'_[1]' in .co_names using builtin compile() in Python 2.6
Ned Batchelder
ned at nedbatchelder.com
Wed Nov 27 16:26:46 EST 2013
On 11/27/13 3:44 PM, Chris Kaynor wrote:
> On Wed, Nov 27, 2013 at 12:09 PM, Ned Batchelder <ned at nedbatchelder.com
> <mailto:ned at nedbatchelder.com>> wrote:
>
> * Is there perhaps a better way to achieve what I'm trying to do?
>
> What I'm really after, is to check that python expressions
> embedded in text files are:
> - well behaved (no syntax errors etc)
> - don't accidentally access anything it shouldn't
> - I serve them with the values they need on execution
>
>
> I hope you aren't trying to prevent malice this way: you cannot
> examine a piece of Python code to prove that it's safe to execute.
> For an extreme example, see: Eval Really Is Dangerous:
> http://nedbatchelder.com/blog/__201206/eval_really_is___dangerous.html
> <http://nedbatchelder.com/blog/201206/eval_really_is_dangerous.html>
>
> In your environment it looks like you have a whitelist of
> identifiers, so you're probably ok.
>
>
> I just tested the crash example from that link in Python 2.7.5 win64 and
> the co_names from the compiled code is empty. Therefore, a simple
> whitelist would not catch that problematic code (and likely any other
> global access done correctly). Even a simple test of making sure that at
> least one (or any number of) valid identifier exists would be
> insufficent, as you can merely tack on a ",a" to add "a" to the
> co_names, and thus for any other variable.
Ah, right you are! I neglected to go back and examine the dangerous
code. So eval really is dangerous!
--Ned.
>
> Basically, even with a pure whitelist, there is likely no possible way
> to make eval/exec safe, unless you also eliminate the ability to make
> literals.
>
> Chris
>
>
More information about the Python-list
mailing list