linux-kernel - Re: [PATCH] x86/kconfig/32: Mark CONFIG

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Date:	Thu, 9 Jul 2015 10:57:03 -0700
From:	Andy Lutomirski <luto@...capital.net>
To:	Pavel Machek <pavel@....cz>
Cc:	Thomas Gleixner <tglx@...utronix.de>,
	Kees Cook <keescook@...omium.org>,
	Borislav Petkov <bp@...en8.de>,
	Arjan van de Ven <arjan@...ux.intel.com>,
	Oleg Nesterov <oleg@...hat.com>,
	LKML <linux-kernel@...r.kernel.org>, X86 ML <x86@...nel.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Peter Zijlstra <peterz@...radead.org>
Subject: Re: [PATCH] x86/kconfig/32: Mark CONFIG_VM86 as BROKEN

On Jul 9, 2015 2:03 AM, "Pavel Machek" <pavel@....cz> wrote:
>
> On Wed 2015-07-08 16:00:48, Thomas Gleixner wrote:
> > On Tue, 7 Jul 2015, Arjan van de Ven wrote:
> >
> > > On 7/7/2015 6:25 PM, Andy Lutomirski wrote:
> > > > VM86 is entirely broken if ptrace, syscall auditing, or NOHZ_FULL is
> > > > in use.  The code is a big undocumented mess, it's a real PITA to
> > > > test, and it looks like a big chunk of vm86_32.c is dead code.  It
> > > > also plays awful games with the entry asm.
> > > >
> > > > No one should be using it anyway.  Use DOSBOX or KVM instead.
> > > >
> > > > Mark it BROKEN.  I want to remove some (obviously incorrect) exit
> > > > asm that it depends on, and I don't want to figure out how to run
> > > > severely obsolete programs just to test something that no one uses
> > > > for anything other than exploits anyway.
> > > >
> > >
> > > while it is never great to deprecate features, in this case I am not sure
> > > there is another choice unless someone steps up to seriously revamp this code.
> > > (and look at it from a PREEMPT, NO_HZ etc etc angle)
> >
> > Aside of being broken in so many aspects it's even more obsolete than
> > 386 support, we should just remove it right away.
>
> Bad news for you:
>
> vbetool-0.5/lrmi.c:#include <asm/vm86.h>
> vbetool-0.5/lrmi.c:#include <sys/vm86.h>
> vbetool-0.5/lrmi.c:#include <machine/vm86.h>
> vbetool-0.5/lrmi.c:         struct vm86_struct vm;
> vbetool-0.5/lrmi.c:                struct vm86_init_args init;
> ...
> vbetool-0.5/lrmi.c:lrmi_vm86(struct vm86_struct *vm)
> vbetool-0.5/lrmi.c:#define lrmi_vm86 vm86
> vbetool-0.5/lrmi.c:        fputs("vm86() failed\n", stderr);
> vbetool-0.5/lrmi.c:run_vm86(void)
> vbetool-0.5/lrmi.c:             vret = lrmi_vm86(&context.vm);
> vbetool-0.5/lrmi.c:vm86_callback(int sig, int code, struct sigcontext
> *sc)
> vbetool-0.5/lrmi.c:vm86_callback(int sig, int code, struct sigcontext
> *sc)
> vbetool-0.5/lrmi.c:run_vm86(void)
> vbetool-0.5/lrmi.c:             fprintf(stderr, "run_vm86: callback
> already installed\n");
>
> vbetool depends on it, and s2ram depends on vbetool. When we get
> proper kernel drivers, this one will be solved, but it is not "more
> obsolete than 386".
>

vmetool has an x86 emulator.  As far as I know, it's been there for a
long time, and I'd be surprised if it doesn't work on CONFIG_VM86=n
kernels.  That being said, the code is kind of tangled and it's not
quite clear to me what's going on.

See: http://www.codon.org.uk/~mjg59/libx86/

Perhaps we should instead move CONFIG_VM86 out of EXPERT, default it
to n, and suggest that everyone running a reasonably modern distro
(2006 and up?) turn it off.

--Andy
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