Re: Stack Allocations

["Jeff" <lists () jeff ath cx>]

Hello,

I ran into the same question when I first discovered buffer overflow papers.
I asked around at the time and the most I could figure out was that it's
some kind of "pillow" of caution.  Seems kinda strange to me too that the C
compiler gives you ANY leeway like that at all, but it does.  I don't know
why, but I know that not many people know why either :)

Jeff
----- Original Message -----
From: <msoda () aspre net>
To: <vuln-dev () securityfocus com>
Sent: Monday, July 09, 2001 6:27 AM
Subject: Stack Allocations


> Hey all,
>
> I have been reading up on buffer overflows and have noticed something odd
> with gcc assembly output.  Consider the following:
>
> void func()
> {
>    char buf[15];
> }
>
> main()
> {
>    func();
> }
>
> When running 'gcc -S' it shows that 24 bytes are allocated on the stack
> for buf[].  I thought it should allocate only 16 bytes.  It works fine, it
> just makes no sense to me.  If I tweak the assembly and change it to 16
> bytes and also change the offsets to %ebp that reference it, it works fine
> also.
>
> Does anyone know why gcc does this?  My need to understand everything is
> killing me!
>
> -Marc