overflow checks for memory gas calculations

Author: jakelang

src/eei.cpp

@@ -198,8 +198,9 @@ string toHex(evmc_uint256be const& value) {
       HERA_DEBUG << "useGas " << gas << "\n";
 
       takeGas(gas);
-      // FIXME: this may overflow
-      takeGas(gas * memory.size() / GasSchedule::memoryPageSize * GasSchedule::memoryCostPerPage);
+
+      heraAssert((ffsl(gas) + ffsl(memory.size()) <= 64), "Memory gas calculation overflow."); //may need to find alternative to ffsl for cross-libc portability
+      takeGas(gas * memory.size() / GasSchedule::memoryPageSize * GasSchedule::memoryCostPerPage + 1); //round gas cost up
 
       return Literal();
     }