Understanding OpenZeppelin's Context contract

Question

I'm new to solidity and trying to understand OpenZeppelin's Context contract. At the time of writing, the documentation says:

While these are generally available via msg.sender and msg.data, they should not be accessed in such a direct manner, since when dealing with meta-transactions the account sending and paying for execution may not be the actual sender (as far as an application is concerned).

This contract is only required for intermediate, library-like contracts.

I kind of understand at a high level that msg.sender can be different from the actual sender for a meta-transaction, but I do not understand how using Context's _msgSender() or _msgData() helps with that. Both functions are simply returning msg.sender and msg.data and they look no different from directly accessing msg.sender or msg.data.

OpenZeppelin's Ownable uses _msgSender() in its constructor and modifier onlyOwner() but I don't understand how that's different from hardcoding msg.sender.

A specific example where calling _msgSender() is different from msg.sender would be very helpful!

Answer 1

This question has been asked and answered on the OZ forum here : https://forum.openzeppelin.com/t/help-understanding-contract-context/10579.

The _msg.sender and _msg.data functions are in fact overridden in the OZ smart contract ERC2771Context which implements the EIP-2771 : Secure Protocol for Native Meta Transactions.

_msg.sender here allows a recipient contract to retrieve the original sender in the case of a meta-transaction sent by a trusted forwarder :

function _msgSender() internal view virtual override returns (address sender) {
    if (isTrustedForwarder(msg.sender)) {
        // The assembly code is more direct than the Solidity version using `abi.decode`.
        assembly {
            sender := shr(96, calldataload(sub(calldatasize(), 20)))
        }
    } else {
        return super._msgSender();
    }
}

From the EIP-2771 :

The Trusted Forwarder is responsible for calling the Recipient contract and MUST append the address of the Transaction Signer (20 bytes of data) to the end of the call data.