[Ethereum] In-line assembly error

assemblysolidity

I am checking if an address is a smart contract. Similarly to many articles and example-codes out there, i do this within my function:

uint32 size;
assembly{
    size := extcodesize(_to)
}
if(size > 0){}

However, when I do this, as so many others seem to do, I get this error:

security/no-inline-assembly: Avoid using Inline Assembly.

I seem to struggle to find a solution to this. Even in the solidity 0.5.0 documentation they describe inline assembly, so seems weird that it is not allowed. Has anyone else encountered this, and found a solution to it?

Best Answer

If you're using Remix, this should only give you the following warning, not an error. It gives you an idea as to why you should avoid assembly (presumably unless you know what you're doing).

The Contract uses inline assembly, this is only advised in rare cases. Additionally static analysis modules do not parse inline Assembly, this can lead to wrong analysis results.

If you're using solc directly, then I think you'll need to play around with the solhint configuration, specifically under the Security Rules section. (See the no-inline-assembly rule.) You should be able to set it to 'false'. (Though on your head be it.)

For your specific piece of code, you need to be careful - it might not do what you expect it to do. See the Consensys Best Practices for why:

Avoid using extcodesize to check for Externally Owned Accounts

Related Solutions

[Ethereum] understanding mload assembly function

mload(0xAB) Loads the word (32byte) located at the memory address 0xAB. e.g mload(0x60) loads the word located at 0x60 memory address.

let's code to understand more :

 function f ()  
    {

        assembly {
            let freemem_pointer := mload(0x40)
            mstore(add(freemem_pointer,0x00),"36e5236fcd4c61044949678014f0d085")
            mstore(add(freemem_pointer,0x20),"36e5236fcd4c61044949678014f0d086")
            let arr1:= mload(freemem_pointer) //read first string 
            mstore(add(freemem_pointer,0x40),arr1)


        }
    }

this results in the flowing memory state :

This previous code stores tow strings (two memory words 32bytes each) in the free memory space. The destinations memory addresses are obtained by adding an offset of 0bytes for the first and of 0x20 for the second to the free memory pointer address(located in the memory address 0x40).
in the EVM The 6 first words in the memory are reserved and the 0x40-0x50 memory words are allocated to the free memory pointer.

Details : mload and mstore are defined in details in the yellow paper :

[a...b) signifies the bytes of memory starting at position a up to (excluding) position b. however b] means b included

[Ethereum] Solidity to inline assembly

I'd do something like this, using call() to do an external function call, passing the msg.value inside it.

function () payable {
    bytes4 sig = bytes4(keccak256("()")); // function signature

    assembly {
        let x := mload(0x40) // get empty storage location
        mstore ( x, sig ) // 4 bytes - place signature in empty storage

        let ret := call (gas, 
            beneficiary,
            msg.value, 
            x, // input
            0x04, // input size = 4 bytes
            x, // output stored at input location, save space
            0x0 // output size = 0 bytes
        )

        mstore(0x40, add(x,0x20)) // update free memory pointer
    }
}

Best Answer

Related Solutions

[Ethereum] understanding mload assembly function

[Ethereum] Solidity to inline assembly

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