Decide if reentrancy can be allowed for certain VMs

[ma2bd]

[MathieuDutSik]

Or certain apps? This could conceivably be part of the application description

[MathieuDutSik]

Reentrancy is used in some DeFi applications for Flash loans. The code for the provider will look like this:

function flashLoan(address receiver, uint256 amount, bytes calldata data) external {
    uint256 balanceBefore = address(this).balance;
    (bool ok, ) = receiver.call{ value: amount }(
        abi.encodeWithSignature("executeOperation(uint256,bytes)", amount, data)
    );
    require(ok, "callback failed");
    require(address(this).balance >= balanceBefore + fee, "loan not repaid");
}

So, you call your lending protocol (Uniswap, Morpho, etc.) with your callback information, and it does a reentrant call on your contract and checks that it went correctly. The operation is done completely or not at all.

[ma2bd]

Or certain apps? This could conceivably be part of the application description

Yes, technically it should be very easy to have two Ethereum VM configurations: one with reentrancy and one without.