242 lines
9.4 KiB
Markdown
242 lines
9.4 KiB
Markdown
---
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eip: 1702
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title: Generalized Account Versioning Scheme
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author: Wei Tang (@sorpaas)
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discussions-to: https://github.com/sorpaas/EIPs/issues/2
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status: Stagnant
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type: Standards Track
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category: Core
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created: 2017-12-30
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---
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## Simple Summary
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Introduce account versioning for smart contracts so upgrading the VM
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or introducing new VMs can be easier.
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## Abstract
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This defines a method of hard forking while maintaining the exact
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functionality of existing account by allowing multiple versions of the
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virtual machines to execute in the same block. This is also useful to
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define future account state structures when we introduce the on-chain
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WebAssembly virtual machine.
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## Motivation
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By allowing account versioning, we can execute different virtual
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machine for contracts created at different times. This allows breaking
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features to be implemented while making sure existing contracts work
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as expected.
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Note that this specification might not apply to all hard forks. We
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have emergency hard forks in the past due to network attacks. Whether
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they should maintain existing account compatibility should be
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evaluated in individual basis. If the attack can only be executed once
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against some particular contracts, then the scheme defined here might
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still be applicable. Otherwise, having a plain emergency hard fork
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might still be a good idea.
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## Specification
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### Account State
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Re-define account state stored in the world state trie to have 5
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items: `nonce`, `balance`, `storageRoot`, `codeHash`, and
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`version`. The newly added field `version` is a 256-bit **scalar**. We
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use the definition of "scalar" from Yellow Paper. Note that this is
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the same type as `nonce` and `balance`, and it is equivalent to a RLP
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variable-sized byte array with no leading zero, of maximum length 32.
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When `version` is zero, the account is RLP-encoded with the first 4
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items. When `version` is not zero, the account is RLP-encoded with 5
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items.
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Account versions can also optionally define additional account state
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RLP fields, whose meaning are specified through its `version`
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field. In those cases, the parsing strategy is defined in "Additional
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Fields in Account State RLP" section.
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### Contract Execution
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When fetching an account code from state, we always fetch the
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associated version field together. We refer to this as the *code's
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version* below. The code of the account is always executed in the
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*code's version*.
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In particular, this means that for `DELEGATECALL` and `CALLCODE`, the
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version of the execution call frame is the same as
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delegating/receiving contract's version.
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### Contract Deployment
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In Ethereum, a contract has a deployment method, either by a contract
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creation transaction, or by another contract. If we regard this
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deployment method a contract's *parent*, then we find them forming a
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family of contracts, with the *root* being a contract creation
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transaction.
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We let a family of contracts to always have the same `version`. That
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is, `CREATE` and `CREATE2` will always deploy contract that has the
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same `version` as the *code's version*.
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In other words, `CREATE` and `CREATE2` will execute the init code
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using the current *code's version*, and deploy the contract of the
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current *code's version*. This holds even if the to-be-deployed code
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is empty.
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### Validation
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A new phrase, *validation* is added to contract deployment (by
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`CREATE` / `CREATE2` opcodes, or by contract creation
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transaction). When `version` is `0`, the phrase does nothing and
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always succeeds. Future VM versions can define additional validation
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that has to be passed.
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If the validation phrase fails, deployment does not proceed and return
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out-of-gas.
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### Contract Creation Transaction
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Define `LATEST_VERSION` in a hard fork to be the latest supported VM
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version. A contract creation transaction is always executed in
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`LATEST_VERSION` (which means the *code's version* is
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`LATEST_VERSION`), and deploys contracts of `LATEST_VERSION`. Before a
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contract creation transaction is executed, run *validation* on the
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contract creation code. If it does not pass, return out-of-gas.
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### Precompiled Contract and Externally-owned Address
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Precompiled contracts and externally-owned addresses do not have
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`version`. If a message-call transaction or `CALL` / `CALLCODE` /
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`STATICCALL` / `DELEGATECALL` touches a new externally-owned address
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or a non-existing precompiled contract address, it is always created
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with `version` field being `0`.
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### Additional Fields in Account State RLP
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In the future we may need to associate more information into an
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account, and we already have some EIPs that define new additional
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fields in the account state RLP. In this section, we define the
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parsing strategy when additional fields are added.
