EIP1985 - Sane limits for certain EVM parameters

# Abstract

Introduce an explicit value range for certain EVM parameters (such as gas limit, block number, block timestamp, size field when returning/copying data within EVM). Some of these already have an implicit value range due to various (practical) reasons.

# Motivation

Having such an explicit value range can help in creating compatible client implementations, in certain cases it can also offer minor speed improvements, and can reduce the effort needed to create consensus critical test cases by eliminating unrealistic edge cases.

# Specification

If block.number >= {FORK_BLOCK}, the following value ranges are introduced. They restrict the results (i.e. values pushed to the stack) of the instructions listed below.

  1. gas, gas limit, block gas limit is a range between 0 and 0x7fffffffffffffff (2**63 - 1, 9223372036854775807). It affects the following instructions:

    • GASLIMIT (0x45),
    • GAS (0x5a).
  2. block number, timestamp is a range between 0 and 0x7fffffffffffffff (2**63 - 1, 9223372036854775807). It affects the following instructions:

    • TIMESTAMP (0x42),
    • NUMBER (0x43).
  3. account address is a range between 0 and 0xffffffffffffffffffffffffffffffffffffffff (2**160 - 1, 1461501637330902918203684832716283019655932542975) i.e. the address occupies the 160 low bits of the 256-bit value and the remaining top 96 bits must be zeros. It affects the following instructions:

    • ADDRESS (0x30),
    • ORIGIN (0x32),
    • CALLER (0x33),
    • COINBASE (0x41),
    • CREATE (0xf0),
    • CREATE2 (0xf5).
  4. buffer size, code size, memory size is a range between 0 and 0xffffffff (2**32 - 1, 4294967295). It affects the following instructions:

    • CALLDATASIZE (0x36),
    • CODESIZE (0x38),
    • EXTCODESIZE (0x3b),
    • RETURNDATASIZE (0x3d),
    • MSIZE (0x59),
    • PC (0x58).

# Rationale

These limits have been:

Most of the limits proposed in this document have been previously explored and tested in EVMC (opens new window).

Using the 2**63 - 1 constant to limit some of the ranges:

  • allows using signed 64-bit integer type to represent it, what helps programming languages not having unsigned types,
  • makes arithmetic simpler (e.g. checking out-of-gas conditions is simple as gas_counter < 0).

# Timestamp

The Yellow Paper (opens new window) defines the timestamp in block as "A scalar value equal to the reasonable output of Unix’s time() at this block’s inception". IEEE Std 1003.1-2001 (POSIX.1) leaves that definition implementation defined.

# Addresses

The size of addresses is specified in the Yellow Paper (opens new window) as 20 bytes. E.g. the COINBASE instruction is specified to return Hc ∈ 𝔹20 which has 20 bytes.

# Memory size

Memory expansion cost is not linear and is determined by the following formula: cost = cost_per_word * number_of_words + (number_of_words ^ 2 / 512)

Expanding to over 2^32 - 1 bytes would cost 35184774742016 gas. This number fits into the gas limit imposed above (2 ^ 63 - 1) and would cost around 35184 Ether in a transaction to exhaust, with a 1 GWei gas cost, which can be attained on mainnet.

However, setting the limit 2^32 - 1 is beneficial from a VM design perspective and we believe limiting memory should be done via carefully selecting the block gas limit.

# Code size

EIP-170 has implemented a code size limit of 0x6000, however even before that, it was practically impossible to deploy a code blob exceeding 2**32 - 1 bytes in size.

# Comparing current implementations

# Backwards Compatibility

All of these limits are already enforced mostly through the block gas limit. Since the out of range case results in a transaction failure, there should not be a change in behaviour.

# Test Cases


# Implementation


# References


  1. Does the gas limit apply to the gas argument for call instructions?

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