what can you do with the gas that is sent to a fall back function - solidity

Question

am am learning about transfer and send in solidity, and the fallback function. To my understanding, when you send ether to a smart contract, you also send some gas to the fallback function as well. Why is this mechanism in place? I thought gas was used to pay validators, how come there is gas now stored in the contract + what is this gas in the contract used for now? Thanks

Answer 1

Each operation costs some predefined amount of gas. The combined amount - no matter how many internal transactions are performed within the main transaction - is always paid by the main transaction sender.
The transfer() function (in the current Solidity version 0.8) allows the called address to spend only 2300 gas, preventing the reentrancy attack.
This amount is enough to emit an event but it's not sufficient to perform another call - for example back to the caller contract, that would allow for the reentrancy attack.
So to answer your original question:
what can you do with the gas that is sent to a fall back function
From the called contract perspective: You can only spend it to perform few basic operations such as to emit an event. But you cannot sell it back to ETH and store it in the called contract, nor call another contract (including the sender).
contract Called {
event Received(uint256 amount);
fallback() external payable {
// ok
emit Received(msg.value);
// fail - costs more than the 2300 limit
msg.sender.call("");
}
}