Does it make sense to require token value to be anything but negative to prevent hack (more details below...)?

Question

I was asked to validate a smart contract for a token and a function that puzzled me looks like that:

     function transferFrom(address _from, address _to, uint tokens)

        {
            require(balanceOf[_from] >= tokens);

            if (_from != msg.sender && allowance[_from][msg.sender] != uint(-8523)) {
                require(allowance[_from][msg.sender] >= tokens);
                allowance[_from][msg.sender] -= tokens;
            }

        _transfer(_from, _to, tokens);


}

So the logic is: If and only if both the sender is different than _from and the approval is for anything other than -8523 then make sure _from has approved message sender to transfer the specified amount of tokens and also subtract them from the allowance.

Which of course leads to the question: is it technically possible for uint to be negative and can someone specify allowance for negative uint which is namely -8523? What if uint is send in hexadecimal or so? Thanx!

Answer 1

Some tokens use a special value in allowance to minimize gas fees when used in combination with approve/transferFrom.

If allowance is that special value it will skip updating it, saving one storage modification. If you perform multiple transactions for example in decentralized exchanges you will have important savings over time.

It is a "hackish" way to minimize gas usage. I don't like it I'd prefer to have a separate flag and for the user to explicitely enable it. Otherwise you have to make sure the value will never be generated under normal circumstances.