Behavior of ERC165Checker when less than 30k gas available

[wighawag]

EIP-165 stipulate that supportsInterface can use up to 30,000 gas.

But as you can see in openzeppelin implementation here, the call is executed without making sure 30,000 gas is indeed given to the call. Remember, the gas provided as part of the STATIC_CALL is just a maximum.

And because of EIP-150 behaviour, it is possible for supportsInterface to get less gas than required for it to complete (and thus throw which is interpreted wrongly as non-implementation) while the rest of the transaction continue and complete.

I described the issue in more details here as the issue is also present in the example implementation described at EIP-165.

Various solution are presented here but the best option is EIP-1930 which also solve issue present in other use cases like meta-transactions.

Also find some test case regarding EIP-165 here

[nventuro]

Hello @wighawag, thank you for reporting this!

How would you suggest to change the implementation? If, as you've mentioned, that function is called with less than 30k gas available, then the execution will error out with an 'out of gas' error, which is different from a regular revert. However, if there's indeed not enough gas, what could the contract do, other than erroring out early?

[wighawag]

Hi @nventuro I mention various solution available today on EIP-1930 as well as on EIP-165 discussion

It can

• check for gas (via gasleft()) before the call but need to account for the gas required between the call to gasleft() and the actual call
• check for gas afterward assuming supportsinterface is not quitting early based on the gas passed in,

But the best solution is to add functionality in the EVM for that : see EIP-1930

[nventuro]

check for gas (via gasleft()) before the call but need to account for the gas required between the call to gasleft() and the actual call

What is the intent behind this? What should a contract do if it has less than 30k gas available to make the call?

check for gas afterward assuming supportsinterface is not quitting early based on the gas passed in,

30k will be sent if they are available: the only way for the implementer to quit early is if the transaction doesn't have enough gas, in which case the caller will revert with an out of gas error.

[wighawag]

check for gas (via gasleft()) before the call but need to account for the gas required between the call to gasleft() and the actual call

What is the intent behind this? What should a contract do if it has less than 30k gas available to make the call?

_callERC165SupportsInterface can be

function _callERC165SupportsInterface(address account, bytes4 interfaceId)
    private
    view
    returns (bool success, bool result)
{
    bytes memory encodedParams = abi.encodeWithSelector(_INTERFACE_ID_ERC165, interfaceId);

    uint256 gasAvailable = gasleft() - E;
    require(gasAvailable - gasAvailable / 64  >= 30000, "not enough gas provided")
    // solhint-disable-next-line no-inline-assembly
    assembly {
        let encodedParams_data := add(0x20, encodedParams)
        let encodedParams_size := mload(encodedParams)

        let output := mload(0x40)    // Find empty storage location using "free memory pointer"
        mstore(output, 0x0)

        success := staticcall(
            30000,                   // 30k gas
            account,                 // To addr
            encodedParams_data,
            encodedParams_size,
            output,
            0x20                     // Outputs are 32 bytes long
        )

        result := mload(output)      // Load the result
    }
}

where E is the gas required for the operations between the call to gasleft() and the actual call.
Unfortunately this computation will be dependent on gas pricing. And as such an overestimation is required.

For the intent, the idea is that if _callERC165SupportsInterface can't ensure that supportsInterface will receive 30,000 gas, it has to revert the call since it cannot be sure whether supportsInterface throw because it did not received enough gas or simply because it does not implement supportsInterface

check for gas afterward assuming supportsinterface is not quitting early based on the gas passed in,

30k will be sent if they are available: the only way for the implementer to quit early is if the transaction doesn't have enough gas, in which case the caller will revert with an out of gas error.

While it is unlikely in the context of ERC-165 I wanted to make it clear that the "after-the-call" check does not work if the implementer of supportsInterface is doing something like require(gasleft() > X as in that case it could quit earlier because of a lack of gas, while not using all gas provided. If that happen the check gasleft() > 30,000 / 63 will not be sufficient since while less than 30,000 was given there could be more than 30,000/63 left.

[frangio]

Thanks for bringing this up @wighawag. Super interesting.

Unfortunately this computation will be dependent on gas pricing. And as such an overestimation is required.

To be clear, by "gas pricing" you mean the gas cost of each opcode, right?

[frangio]

I'm looking at the EIP and it seems rather clear that a reverting supportsInterface should only be interpreted to mean "false" when querying about supporting ERC165 itself.

How to Detect if a Contract Implements ERC-165

1. The source contract makes a STATICCALL to the destination address with input data: 0x01ffc9a701ffc9a700000000000000000000000000000000000000000000000000000000 and gas 30,000. This corresponds to contract.supportsInterface(0x01ffc9a7).
2. If the call fails or return false, the destination contract does not implement ERC-165.
3. If the call returns true, a second call is made with input data 0x01ffc9a7ffffffff00000000000000000000000000000000000000000000000000000000.
4. If the second call fails or returns true, the destination contract does not implement ERC-165.
5. Otherwise it implements ERC-165.

How to Detect if a Contract Implements any Given Interface

1. If you are not sure if the contract implements ERC-165, use the above procedure to confirm.
2. If it does not implement ERC-165, then you will have to see what methods it uses the old-fashioned way.
3. If it implements ERC-165 then just call supportsInterface(interfaceID) to determine if it implements an interface you can use.

Note how the second list doesn't mention anything about a failing call. This impies that reverts must be propagated.

So there are two bugs in our implementation:

1. ERC165Checker._supportsERC165 is affected by the gas-related bug reported in this issue.
2. ERC165Checker._supportsInterface is not propagating reverts like it should be.

[frangio]

The second bug is easy to fix. The first one is the tricky one.

the "after-the-call" check does not work if the implementer of supportsInterface is doing something like require(gasleft() > X)

@wighawag In this case it is the implementer who is buggy. Because of the following line from the EIP:

If the call fails or return false, the destination contract does not implement ERC-165.

This implies that if the implementer indeed does require(gasleft() > X) it should be interpreted as not implementing ERC165. (This definitely deserves special mention in the EIP.)

With this in mind, do you think an after-the-call check would work? I will give this a bit more thought and get back.

[nventuro]

Thanks for reporting this @wighawag, I had forgotten about the 63/64ths rule.

In the worst case, a contract needs 30k gas to execute its function, and is provided only 29,999. This means that the caller will end up with 476 gas (29999 / 63) after the (failed) call. I don't think 476 gas is enough for a contract to perform any significant action (and this is the absolute worst case), but it is enough for a view function to return, which may be used to trick whomever is calling (maybe a dApp's UI)? We should therefore definitely fix this.

[frangio]

Pasting here @wighawag's suggestion from ethereum/EIPs#881 (comment):

check for gasleft() after the call

// execute STATIC_CALL with 30000 gas
require(gasleft() > 30000/63, "not enough gas left");

This works because if the call throw because of not enough gas, the amount of gas left will be lower than 30000/63. But this also require for supportsInterface to not have code like require(gasleft() > X) since this in that case, the gas left after the call would be bigger than what it would be if all the gas was used
This does not depend on gas pricing except for the 1/64 behavior of EIP-150

Given the considerations above, I think this is the best course of action.

[nventuro]

@frangio is the idea behind that check to not require the caller tx to have over 30k gas? Shouldn't we also only revert if the call failed? i.e. if the call failed and it possibly run out of gas (there's less than 30k/63 remaining), then we revert.

[frangio]

i.e. if the call failed and it possibly run out of gas (there's less than 30k/63 remaining), then we revert.

Yes. Why "possibly" though?