Turning String into Numerical Equation

[Michael Spencer]

Giovanni Bajo wrote:
> Michael Spencer wrote:
> 
> 
>>>In fact, I believe my solution to be totally safe,
>>
>>That's a bold claim!  I'll readily concede that I can't access
>>func_globals from restricted mode eval (others may know better).  But
>>your interpreter is still be vulnerable to DOS-style attack from
>>rogue calculations or quasi-infinite loops.
> 
> 
> 
> Yes, but I don't see your manually-rolled-up expression calculator being
> DOS-safe. I believe DOS attacks to be a problem whenever you want to calculate
> the result of an expression taken from the outside. What I was trying to show
> is that my simple one-liner is no worse than a multi-page full-blown expression
> parser and interpreter.

Fair point that brevity is itself valuable in achieving security.  It isn't 
worth using my "manually-rolled-up expression calculator" simply to deny access 
to func_globals as you have demonstrated.

However, the advantage of the MRUEP is that every operation is evaluated 
individually.  In the example I showed, loops are disabled, attribute access is 
disabled.  Numeric inputs and intermediate results can be checked easily for 
boundedness (though they are not in the example I gave).  This sort of 
fine-grain control is very much harder to do with a straight eval model.

Cheers

Michael