From: Gennady <bystr@...>
Date: 2003-01-29T13:44:15+09:00
Subject: $SAFE level mutation for tained classes

Hi, fellow Rubyists

What is the rationale behind the following change in SAFE level when 
invoking a method with "call":

class Test
   attr_accessor :value
   def show
     "Safe level: #{$SAFE}"
   end
end

Test.taint
t = Test.new

p t.show  # ==> Safe level: 0
p t.method(:show).call # ==> Safe level: 4

So every time you invoke a method with "call", safe level is set to 4 
for the duration of the call with all consequences -- when you invoke 
value= with
t.method(:value=).call(1), you get an error:

ttt.rb:17:in `value=': Insecure: can't modify instance variable 
(SecurityError)
         from ttt.rb:17:in `call'
         from ttt.rb:17

Even puts from a so invoked method fails:

ttt.rb:5:in `write': Insecure operation `write' at level 4 
(SecurityError)
         from ttt.rb:5:in `puts'
         from ttt.rb:5:in `show'
         from ttt.rb:15:in `call'
         from ttt.rb:15

I came across this behavior when experimenting with safe level 3, where 
all objects are created tainted.

Thanks in advance for any suggestions and/or enlightments.
Gennady.

P.S. I checked in eval.c, there ruby_safe_level is explicitly set to 4 
in method_call().