an external layer, very similar to what is being explored with the “v2” here. But introduces the L1 blockhash, instead of a beacon-chain block root.

System transaction comparison

And instead of extending the EVM interface with the external blockroot as explicit argument, and instead of modifying the block-header, we use our “deposit tx type” (like 1559 tx, without signature) (specs). This deposit-tx is inserted at the top of the block, and passed in through a transactions-list via Payload-Attributes in the Engine API (this same functionality is used to reproduce blocks from inputs only, as only inputs are made available on L1).

Predeploy, not precompile

The transaction is then processed by a “predeploy” (specs). The bytecode of a predeployed EVM contract is present starting at genesis of the chain, not deployed by a regular user.
This “predeploy” mechanism is the same trick as what I implemented in testnet tools to embed the beacon deposit-contract with initial storage at a special address, which e.g. is going into the deposit-contract in Holesky at 0x000000006465706F73697420636f6E7472616374 (“deposit contract” in utf8).
This process is described here as well as in this merge-at-genesis tutorial by Afri.

In the case of EIP-4788 the block-header is extended, so the tx itself does not really have to be a tx, but the pattern of calling a predeploy, instead of a precompile, definitely works.

Bridge usage

And we’re considering to extend the predeploy contract with a ring-buffer: it’s very useful to retain recent history, and expose that history to other contracts, rather than just the very latest L1 blockhash (or beacon block root in this case). Bridges and other tools may build their txs for a certain beacon block-root, and want to interact with it like e.g. doing a merkle proof, without having to “remember” it with a prior transaction that persists the latest beacon-root first. Maybe this type of functionality should be considered for the beacon block root predeploy as well?

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With the parent_beacon_block_root hashed into the block_hash, there is an on-chain commitment that is exposed to EVM via the blockhash opcode. Hence, contracts have everything they need to validate the parent block root via calldata by simply pushing in the full block header or a matching zk proof.

Not sure if the system contract should be included without it being absolutely necessary, or before there being applications making use of it. The contract just feels like adding relatively heavy infrastructure that will be hard to modify in the future.

Given the case that we have a proof of any property of a block whose timestamp is out of the current state of a ring buffer, i.e., it was included in the blockchain too long ago, what approach should we use for the proof?

My first thought is for a Prover contract to have a storage cache with (timestamp, root) “checkpoints,” which is populated by calling the EIP-presented contract. Then anyone can bring a proof of an arbitrary root against some checkpoint in the cache, effectively adding a new checkpoint to the cache. Eventually, we can use any proof for any block root, if I understand correctly.

Is there a more convenient approach? Or am I wrong in something?

EIP-4788:Beacon block root in the EVM with @ralexstokes

is there a timeline of when this will be released?