0.8.20 solidity change #41
Description
/**
* @dev Checks if the requested gas was correctly forwarded to the callee.
*
* As a consequence of https://eips.ethereum.org/EIPS/eip-150[EIP-150]:
* - At most `gasleft() - floor(gasleft() / 64)` is forwarded to the callee.
* - At least `floor(gasleft() / 64)` is kept in the caller.
*
* It reverts consuming all the available gas if the forwarded gas is not the requested gas.
*
* IMPORTANT: The `gasLeft` parameter should be measured exactly at the end of the forwarded call.
* Any gas consumed in between will make room for bypassing this check.
*/
function _checkForwardedGas(uint256 gasLeft, ForwardRequestData calldata request) private pure {
// To avoid insufficient gas griefing attacks, as referenced in https://ronan.eth.limo/blog/ethereum-gas-dangers/
//
// A malicious relayer can attempt to shrink the gas forwarded so that the underlying call reverts out-of-gas
// but the forwarding itself still succeeds. In order to make sure that the subcall received sufficient gas,
// we will inspect gasleft() after the forwarding.
//
// Let X be the gas available before the subcall, such that the subcall gets at most X * 63 / 64.
// We can't know X after CALL dynamic costs, but we want it to be such that X * 63 / 64 >= req.gas.
// Let Y be the gas used in the subcall. gasleft() measured immediately after the subcall will be gasleft() = X - Y.
// If the subcall ran out of gas, then Y = X * 63 / 64 and gasleft() = X - Y = X / 64.
// Under this assumption req.gas / 63 > gasleft() is true is true if and only if
// req.gas / 63 > X / 64, or equivalently req.gas > X * 63 / 64.
// This means that if the subcall runs out of gas we are able to detect that insufficient gas was passed.
//
// We will now also see that req.gas / 63 > gasleft() implies that req.gas >= X * 63 / 64.
// The contract guarantees Y <= req.gas, thus gasleft() = X - Y >= X - req.gas.
// - req.gas / 63 > gasleft()
// - req.gas / 63 >= X - req.gas
// - req.gas >= X * 63 / 64
// In other words if req.gas < X * 63 / 64 then req.gas / 63 <= gasleft(), thus if the relayer behaves honestly
// the forwarding does not revert.
if (gasLeft < request.gas / 63) {
// We explicitly trigger invalid opcode to consume all gas and bubble-up the effects, since
// neither revert or assert consume all gas since Solidity 0.8.20
// https://docs.soliditylang.org/en/v0.8.20/control-structures.html#panic-via-assert-and-error-via-require
/// @solidity memory-safe-assembly
assembly {
invalid()
}
}
}