Stack overflow issue in scalecodec while decoding inputs

[mmostafas]

There is an stack overflow issue in the parity-scale-codec crate that crashes while decoding inputs. We ran into this issue when decoding an XCM call but we believe it can be triggered from other code paths as well.

Here is a tests that triggers this bug:

#[test]
fn decode_instruction_segfault() {
	use parity_scale_codec::DecodeLimit;
	let mut decoded = VersionedXcm::V3(v3::Xcm::<()>::new());
	let encoded = hex_literal::hex!("03f800000100000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000151515151515151515151515151515151515151515151515151515151515151515151515151515151515151515151515151515151515151515151515151515151515151515151515151515151515151515151515151515151515151515151515151515151515151515151515151515151515151515151515151515151515151515151515151515151515151515151515151515151515151515151515151515151515151515151515");

	decoded  = DecodeLimit::decode_with_depth_limit(20, &mut encoded.as_slice()).unwrap();
}

Please note that the depth limit of 20 is used here which is relatively low.

There is also a similar issue submitted here.

[koute]

@mmostafas Could you check whether this PR fixes your issue?

#426

[mmostafas]

Thank you @koute,

Unfortunately #426 did not fix our issues. If you want to reproduce the bug, you can do so by adding the provided test above to the unit tests for the XCM.

Please let me know if you need more info.

[koute]

@mmostafas Thanks for the explanation on how to reproduce it!

I confirm I can reproduce the issue. I'll fix it.

[koute]

I've found why this happens; it's due to this upstream rustc bug:

rust-lang/rust#34283

The Instruction enum is 1472 bytes big and has a ton of variants. When it is decoded there's a match over every variant, and for every variant it seems that extra stack space is consumed proportional to how big the enum is. Then nest it a few times and you get a stack overflow.

[koute]

Okay, I think I fixed it this time!

[mmostafas]

I can confirm this fixes our issues. Thanks @koute.