fix(vm): set callcode to use readOnly mode for precompile calls (evmos#24)

Author: MalteHerrmann

core/vm/evm.go

@@ -208,6 +208,7 @@ func (evm *EVM) Call(caller ContractRef, addr common.Address, input []byte, gas
 		}
 	}
 
+	// It is allowed to call precompiles, even via call -- as opposed to callcode, staticcall and delegatecall it can also modify state
 	if isPrecompile {
 		ret, gas, err = evm.RunPrecompiledContract(p, caller, input, gas, value, false)
 	} else {
@@ -270,9 +271,9 @@ func (evm *EVM) CallCode(caller ContractRef, addr common.Address, input []byte,
 		}(gas)
 	}
 
-	// It is allowed to call precompiles, even via delegatecall
+	// It is allowed to call precompiles, even via callcode, but only for reading
 	if p, isPrecompile := evm.Precompile(addr); isPrecompile {
-		ret, gas, err = evm.RunPrecompiledContract(p, caller, input, gas, value, false)
+		ret, gas, err = evm.RunPrecompiledContract(p, caller, input, gas, value, true)
 	} else {
 		addrCopy := addr
 		// Initialise a new contract and set the code that is to be used by the EVM.