Ethereum Smart contract calling a function in another smart contract that has a address payable return value give error

Question

I am new to solidity. Anyway, I have written the following smart contract:

pragma solidity >=0.4.22 <0.7.0;

contract SP {

    address BlockAddress;
    bytes32 _Mhash;

    function verifyHash_ASM(bytes32 _MHash) public returns (address payable Validator_address) {

        bytes4 sig = bytes4(keccak256("verifyHash(string)"));
        assembly {
            // move pointer to free memory spot
            let ptr := mload(0x40)
            // put function sig at memory spot
            mstore(ptr,sig)
            // append argument after function sig
            mstore(add(ptr,0x04), _MHash)

            let result := call(
              15000, // gas limit
              sload(BlockAddress_slot),  // to addr. append var to _slot to access storage variable
              0, // not transfer any ether
              ptr, // Inputs are stored at location ptr
              0x20, // Inputs are 32 bytes long
              ptr,  //Store output over input
              0x20) //Outputs are 32 bytes long

            if eq(result, 0) {
                revert(0, 0)
            }

            Validator_address := mload(ptr) // Assign output to Validator_address var
            mstore(0x40,add(ptr,0x24)) // Set storage pointer to new space

        }
        //PayTokens(Validator_address, User_address, RepTokens);
        //return Validator_address;
    }

When I call the verifyHash_ASM function and parse the byte32 value 0x33c897fb3febb48f0a8a9ff79518057a9943ed879d644bba81a8a1050ccfaa4f which is equivalent to the one assigned in another contract and call the same function in that contract, I get the following error:

"transact to SP.verifyHash_ASM errored: VM error: revert. revert The transaction has been reverted to the initial state. Note: The called function should be payable if you send value and the value you send should be less than your current balance. Debug the transaction to get more information."

The status is: "0x0 Transaction mined but execution failed"

Please note that I executed the function without values in Remix. I have also tried executing this by adding gas values and changing the function to "payable",but I still got this error.

The compiler I am using is 0.5.11 and the execution environment is JavaScript VM in Remix.

Your help and support is much appreciated

Thank you

Answer 1

It looks like you have two bugs. First:

bytes4 sig = bytes4(keccak256("verifyHash(string)"));

The actual function takes a bytes32, not a string, so this should be:

bytes4 sig = bytes4(keccak256("verifyHash(bytes32)"));

Second:

0x20, // Inputs are 32 bytes long

The input is actually 36 bytes long. 4 bytes for the function selector, and then a single bytes32 parameter. So this should be:

0x24, // Inputs are 36 bytes long

Once I made those changes, this code worked as expected.

---

As an aside, is there a reason you're doing this in assembly? I would have expected something more like this:

interface IValidator {
    function verifyHash(bytes32 hash) external view returns (address);
}

contract SP {
    IValidator validator;
    ...
    function verifyHash(bytes32 hash) external view returns (address) {
        return validator.verifyHash(hash);
    }
}