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* Check the RLP list length, if it is 4, then set account version to
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`0`, and do not parse any additional fields.
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* If the RLP list length more than 4, set the account version to the
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scalar at position `4` (counting from `0`).
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* Check version specification for the number of additional fields
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defined `N`, if the RLP list length is not equal to `5 + N`,
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return parse error.
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* Parse RLP position `5` to `4 + N` as the meaning specified in
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additional fields.
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## Extensions
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In relation to the above "Specification" section, we have defined the
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base account versioning layer. The base account versioning layer is
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already useful by itself and can handle most EVM improvements. Below
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we define two specifications that can be deployed separately, which
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improves functionality of base layer account versioning.
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Note that this section is provided only for documentation
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purpose. When "enabling EIP-1702", those extensions should not be
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enabled unless the extension specification is also included.
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- [44-VERTXN: Account Versioning Extension for Contract Creation
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Transaction](https://specs.corepaper.org/44-vertxn/)
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- [45-VEROP: Account Versioning Extension for CREATE and
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CREATE2](https://specs.corepaper.org/45-verop/)
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## Usage Template
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This section defines how other EIPs might use this account
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versioning specification. Note that currently we only define the usage
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template for base layer.
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Account versioning is usually applied directly to a hard fork
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meta. EIPs in the hard fork are grouped by the virtual machine
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type, for example, EVM and eWASM. For each of them, we define:
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* **Version**: a non-zero scalar less than `2^256` that uniquely
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identifies this version. Note that it does not need to be
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sequential.
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* **Parent version**: the base that all new features derived
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from. With parent version of `0` we define the base to be legacy
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VM. Note that once a version other than `0` is defined, the legacy
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VM's feature set must be frozen. When defining an entirely new VM
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(such as eWASM), parent version does not apply.
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* **Features**: all additional features that are enabled upon this
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version.
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If a meta EIP includes EIPs that provide additional account state RLP
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fields, we also define:
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* **Account fields**: all account fields up to the end of this meta
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EIP, excluding the basic 5 fields (`nonce`, `balance`,
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`storageRoot`, `codeHash` and `version`). If EIPs included that are
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specific to modifying account fields do not modify VM execution
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logic, it is recommended that we specify an additional version whose
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execution logic is the same as previous version, but only the
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account fields are changed.
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## Rationale
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This introduces account versioning via a new RLP item in account
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state. The design above gets account versioning by making the contract
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*family* always have the same version. In this way, versions are only
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needed to be provided by contract creation transaction, and there is
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no restrictions on formats of code for any version. If we want to
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support multiple newest VMs (for example, EVM and WebAssembly running
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together), then this will requires extensions such as 44-VERTXN and
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45-VEROP.
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Alternatively, account versioning can also be done through:
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* **[26-VER](https://specs.corepaper.org/26-ver/)** and
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**[40-UNUSED](https://specs.corepaper.org/40-unused/)**: This makes an
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account's versioning soly dependent on its code header prefix. If
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with only 26-VER, it is not possible to certify any code is valid,
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because current VM allows treating code as data. This can be fixed
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by 40-UNUSED, but the drawback is that it's potentially backward
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incompatible.
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* **EIP-1891**: Instead of writing version field into account RLP
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state, we write it in a separate contract. This can accomplish the
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same thing as this EIP and potentially reduces code complexity, but
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the drawback is that every code execution will require an additional
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trie traversal, which impacts performance.
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## Backwards Compatibility
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Account versioning is fully backwards compatible, and it does not
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change how current contracts are executed.
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## Discussions
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### Performance
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Currently nearly all full node implementations uses config parameters
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to decide which virtual machine version to use. Switching virtual
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machine version is simply an operation that changes a pointer using a
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different set of config parameters. As a result, this scheme has
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nearly zero impact to performance.
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### WebAssembly
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This scheme can also be helpful when we deploy on-chain WebAssembly
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virtual machine. In that case, WASM contracts and EVM contracts can
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co-exist and the execution boundary and interaction model are clearly
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defined as above.
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## Test Cases and Implementations
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To be added.
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## References
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The source of this specification can be found at
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[43-VER](https://specs.corepaper.org/43-ver/).
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## Copyright
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Copyright and related rights waived via [CC0](../LICENSE.md).
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