Limit and meter initcode
EIP-3860 introduces a maximum size limit of 49152 for initcode and charges 2 gas for every 32-byte chunk of initcode to represent the cost of jumpdest-analysis. This is to address the issue of linearly scaling work during contract creation and to prevent the possibility of crafting malicious initcode.
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Original
Abstract
We extend EIP-170 by introducing a maximum size limit for initcode
(MAX_INITCODE_SIZE = 2 * MAX_CODE_SIZE = 49152
).
Furthermore, we introduce a charge of 2
gas for every 32-byte chunk of initcode
to represent the cost of jumpdest-analysis.
Lastly, the size limit results in the nice-to-have property that EVM code size, code offset (PC
), and jump offset fits a 16-bit value.
Motivation
During contract creation the client has to perform jumpdest-analysis on the initcode
prior to execution. The work performed scales linearly with the size of the initcode
. This work currently is not metered, nor is there a protocol enforced upper bound for the size.
There are three costs charged today:
- Cost for calldata aka
initcode
: 4 gas for a byte with the value of zero, and 16 gas otherwise. - Cost for the resulting deployed code: 200 gas per byte.
- Cost of address calculation (hashing of code) in case of
CREATE2
only: 6 gas per word.
Only the first cost applies to initcode
, but only in the case of contract creation transactions. For the case of CREATE
/CREATE2
there is no such cost, and it is possible to programmatically generate variations of initcode
in a relatively cheap manner. In the past it was possible to craft malicious initcode
due to a vulnerability fixed in 2017 by geth 1.6.5.
Furthermore, the lack of a limit has caused lengthy discussions for some EVM proposals, influencing the design, or even causing a delay or cancellation of a feature.
We are motivated by three reasons:
- Ensuring
initcode
is fairly charged (most importantly cost is proportional toinitcode
's length) to minimize the risks for the future. - To have a cost system which is extendable in the future.
- To simplify EVM engines by the explicit limits (code size, code offsets (
PC
), and jump offsets fit 16-bits).
Specification
Parameters
Constant | Value |
---|---|
INITCODE_WORD_COST | 2 |
MAX_INITCODE_SIZE | 2 * MAX_CODE_SIZE |
Where MAX_CODE_SIZE
is defined by EIP-170 as 24576
.
We define initcode_cost(initcode)
to equal INITCODE_WORD_COST * ceil(len(initcode) / 32)
.
Rules
- If length of transaction data (
initcode
) in a create transaction exceedsMAX_INITCODE_SIZE
, transaction is invalid. (Note that this is similar to transactions considered invalid for not meeting the intrinsic gas cost requirement.) - For a create transaction, extend the transaction data cost formula to include
initcode_cost(initcode)
. (Note that this is included in transaction intrinsic cost, i.e. transaction with not enough gas to cover initcode cost is invalid.) - If length of
initcode
toCREATE
orCREATE2
instructions exceedsMAX_INITCODE_SIZE
, instruction execution exceptionally aborts (as if it runs out of gas). - For the
CREATE
andCREATE2
instructions charge an extra gas cost equaling toinitcode_cost(initcode)
. This cost is deducted before the calculation of the resulting contract address and the execution ofinitcode
. (Note that this means before or at the same time as the hashing cost is applied inCREATE2
.)
Rationale
Gas cost constant
The value of INITCODE_WORD_COST
is selected based on performance benchmarks of differing worst-cases per implementation. The baseline for the benchmarks is the performance of KECCAK256
hashing in geth 1.10.9, which matches the 70 Mgas/s gas limit target on a 4.0 GHz x86_64 CPU.
EVM | version | MB/s | B/CPUcycle | CPUcycle/B | cost of 1 B | cost of 32 B |
---|---|---|---|---|---|---|
geth/KECCAK256 | 1.10.9 | 357 | 1.8 | 0.6 | 0.2 | 6.0 |
geth | 1.10.9 | 1091 | 5.5 | 0.2 | 0.1 | 2.0 |
evmone/Baseline | 0.8.2 | 727 | 3.7 | 0.3 | 0.1 | 2.9 |
evmone/Advanced | 0.8.2 | 155 | 0.8 | 1.3 | 0.4 | 13.8 |
Gas cost per word (32-byte chunk)
We have chosen the cost of 2 gas per word based on Geth's implementation and comparing with KECCAK256
performance. This means the per byte cost is 0.0625
. While fractional gas costs are not permitted in the EVM, we can approximate it by charging per-word.
Moreover, calculating gas per word is compatible with the calculation of CREATE2
's hashcost of EIP-1014. Therefore, the same implementation may be used for CREATE
and CREATE2
with different cost constants: before activation 0
for CREATE
and 6
for CREATE2
, after activation 2
for CREATE
and 6 + 2
for CREATE2
.
Reason for size limit of initcode
Estimating and creating worst case scenarios is easier with an upper bound in place, given one parameter for the search is greatly reduced. This allows for selecting a much more optimistic gas per byte.
Should there be no upper bound, the cost would need to be higher accounting for unknown unknowns. Given most initcode (TODO: state maximum initcode size resulting in deployment seen on mainnet here) does not exceed the proposed limit, penalising contracts by overly conservative costs seems unnecessary.
Effect of size limit of initcode
In most, if not all cases when a new contract is being created, the resulting runtime code is copied from the initcode itself. For the basic case the 2 * MAX_CODE_SIZE
limit allows MAX_CODE_SIZE
for runtime code and another MAX_CODE_SIZE
for contract constructor code. However, the limit may have practical implications for cases where multiple contracts are deployed in a single create transaction.
Initcode cost for create transaction
The initcode cost for create transaction data (0.0625 gas per byte) is negligible compared to the transaction data cost (4 or 16 gas per byte). Despite that, we decided to include it in the specification for consistency, and more importantly for forward compatibility.
How to report initcode limit violation?
We specified that initcode size limit violation for CREATE
/CREATE2
results in exceptional abort of the execution. This places it in the group of early out-of-gas checks, including: stack underflow, memory expansion, static call violation, initcode hashing cost, and initcode cost introduced by this EIP. They precede the later "light" checks: call depth and balance. The choice gives consistency to the order of checks and lowers implementation complexity (out-of-gas checks can be performed in any order).
Backwards Compatibility
This EIP requires a "network upgrade", since it modifies consensus rules.
Already deployed contracts should not be effected, but certain transactions (with initcode
beyond the proposed limit) would still be includable in a block, but result in an exceptional abort.
Test Cases
Tests should include the following cases:
- Creation transaction with gas limit enough to cover initcode cost
- Creation transaction with gas limit enough to cover intrinsic cost except initcode cost
CREATE
/CREATE2
/creation transaction withlen(initcode)
atMAX_INITCODE_SIZE
CREATE
/CREATE2
/creation transaction withlen(initcode)
atMAX_INITCODE_SIZE+1
Security Considerations
For client implementations, this EIP makes attacks based on jumpdest-analysis less problematic, so should increase the robustness of clients.
For layer 2, this EIP introduces failure-modes where there previously were none. There could exist factory-contracts which deploy multi-level contract hierarchies, such that the code for multiple contracts are included in the initcode of the first contract. The author(s) of this EIP are not aware of any such contracts.
Currently, on London, with 30M
gas limit, it would be possible to trigger jumpdest-analysis of a total ~1.3GB
of initcode. With this EIP, the cost for such an attack would increase by roughly 80M
gas.
Copyright
Copyright and related rights waived via CC0.
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