[Ethereum] How to use delegatecall assembly with a storage pointer to a struct

assemblydelegatecallsolidity

I am trying to call a library function using assembly and the delegatecall opcode. The library function accepts two parameters, the state object storage pointer and an address, it returns a single uint.

I can tell if the op fails because the data at in_pos is untouched, however when it does work it always returns zero even if the function being called simply returns a 0x01 value.

state is a struct that is shared by the calling contract and the library, its position is 0

address t = 0xTargetLibraryAddress;
address a = 0xAccount;
bytes32 result;
assembly {
   let in_pos := mload(0x40)
   mstore(in_pos, 0x12345678) //a valid func sig within the library
   mstore(add(in_pos, 0x04), state)
   mstore(add(in_pos, 0x24), a)
   let success := delegatecall(sub(gas, 10000), t, in_pos, 0x44, in_pos, 0x20)
   result := mload(in_pos)
   mstore(0x40, add(in_pos, 0x44))
}

Any suggestions?

Best Answer

From what I can gather mstore() will write in 32 byte chunks, therefore mstore(in_pos, 0x04) is still in the same space as in_pos and will overwrite the 4 bytes function signature.

What I ended up doing is writing the 4 bytes in the first 32 bytes, the 1st arg in the 2nd 32 bytes and the 2nd arg in the 3rd 32 bytes, then called delegatecall pointing to the start of the 4 byte signature by using a 28 byte offset.

delegatecall(sub(gas, 1000), t, add(in_pos, 28), 0x44, in_pos, 0x20)

Related Solutions

Solidity – How to Get Delegatecall to Return Data

The issue is that the response variable is not a memory pointer, it's a stack variable. When you use it as the pointer for your out data in the delegatecall, you're actually just writing the data to the 0 memory location. This doesn't cause errors because that space is reserved for "scratch space", but it also never loads the result into your return variable.

What you want to do is

assembly {
        let status := delegatecall(sub(gas,5000), dest, add(calldata, 0x20), mload(calldata), 0, 32)
        response := mload(0)
        jumpi(invalidJumpLabel, iszero(status))
}

[Ethereum] Use of delegatecall with msg.data

The fallback function gets called when no other function matches the calldata signature. In this code, the fallback functions is used as a universal function.

This is done by forwarding the calldata to the contract at currentVersion. Forwarding the calldata to another smart contract results in it running the proper funcion. There are two ways to do this: address.call and address.delegatecall. The difference between those is the context in which the receiving contract is going to run the code.

With delegatecall, the receiving contract accesses the Relay contract's storage. This way, the receiving contract can alter all fields of the Relay.

Effectively, by updating currentVersion, Relay can update it's code without changing it's address or migrating it's storage contents.

Docs:

Best Answer

Related Solutions

Solidity – How to Get Delegatecall to Return Data

[Ethereum] Use of delegatecall with msg.data

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