EIP-3155: EVM trace specification

Abstract

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Introduce a new JSON standard for EVM traces during execution of state tests.

Motivation

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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-Ethereum, 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 goals:

• 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 to find 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

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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-ethereum's 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
memSize	Number	Size of memory array
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

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

Example:

• The stack, memory and memSize are the values before execution of the op.
• All array attributes (stack, memory) MUST be initialized to empty arrays ("stack":[]) NOT to null.
• If the CUT will not be outputting 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:

Summary and Error Handling

At the end of execution, the CUT MUST print summary 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

Optional Fields

Name	Type	Explanation
time	Number	Time in nanoseconds needed to execute the transaction
fork	String	Name of the fork rules used for execution

Example:

Rationale

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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

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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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Security Considerations

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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

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Copyright and related rights waived via CC0.