modifier function

Question

Can someone help me on what exactly the modifier is doing in this contract and why the message data length is increased by 4?

contract NonPayloadAttackableToken {
    modifier onlyPayloadSize(uint size) {
        assert(msg.data.length == size + 4);
        _;
    }

    function transfer(address _to, uint256 _value) onlyPayloadSize(2 * 32) {
    // do stuff
    }
}

Answer 1

The + 4 is because the hash of the function signature is 4 bytes, so the modifier is only checking the length of the function payload. This modifier seems to be validating that the msg is size bytes, ignoring the function signature.