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Re: Restricting eval

From: Panicz Maciej Godek
Subject: Re: Restricting eval
Date: Sun, 23 Mar 2014 19:33:33 +0100


2014-03-23 19:07 GMT+01:00 Grant Rettke <address@hidden>:
> On Sat, Nov 30, 2013 at 2:26 PM, Panicz Maciej Godek
> <address@hidden> wrote:
>> And hence my question: is there any way to restrict
>> the execution environment of eval, e.g. to specify
>> which symbols should be available? (For security
>> reasons, I wouldn't want functions like "system"
>> or "exit" to be present in that environment)
>> Or perhaps there's some better way to do that?
> How did you end up achieving your goal?

Oh, with Guile it turned out to be a piece of cake ;]
It's thanks to first-class modules and the fact that a module can be
provided as the second argument to eval.
Guile actually has e.g. (ice-9 safe-r5rs), which exports a safe subset
of Scheme, and (ice-9 null), which provides the most basic syntactic
bindings and no functions whatsoever.

So it is possible to either create a module in runtime using
make-fresh-user-module and add all the necessary bindings, or to have
some regular module prepared and obtain it using resolve-module.

This is more or less how I did it, but I have to admit that I did
neglect the security issues and designed the system to work rather
than to make it hacker-proof.

The bad news is that the module API isn't officially documented and
one needs to read the boot-9.scm file to figure out how it works (the
code is a good read, though).


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