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import HistoricalFact from './_gpv2.md'

GPv2VaultRelayer

Architecture​

The GPv2VaultRelayer contract is an important component used to protect user funds from malicious solvers. As previously mentioned, the GPv2Settlement contract allows using arbitrary on-chain liquidity through interactions (such as performing a swap on Balancer V2, or performing a Paraswap trade). If Vault and ERC20 allowances were made directly to a GPv2Settlement contract, a malicious solver could drain user funds through the interaction mechanism. However, since these allowances are made to the GPv2VaultRelayer contract and interactions to the contract are strictly forbidden, malicious solvers have no direct access to user funds. The GPv2Settlement contract uses the GPv2VaultRelayer to withdraw user tokens only as part of the trade, which contains strong guarantees that the user's signed order parameters are respected.

The GPv2VaultRelayer has access to user balances through 3 mechanisms:

  1. Fallback ERC20 Allowances
  2. Balancer External Balances
  3. Balancer Internal Balances

Guarantees and Invariants​

  • The GPv2VaultRelayer is only able to transfer ERC20 tokens to the GPv2Settlement contract

Fallback ERC20 Allowances​

The third and final method of approving tokens for CoW protocol is to use direct ERC20 allowances to the GPv2VaultRelayer. This works like most other trading protocols, where for each token you want to sell, an allowance must first be approved for the GPv2VaultRelayer contract.

Orders with the sellTokenBalance flag set to erc20 will withdraw using this process. The buyTokenBalance flag can also be set to erc20 in order to receive trade proceeds directly in ERC20 amounts.

Balancer External Balances​

The first mechanism that the GPv2VaultRelayer contract can use to withdraw user ERC20 tokens is through Vault external balances. This works by having an ERC20 allowance for the Balancer Vault, and a relayer approval for the GPv2VaultRelayer contract.

This allowance and approval combination allows the GPv2VaultRelayer contract to transfer ERC20 tokens through the Vault. Roughly speaking, the process works in the following way:

  1. GPv2VaultRelayer request to the Balancer Vault an ERC20 transfer from the user account to the GPv2Settlement contract
  2. The Balancer Vault verifies that the GPv2VaultRelayer contract is:
    a. Authorized by Balancer governance to act as a relayer
    b. The user has set an approval for that specific relayer
  3. The Balancer Vault issues an ERC20 transfer from the user account to the GPv2Settlement contract using the Vault's existing ERC20 allowance

This system for withdrawing user funds has several advantages such as:

  • It can reuse existing Vault ERC20 allowances and doesn't require new ones specific to the CoW Protocol.
  • Upgrades to the CoW Protocol contract would only require a single relayer approval for all tokens instead of individual ERC20 approvals for each token being traded.
  • The GPv2VaultRelayer approval can be revoked by a single transaction to the Vault instead of multiple transactions to each ERC20 token for which the user wants to remove the approval.

Orders with the sellTokenBalance flag set to external will withdraw using this process.

Balancer Internal Balances​

The second mechanism is to use balances internal to the Vault. The Balancer V2 vault can accrue ERC20 token balances and keep track of them internally in order to allow extremely gas-efficient transfers and swaps. The CoW Protocol contracts can make use of this in order to decrease the gas cost of settling a user order on-chain. In order for this to work, the user must approve the GPv2VaultRelayer contract and have internal Vault balances available.

Internal balances can be withdrawn from the Vault at any time for their ERC20 equivalent amounts.

Orders with the sellTokenBalance flag set to internal will withdraw using this process. The buyTokenBalance flag can also be set to internal in order to receive trade proceeds in internal balances instead of ER20 token balances.

Data Types and Storage​

Nil

Functions​

For the Protocol​

transferFromAccounts​

This function is used for transferring ERC20 tokens from users to the GPv2Settlement contract in the course of settling a batch auction.

function transferFromAccounts(
   GPv2Transfer.Data[] calldata transfers
) external onlyCreator {
   vault.transferFromAccounts(transfers, msg.sender);
}

batchSwapWithFee​

This function is used in the course of settling a single trade on-chain. It is called by the GPv2Settlement contract and is used to perform a batch swap on the Balancer V2 vault. The function is defined as:

function batchSwapWithFee(
   IVault.SwapKind kind,
   IVault.BatchSwapStep[] calldata swaps,
   IERC20[] memory tokens,
   IVault.FundManagement memory funds,
   int256[] memory limits,
   uint256 deadline,
   GPv2Transfer.Data calldata feeTransfer
) external onlyCreator returns (int256[] memory tokenDeltas)

Indexing​

Nil

Off-chain​

Nil