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From: "Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de>
Subject: Re: Presentations on-line - Ada&SPARK for Education&Research
Newsgroups: comp.lang.ada
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References: <hmjvj6$orr$1@asgard.cs.kuleuven.be>
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Date: Fri, 5 Mar 2010 10:19:06 +0100
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Date: 2010-03-05T10:19:03+01:00
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On Fri, 05 Mar 2010 09:03:08 +0100, Georg Bauhaus wrote:

> True?  With some effort, it seems possible to break an Ada
> monitor implemented as a protected object.
> 
> package Monitor is
> 
>     type SynchNode;
>     type Linkage is access all synchnode;
>     type data is access all integer;
> 
>     protected type synchnode is
>        procedure Link (x : Linkage);
>        procedure Expose (N : out Data);
>     private
>        Outgoing : aliased Integer := 0;
>     end SynchNode;
> 
> end Monitor;
> 
> package body monitor is
> 
>     protected body synchnode is
>        procedure link (X : Linkage) is
>           View : Data; -- X's data
>        begin
>           X.Expose(View);
>           View.all := View.all + 1;  -- <-- unprotected
>        end link;
> 
>        procedure Expose (N : out Data) is
>        begin
>           N := Outgoing'unchecked_access;

monitor.adb:13:25: warning: possible unprotected access to protected data

Which pins down broken design.

>        end expose;
>     end SynchNode;
> 
> end Monitor;

With even less efforts you can break anything:

   X : SynchNode;
   Y : Float;
   for Y'Address use X'Address;
begin
   Y := sqrt (Y);
----------------------------------

1. Do not use pointers
2. Do not use global variables

-- 
Regards,
Dmitry A. Kazakov
http://www.dmitry-kazakov.de