Get return data from delegatecall

Question

I'm using a low level delegatecall to forward my calls to a library.

I need to (in this order) :

1. load bytes4 sig into _message from memory

2. load bounty (STRUCT) into _message from storage

3. load bytes _data into _message

4. Delegatecall with the new message

5. Get return valuesThe contract works (eg the getBounty function will work) but I'm having trouble with the assembly in my forwarder() function. https://ethfiddle.com/po3wHGQraJ

I'm having trouble making my forwarder (in assembly) work properly however.

  function forward(bytes32 _sig, bytes memory _data) public payable {
    bytes memory data = _data;
    bytes4 sig = bytes4(_sig);
    address _impl = impl;
    require(_impl != address(0));
    assembly {
      let _message := mload(0x40) //find empty memory location
      mstore(_message, sig) //place sig at beginning of empty storage
      //add _bounty
      mstore(add(_message, 4), sload(bounty_slot))
      //add _data
      calldatacopy(add(_message, 36), 100, sub(calldatasize,100))
      //mstore(add(_message, 36), data)
      let size := returndatasize
      let result := delegatecall(gas, _impl, _message, msize, 0, 0)
      //return data ??????
      returndatacopy(_message, 0, size)
      switch result
      case 0 {revert(_message, size) }
      default { return(_message, size) }
    }
  }

This is what the storage looks like:

0x00
0x687474703a2f2f6170692e6269746275636b65742e636f6d2f33303433303430
0xe1bb4acb579f780cbaf21cef9ac3641b8e3fcd69
0x5b7eaef7
0x3334303330340000000000000000000000000000000000000000000000000000
0x012a05f200
0x00
0x00
0x059ce9161d4fb0
0x00
0x00
0x00
0x00
0x00
0x00
0x00
0x00
0x00

Answer 1

The returndatacopy is the right approach, but you have an issue with _data.

The _data variable is a pointer to memory, since bytes is an array type. You need to copy the data into the message buffer.

This can be done either by making the function external then using calldatacopy, or you can use a loop to copy word-by-word, or you can use the identity precompiled contract at address 0x04