From: "Skye Shaw!@#$" <skye.shaw@...>
Date: 2007-08-08T16:45:04+09:00
Subject: Re: non-constant strings

On Aug 7, 2:41 pm, Dmitry Bilunov <km...@kmeaw.com> wrote:
> 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?

This will result in a no method error, no?

>       @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

Well, this is not the fault of the language, rather your SecureRunner
class.

def run
     # Only safe commands should be run
     if @command.tainted?
        raise RuntimeError, "Security check failed!"
     end
     system(@command)
end

> The same can be done to fields of instances, which are exported as
> read-only (attr_reader).

This is the case any language where arguments are passed by reference.

> but what is the reason Ruby has non-constant strings

You mean mutable strings. Ruby does have constant strings:

irb(main):001:0> CONST="assbasscass"
=> "assbasscass"
irb(main):002:0> CONST=123
(irb):2: warning: already initialized constant CONST
=> 123

The String class wraps an array of chars, realloc()'in as necessary
(ruby hackers correct me if I've mistakin).

Consider this in Java:

class SomeClassThatRequiredAlotOfTyping
{
  private StringBuffer sb
  //....
  public void printBuffer()  { System.out.println(sb); }
}

StringBuffer sb = new StringBuffer("Dont Chnage!");

SomeClassThatRequiredAlotOfTyping clazz = new
SomeClassThatRequiredAlotOfTyping(sb);
// I'll show that private "read-only" var who's in charge!
sb.delete(0,sb.length());
sb.append("VB6, its ByVal keyword rocked... Not!");
clazz.printBuffer();

Hope that helps.