Consider a declarative API to describe XCM barrier conditions

[KiChjang]

Currently, the way we set up XCM barriers is done in a very procedural and imperative manner:

polkadot/xcm/xcm-builder/src/barriers.rs

Lines 53 to 92 in 903c6f2

	pub struct AllowTopLevelPaidExecutionFrom<T>(PhantomData<T>);
	impl<T: Contains<MultiLocation>> ShouldExecute for AllowTopLevelPaidExecutionFrom<T> {
	fn should_execute<Call>(
	origin: &MultiLocation,
	message: &mut Xcm<Call>,
	max_weight: Weight,
	_weight_credit: &mut Weight,
	) -> Result<(), ()> {
	log::trace!(
	target: "xcm::barriers",
	"AllowTopLevelPaidExecutionFrom origin: {:?}, message: {:?}, max_weight: {:?}, weight_credit: {:?}",
	origin, message, max_weight, _weight_credit,
	);
	ensure!(T::contains(origin), ());
	let mut iter = message.0.iter_mut();
	let i = iter.next().ok_or(())?;
	match i {
	ReceiveTeleportedAsset(..) |
	WithdrawAsset(..) |
	ReserveAssetDeposited(..) |
	ClaimAsset { .. } => (),
	_ => return Err(()),
	}
	let mut i = iter.next().ok_or(())?;
	while let ClearOrigin = i {
	i = iter.next().ok_or(())?;
	}
	match i {
	BuyExecution { weight_limit: Limited(ref mut weight), .. } if *weight >= max_weight => {
	*weight = max_weight;
	Ok(())
	},
	BuyExecution { ref mut weight_limit, .. } if weight_limit == &Unlimited => {
	*weight_limit = Limited(max_weight);
	Ok(())
	},
	_ => Err(()),
	}
	}
	}

Since we've already have cases where a misconfigured barrier condition has led to privilege escalation attacks, it is therefore prudent to consider whether this should be the way in which we manage the complexity involved in configuring the barrier.

Due to the security implications, we can even begin to think of the barrier as the firewall, the first line of defense against malicious actors, and so barrier conditions should:

• be easily maintainable and auditable;
• hide complexity when it is not necessary to be surfaced;
• simple to use and configure

One of the suggestions made by @apopiak was to have an API that resembles something like the following:

message
	.match_next_inst(|inst| matches!(inst, WithdrawAsset(..))
	.match_while(|inst| matches!(inst, ClearOrigin))
	.match_next_inst( /* ... */ )

Files: xcm/xcm-builder/src/barriers.rs, xcm/src/v3/mod.rs

[Polkadot-Forum]

This issue has been mentioned on Polkadot Forum. There might be relevant details there:

https://forum.polkadot.network/t/polkadot-release-analysis-v0-9-40/2468/1