Abstract

Introduce code validation at contract creation time for EOF formatted (EIP-3540) contracts. Reject contracts which contain truncated PUSH-data or undefined instructions. Legacy bytecode (code which is not EOF formatted) is unaffected by this change.

Motivation

Currently existing contracts require no validation of correctness and EVM implementations can decide how they handle truncated bytecode or undefined instructions. This change aims to bring code validity into consensus, so that it becomes easier to reason about bytecode. Moreover, EVM implementations may require fewer paths to decide which instruction is valid in the current execution context.

If there's a desire to introduce new instructions without bumping the EOF version, having undefined instructions already deployed could potentially break such contracts, as some instructions might change their behavior. Rejecting to deploy undefined instructions allows introducing new instructions with or without bumping the EOF version.

EOF1 forward compatibility

The EOF1 format provides following forward compatibility properties:

1. New instructions can be defined for previously unassigned opcodes. These instructions may have immediate values.
2. Mandatory EOF sections may be made optional.
3. New optional EOF sections may be introduced. They can be placed in any order in relation to previously defined sections.

Specification

Remark: We rely on the notation of initcode, code and creation as defined by EIP-3540.

This feature is introduced on the same block EIP-3540 is enabled, therefore every EOF1-compatible bytecode MUST be validated according to these rules.

1. Previously deprecated instructions CALLCODE (0xf2) and SELFDESTRUCT (0xff) are invalid and their opcodes are undefined.
2. At contract creation time instructions validation is performed on both initcode and code. The code is invalid if any of the checks below fails. For each instruction:
   1. Check if the opcode is defined. The INVALID (0xfe) is considered defined.
   2. Check if all instructions' immediate bytes are present in the code (code does not end in the middle of instruction).

Rationale

Immediate data

Allowing implicit zero immediate data for PUSH instructions introduces inefficiencies to EVM implementations without any practical use-case (the value of a PUSH instruction at the code end cannot be observed by EVM). This EIP requires all immediate bytes to be explicitly present in the code.

Rejection of deprecated instructions

The deprecated instructions CALLCODE (0xf2) and SELFDESTRUCT (0xff) are removed from the valid_opcodes list to prevent their use in the future.

Backwards Compatibility

This change poses no risk to backwards compatibility, as it is introduced at the same time EIP-3540 is. The validation does not cover legacy bytecode (code which is not EOF formatted).

Test Cases

Contract creation

Each case should be tested for creation transaction, CREATE and CREATE2.

• Invalid initcode
• Valid initcode returning invalid code
• Valid initcode returning valid code

Valid codes

• EOF code containing INVALID
• EOF code with data section containing bytes that are undefined instructions
• Legacy code containing undefined instruction
• Legacy code ending with incomplete PUSH instruction

Invalid codes

• EOF code containing undefined instruction
• EOF code ending with incomplete PUSH instruction
  • This can include PUSH instruction unreachable by execution, e.g. after STOP

Reference Implementation

# The ranges below are as specified in the Yellow Paper.
# Note: range(s, e) excludes e, hence the +1
valid_opcodes = [
    *range(0x00, 0x0b + 1),
    *range(0x10, 0x1d + 1),
    0x20,
    *range(0x30, 0x3f + 1),
    *range(0x40, 0x48 + 1),
    *range(0x50, 0x5b + 1),
    *range(0x60, 0x6f + 1),
    *range(0x70, 0x7f + 1),
    *range(0x80, 0x8f + 1),
    *range(0x90, 0x9f + 1),
    *range(0xa0, 0xa4 + 1),
    # Note: 0xfe is considered assigned.
    0xf0, 0xf1, 0xf3, 0xf4, 0xf5, 0xfa, 0xfd, 0xfe
]

immediate_sizes = 256 * [0]
immediate_sizes[0x60:0x7f + 1] = range(1, 32 + 1)  # PUSH1..PUSH32


# Raises ValidationException on invalid code
def validate_instructions(code: bytes):
    # Note that EOF1 already asserts this with the code section requirements
    assert len(code) > 0

    pos = 0
    while pos < len(code):
        # Ensure the opcode is valid
        opcode = code[pos]
        if opcode not in valid_opcodes:
            raise ValidationException("undefined opcode")

        # Skip immediate data
        pos += 1 + immediate_sizes[opcode]

    # Ensure last instruction's immediate doesn't go over code end
    if pos != len(code):
        raise ValidationException("truncated immediate")

Security Considerations

See Security Considerations of EIP-3540.

Copyright

Copyright and related rights waived via CC0.