From: dohzya <dohzya@...>
Date: 2007-08-08T20:17:00+09:00
Subject: Re: non-constant strings

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Hi,

2007/8/7, Dmitry Bilunov <kmeaw@kmeaw.com>:
>
> Hello. Why does Ruby have non-constant strings? It seems there is a way
> to bypass object encapsulation paradigm and break object integrity. Here
> is any example:
>
> class SecureRunner
> # This class implements a sudo-like
> # runner
>
>   def initialize(command)
>   # Creates an instance. Guaranties, that a command is safe.
>     if command.safe?
>       @comamnd = command
>     else
>       raise RuntimeError, "Security check failed!"
>     end
>   end
>
>   def run
>   # Only safe commands should be run
>     system(@command)
>   end
> end
>
> # This class seems to be safe
> # Here is a way to bypass security check:
>
> command = "some_safe_command"
> runner = SecureRunner.new(command)
> # a command is safe, so check will be passed
>
> command.replace("evil_command")  # BYPASS THE CHECK
>
> runner.run # runs evil_command, that is not safe
>
> The same can be done to fields of instances, which are exported as
> read-only (attr_reader). I know there is a way to fix it (using .clone
> or .dup), but what is the reason Ruby has non-constant strings, as most
> languages (Java, Python) do? Is there a way to disable such behaviour
> ($SAFE will not help, because internal class methods will not be able to
> change instance-variable strings too).
> --
> Posted via http://www.ruby-forum.com/.
>
>
You want to freeze the object @command, not the class String, don't
you ?
So you don't need to use an Immutable_String class, just add
@command.freeze in your code


-- 
Etienne Vallette d'Osia

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