1. 引言

智能合约一旦在以太坊上部署，就不可修改，若合约有漏洞，唯一的解决办法是找到并利用该漏洞，以比黑客更快的速度将资产转移到安全的账号。

尽管智能合约一经部署不可升级，但是，可建立a proxy contract architecture that will allow you to use new deployed contracts as if your main logic had been upgraded。
在这里插入图片描述
即，所有的message calls都将通过Proxy Contract直接指向最新部署的Logic Contract。部署新版本的Logic Contract合约升级时，只需修改Proxy指向新部署的合约地址即可。

Zeppelin提供了3种upgrade proxy模式：

1）Inherited Storage
2）Eternal Storage
3）Unstructured Storage

这3种模式都依赖底层的delegatecalls。Solidity提供了delegatecall函数，但是该函数仅返回true/false来表示调用是否成功，并不支持对返回值进行操作。
这3种模式的主要区别在于，用不同的方式来解决相同的技术难题：how to ensure that the Logic Contract does not overwrite state variables that are used in the proxy for upgradeability。

需要理解一下两个关键要点：

When a function call to a contract is made that it does not support, the fallback function will be called. You can write a custom fallback function to handle such scenarios. The proxy contract uses a custom fallback function to redirect calls to other contract implementations.
Whenever a contract A delegates a call to another contract B, it executes the code of contract B in the context of contract A. This means that msg.value and msg.sender values will be kept and every storage modification will impact the storage of contract A.

在这里插入图片描述
借助calldatacopy和returndatacopy，可实现calldata的传入和结果的传出。