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Origin manipulation

An XCVM contains contextual information while executing XCM instructions. It uses the XcmContext struct to provide them. XcmContext contains information such as the origin of the corresponding XCM, the hash of the message, and the topic of the XCM.

pub struct XcmContext {
	/// The `MultiLocation` origin of the corresponding XCM.
	pub origin: Option<MultiLocation>,
	/// The hash of the XCM.
	pub message_hash: XcmHash,
	/// The topic of the XCM.
	pub topic: Option<[u8; 32]>,
}

In the XCVM, the origin field of the XcmContext indicates which MultiLocation's privilege level that the current program is using to execute. The origin is important for enforcing restrictions and ensuring appropriate execution of the instructions.

There are multiple instructions in XCM that can alter the XcmContext origin field:

  • ClearOrigin
  • DescendOrigin
  • UniversalOrigin
  • AliasOrigin

ClearOrigin​

ClearOrigin

The ClearOrigin instruction clears the origin register in the XCVM. Specifically, it sets the origin field of the XCM context to None. This ensures that subsequent instructions in the XCM cannot use the privilege level of the cleared origin to execute operations.

DescendOrigin​

DescendOrigin(InteriorMultiLocation),

The DescendOrigin instruction is used to change the XcmContext origin to an interior location of the current origin.

This can be useful when executing instructions that require a specific location within the current origin.

Note that the XcmContext origin is a MultiLocation containing an InteriorMultiLocation enum; it can only hold up to a maximum of 8 Junctions, so when we try to execute multiple DescendOrigin instructions which would result in an InteriorMultiLocation containing more than 8 Junctions, a LocationFull error is thrown.

UniversalOrigin​

UniversalOrigin(Junction)

The UniversalOrigin XCM instruction sets the Origin Register to be a child of the Universal Location. The Junction parameter should generally be a GlobalConsensus variant since only these are children of the Universal Location.

Safety Note: Should only be usable if the Origin is trusted to represent a child of the Universal location. In general, no Origin should be able to represent the Universal Location's child which is the root of the local consensus system since it would by extension allow it to act as any location within the local consensus, but it is necessary when bridging XCMs between GlobalConsensus systems.

AliasOrigin​

AliasOrigin(MultiLocation)

The AliasOrigin instruction is similar to the UniversalOrigin instruction, but it is primarily used for account IDs. When executed, it switches out the current origin for the given MultiLocation. THe AliasOrigin instruction would allow to remove certain prefix patterns such as Parent/Parachain(X)/ for certain values of X (thereby allowing sibling chains to use the same account IDs) or Parachain(X)/ (allowing a relay chain to use the account IDs native to its child parachains) or just Parent/ (allowing parachains to use AccountIds of the relay chain).