Received-SPF: softfail (google.com: domain of transitioning linux-kernel-owner@vger.kernel.org does not designate 23.128.96.19 as permitted sender) client-ip=23.128.96.19;
Date:   Wed, 16 Mar 2022 20:22:40 -0700
From:   Kees Cook <keescook@chromium.org>
To:     David Laight <David.Laight@aculab.com>
Cc:     James Jones <linux@theinnocuous.com>,
        Arnd Bergmann <arnd@arndb.de>, Borislav Petkov <bp@alien8.de>,
        the arch/x86 maintainers <x86@kernel.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH] x86: Remove a.out support
Message-ID: <202203162014.FEB1496@keescook>
References: <4c449fab-8135-5057-7d2c-7b948ce130cc@theinnocuous.com>
 <0b31b1d3-852d-6cab-82ae-5eecaec05679@theinnocuous.com>
 <202203151150.1CDB1D8DA@keescook>
 <bfbd9394-161b-0e70-00c5-79d0dd722e08@theinnocuous.com>
 <CAK8P3a28dpyEM2+vM+ePZzeFc539b7w_8FDEoRke-j+3AQVZAA@mail.gmail.com>
 <202203161523.857B469@keescook>
 <5161ace381a74a63b58b0a2a2a2cb57d@AcuMS.aculab.com>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <5161ace381a74a63b58b0a2a2a2cb57d@AcuMS.aculab.com>
Precedence: bulk

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