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List:       bochs-dev
Subject:    Re: [Bochs-developers] gcc bochs-bios v0.2.0
From:       "Kevin O'Connor" <kevin () koconnor ! net>
Date:       2008-03-31 1:26:18
Message-ID: 20080331012618.GA4749 () ohio ! lan
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On Mon, Mar 31, 2008 at 12:36:50AM +0100, Michael Brown wrote:
> Tested on a freshly built git tree (on Mandriva 2007.1, AVOIDCOMBINE=1 
> required, gcc 4.1.2) with a current gPXE snapshot, and it appears to work.  
> Nice job!

Thanks.

> I'm a little wary of the GET_VAR() magic; in particular, the way that 
> there's nothing that prevents the compiler from compiling a direct 
> dereference of a pointer that cannot legitimately be accessed via the 
> current %ds==%ss.

Yes - this concerns me as well.

The 16bit bios callbacks store all variables in two tables - the bios
data area and the extended bios data area.  These areas are accessed
via GET/SET_BDA and GET/SET_EBDA macros - there is no global pointer
to these areas that a developer could mistakenly access directly.  So,
the code does protect against casual accesses to variables.

>In gPXE, we use some preprocessor magic so that 
> out-of-segment variables can be accessed transparently.  I can't 
> immediately think of a solution that would allow you to do this given 
> your constraints, but you could at least have something like:
> 
>   #define CS_VAR(var) _CS_ ## var
>   #define GET_VAR(seg,var) <...dereferencing magic on "_CS_ ## var"...>
>   static int CS_VAR(foo) = 5;

The CS segment is only useful for storing constant data (machines
generally mark the 0xf0000 area that holds the bios code as
read-only).

The data that is stored in the CS segment is mostly constant strings
(eg, the "hello world" in a call like printf("hello world")).  It's
difficult to protect these types of strings; the protection may be
more pain than the problem.

Oh well - I'm hoping documentation, code auditing, and proper testing
will prevent the issue.

Thanks again,
-Kevin

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