I have been trying to understand how assembly code works,spending lots of hours, but I could not.
Please help me
Basically, I am digging into a proxy contract, and I do not really understand calldatacopy and delegatecall.
Would you please explain it ?
fallback() external { bytes4 sig; assembly { sig := calldataload(0) } uint len = _sizes[sig]; address target = _dest; assembly { calldatacopy(0x0, 0x0, calldatasize()) let result := delegatecall(sub(gas(), 10000), target, 0x0, calldatasize(), 0, len) return(0, len) //we throw away any return data } } I do not really understand how the assembly code delegatecall works.
so The EIP-7 said
- gas: the amount of gas the code may use in order to execute;
- to: the destination address whose code is to be executed;
- in_offset: the offset into memory of the input;
- in_size: the size of the input in bytes;
- out_offset: the offset into memory of the output;
- out_size: the size of the scratch pad for the output.
so, let's assume that I deployed "MyContract"
I sent the msg.data "0x360058390000000000000000000000000000000000000000000000000000000000000003" to the "MyContract"
0x36005839 : a function selector 0000000000000000000000000000000000000000000000000000000000000003 : input value However, there was no function selector for the "MyContract",
So the fallback function took care of it!
assembly { calldatacopy(0x0, 0x0, calldatasize()) let result := delegatecall(sub(gas(), 10000), target, 0x0, calldatasize(), 0, len) return(0, len) //we throw away any return data } I used the event to print out calldatasize() and it was 36. I think it was because 4bytes for the function selctor + 32bytes for the input value
calldatacopy(0x0, 0x0, calldatasize()) I guess, In assembly, it will copy the exact same msg.data I sent.
Do I clearly understand calldatacopy?
But what is this for ? It does not even return the value..
delegatecall(sub(gas(), 10000), target, 0x0, calldatasize(), 0, len) Okay, for this part, I understand gas, target address.
But why does the in_offset start at 0 and is the in_size calldatasize?
