Why does Gnosis' MultiSig passes less gas than available in external_call?

Question

Gnosis' MultiSig Wallet substract a fixed amount of gas (34710) when calling an external address.

function external_call(address destination, uint value, uint dataLength, bytes data) internal returns (bool) {
    bool result;
    assembly {
        let x := mload(0x40)   // "Allocate" memory for output (0x40 is where "free memory" pointer is stored by convention)
        let d := add(data, 32) // First 32 bytes are the padded length of data, so exclude that
        result := call(
            sub(gas, 34710),   // 34710 is the value that solidity is currently emitting
                               // It includes callGas (700) + callVeryLow (3, to pay for SUB) + callValueTransferGas (9000) +
                               // callNewAccountGas (25000, in case the destination address does not exist and needs creating)
            destination,
            value,
            d,
            dataLength,        // Size of the input (in bytes) - this is what fixes the padding problem
            x,
            0                  // Output is ignored, therefore the output size is zero
        )
    }
    return result;
}

It seems to reserve some gas for the worst case scenario. But it causes strange behavior in some circumstances.

• gas available less than 34710: sub() will overflow, but EVM fill cap gas to the max available, so the call may succeed.
• gas available slightly more to 34710: sub() returns a small amount of thas that will make the inner call to fail with out of gas.

Is there any reason for doing that instead of just sending all available gas?

Answer 1

This contract is quite "old" (pragma solidity ^0.4.15;). The logic is actually taken from the solidity compiler. When targeting older versions of the evm it was not possible to send along all gas. The solidity code still exists even now: https://github.com/ethereum/solidity/blob/develop/libsolidity/codegen/ExpressionCompiler.cpp#L2343

Solidity changed this with https://github.com/ethereum/solidity/releases/tag/v0.4.18 (in preparation for 0.5.0):

Code Generator: Always use all available gas for calls as experimental 0.5.0 feature (previously, some amount was retained in order to work in pre-Tangerine-Whistle EVM versions)

Why this was initially done in assembly in the MultiSig contract?

This is because in older Solidity versions the msg.data of a Solidity call was padded which caused errors with some contracts. (Guards for short address attacks, see https://vessenes.com/the-erc20-short-address-attack-explained/)