EIP-3155: EVM trace specification
| Authors | Martin Holst Swende (@holiman), Marius van der Wijden (@MariusVanDerWijden) |
|---|---|
| Created | 2020-12-07 |
| Discussion Link | https://ethereum-magicians.org/t/eip-3155-create-evm-trace-specification/5007 |
Table of Contents
Simple Summary
Introduce a new JSON standard for EVM traces during execution of state tests.
Motivation
The Ethereum Virtual Machine executes all smart contract code on ethereum. In order to debug smart contracts and state tests better, a common format was introduced to log every execution step of the EVM. This format was implemented by go-ethereum, parity, nethermind and Besu. Since the common format was not well defined, the implementations differed slightly making it hard to develop adequate tooling which reduces the usefulness of tracing significantly.
This EIP has multiple objectives:
- Move the specification to a more visible place to encourage new clients to implement it
- Strictly define corner cases that were not addressed in the previous version
- Allow for updates to the specification in case new fields are introduced during execution
- Provide sample output
Implementing this EIP in all major clients allows us to create meaningful differential fuzzers that fuzz EVM implementations for the mainnet and all upcoming hardforks. It also helps finding differences in execution quickly in the case of a chain split.
This EIP will enable users to create better differential fuzzing infrastructure to compare the EVM implementations of all major Ethereum clients against each other. This could help to find bugs that are currently present in the client implementations.
Specification
Clients should be able to execute simple transactions as well as code and return traces. In the following, we will call this client CUT (client under test) and use go-ethereums evm binary for code examples.
Datatypes
| Type | Explanation | Example |
|---|---|---|
| Number | Plain json number | "pc":0 |
| Hex-Number | Hex-encoded number | "gas":"0x2540be400" |
| String | Plain string | "opName":"PUSH1" |
| Hex-String | Hex-encoded string | |
| Array of x | Array of x encoded values | |
| Key-Value | Key-Value structure with key and values encoded as hex strings | |
| Boolean | Json bool can either be true or false | "pass": true |
Output
The CUT MUST output a json object for EACH operation.
Required Fields:
| Name | Type | Explanation |
|---|---|---|
pc | Number | Program Counter |
op | Number | OpCode |
gas | Hex-Number | Gas left before executing this operation |
gasCost | Hex-Number | Gas cost of this operation |
stack | Array of Hex-Numbers | Array of all values on the stack |
depth | Number | Depth of the call stack |
returnData | Hex-String | Data returned by function call |
refund | Hex-Number | Amount of global gas refunded |
memSize | Number | Size of memory array |
Optional Fields:
| Name | Type | Explanation |
|---|---|---|
opName | String | Name of the operation |
error | Hex-String | Description of an error (should contain revert reason if supported) |
memory | Array of Hex-Strings | Array of all allocated values |
storage | Key-Value | Array of all stored values |
returnStack | Array of Hex-Numbers | Array of values, Stack of the called function |
Example:
{"pc":0,"op":96,"gas":"0x2540be400","gasCost":"0x3","memory":"0x","memSize":0,"stack":[],"depth":1,"error":null,"opName":"PUSH1"}
The stack, memory and memSize are the values before execution of the op.
All array attributes (stack, returnStack, memory) MUST be initialized to empty arrays ("stack":[],) NOT to null.
If the CUT will not output values for memory or storage then the memory and storage fields are omitted.
This can happen either because the CUT does not support tracing these fields or it has been configured not to trace it.
The memSize field MUST be present regardless of memory support.
Clients SHOULD implement a way to disable recording the storage as the stateroot includes all storage updates.
Clients SHOULD output the fields in the same order as listed in this EIP.
The CUT MUST NOT output a line for the STOP operation if an error occurred:
Example:
{"pc":2,"op":0,"gas":"0x2540be3fd","gasCost":"0x0","memory":"0x","memSize":0,"stack":["0x40"],"depth":1,"error":null,"opName":"STOP"}
Summary and error handling
At the end of execution, the CUT MUST print some summerical info, this info SHOULD have the following fields.
The summary should be a single jsonl object.
Required Fields:
| Name | Type | Explanation |
|---|---|---|
stateRoot | Hex-String | Root of the state trie after executing the transaction |
output | Return values of the function | |
gasUsed | Hex-Number | All gas used by the transaction |
pass | Boolean | Bool whether transaction was executed successfully |
OptionalFields:
| Name | Type | Explanation |
|---|---|---|
time | Number | Time in nanoseconds needed to execute the transaction |
fork | String | Name of the fork rules used for execution |
{"stateRoot":"0xd4c577737f5d20207d338c360c42d3af78de54812720e3339f7b27293ef195b7","output":"","gasUsed":"0x3","successful":"true","time":141485}
Rationale
This EIP is largely based on the previous non-official documentation for EVM tracing. It tries to cover as many corner cases as possible to enable true client compatibility. The datatypes and if a field is optional is chosen to be as compatible with current implementations as possible.
Backwards Compatibility
This EIP is fully backward compatible with ethereum as it only introduces a better tracing infrastructure that is optional for clients to implement.
Clients
This EIP is fully backward compatible with go-ethereum. OpenEthereum, Besu and Nethermind clients would have to change their JSON output of openethereum-evm evmtool and nethtest slightly do adhere to the new and stricter specs. New clients would need to implement this change if they want to be part of the differential fuzzing group.
Test Cases
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Implementation
Implementation in go-ethereum Implementation in OpenEthereum Implementation in Besu Implementation in Nethermind
Security Considerations
Tracing is expensive. Exposing an endpoint for creating traces publicly could open up a denial of service vector. Clients should consider putting trace endpoints behind a separate flag from other endpoints.
Copyright
Copyright and related rights waived via CC0.
Citation
Please cite this document as: