From: Bill Kelly <billk@...>
Date: 2003-08-13T00:58:27+09:00
Subject: Re: $SAFE = 5 and Safe Ruby Misleading?

Hi,

From: "Dave Fayram" <kirindave@lensmen.net>
> Basically, can I evaluate something in a $SAFE = 4 thread, then get it 
> out and work with it meaningfully? The only way I can think of is to 
> try analyzing it in a $SAFE = 3 thread, where an object can't untaint 
> itself, but doing so for an object would be pretty much impossible.

Not sure if this is sophisticated enough for your purpose - I just
needed arbitrary code to be evaluated "safely" at $SAFE = 4, and have
it be able to modify an object passed to it and return [some] result.

def safe_eval(_eval_str, *_args)
  th = Thread.new(_eval_str, *_args) do |eval_str, *args|
    res, warning = nil, nil
    begin
      $SAFE = 4
      res = eval(eval_str)
    rescue Exception => ex
      warning = ex.to_s
    end
    [res, warning]
  end
  th.value
end

The calling code in my case is $SAFE = 1... not that the safe-level
of the calling code should matter, I believe.

For some reason, looking at my calling code, I wanted to pass in
a tainted object so it could be modified directly by the $SAFE = 4
thread via mutator methods (i.e. gsub!)... Though if one didn't
want to allow any object passed to it to be modified, the evaluated
code could presumably dup the object, modify the copy, and return
the modified object as a result...  (Unless that's asking too much
of $SAFE = 4... I don't remember <grin>)

Anyway what I'm doing is essentially:

  some_object_to_be_modified.taint  # allow mutator methods in high SAFE levels
  res, warning = safe_eval(untrusted_code, some_object_to_be_modified)
  if warning
    log warning
    log untrusted_code
  end


Hope this helps,

Bill