linux-kernel - Re: [PATCH] x86: Remove a.out support

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Message-ID: <202203162014.FEB1496@keescook>
Date:   Wed, 16 Mar 2022 20:22:40 -0700
From:   Kees Cook <keescook@...omium.org>
To:     David Laight <David.Laight@...lab.com>
Cc:     James Jones <linux@...innocuous.com>,
        Arnd Bergmann <arnd@...db.de>, Borislav Petkov <bp@...en8.de>,
        the arch/x86 maintainers <x86@...nel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] x86: Remove a.out support

On Thu, Mar 17, 2022 at 02:32:29AM +0000, David Laight wrote:
> From: Kees Cook
> > Sent: 16 March 2022 22:30
> > 
> > On Wed, Mar 16, 2022 at 01:38:31PM +0100, Arnd Bergmann wrote:
> > > is in the end, but it's likely easier than a standalone a.out loader
> > > in user space, or a conversion to ELF format.
> > 
> > Yeah, the exec format is really simple. The only tricky bit was preparing
> > the stack and making sure everything landed in the right place for text
> > and data. James, can you try this? aln and mac run for me, but I'm not
> > actually exercising them beyond dumping argument lists, etc:
> 
> Doesn't that restrict the a.out program to the address space below
> the normal base address for elf programs?
> So you'll only be able to load small programs - which might be ok here.
> 
> OTOH it might be possible to link the 'loader program' to a high
> address - the elf loader will probably just load it.
> Best to link it static to avoid shared lib mmaps
> and probably try to avoid libc calls.
> 
> I was wondering what happens when malloc() starts using
> sbrk() - but I guess it sees the top of the bss for the
> loaded and it all works fine.

I'll wait unless something breaks. :) Right now I just wanted to get aln
and mac working -- and that's a pretty small subset of all a.out
binaries. :)

-- 
Kees Cook