How to sign a transaction send by contract - ethereum

Question

I am trying to understand the basics behind signing an ethereum transaction.
I come across Gnosis's MultiSigWallet :
https://github.com/gnosis/MultiSigWallet/blob/master/contracts/MultiSigWallet.sol
In which, a modifier onlyWallet requires a transaction must be sent from the contract itself, but the transaction is signed by one of the owners of the contracts:
modifier onlyWallet() {
require(msg.sender == address(this));
_;
}
for functions like:
function replaceOwner(address owner, address newOwner)
public
onlyWallet
ownerExists(owner)
ownerDoesNotExist(newOwner)
{...}
I have successfully deployed the contracts on my testnet, and tried its functionalities using their dapp https://wallet.gnosis.pm/#/transactions
However, I cannot understand how a transaction is signed and sent to meet the onlyWallet requirement, since signing a transaction using metamask for example will cause the msg.sender to be my own wallet address.
If possible, an example of this ethereumjs-tx's functions would be much appreciated.
like:
const privateKey = Buffer.from('<private key 1>','hex');
const txParams = {
nonce: web3.utils.toHex(11),
gasPrice: web3.utils.toHex(1000000000),
gasLimit: web3.utils.toHex(300000),
to: '<contract address>',
value: web3.utils.toHex(web3.utils.toWei("1",'ether')),
data: '0x00',
chainId: 1
};
let tx = new EthTx(txParams);
tx.sign(tx.serialize().toString('hex');
web3.eth.sendSignedTransaction(`0x${tx.serialize().toString('hex')}`,
(error, data) => {
if(!error) {
console.log(data);
}else console.log(error);
}
);
Thank you very much

Answer 1

Adding and removing an owner follows the same rules as confirming any transactions from the MultiSigWallet. Assume the wallet was deployed with 3 EOA addresses as owners and requiring 2 confirmations for execution. For any regular transaction, say donate funds from the wallet to a FundRaisingContract, one of the owners would need to first call submitTransaction() passing in the address of FundRaisingContract, the amount of ether, and the hash for the donate function. After submission, you still need 1 of the other owners to confirmTransaction (the submitter is automatically confirmed). Once completed, now executeTransaction can be successfully run which will execute the call to donate() from the MultiSigWallet address.
That last part is what you're looking for to answer your question. To add an owner, repeat the example but use the MultiSigWallet address and the hash of the addOwner method. When one of the other owners confirms the transaction, addOwner will be called and it will pass the onlyWallet modifier check.