linux-kernel - Re: [PATCH 5/6] x86/ptrace: down with test_thread_flag(TIF

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Message-ID: <20160613135020.GA27007@redhat.com>
Date:	Mon, 13 Jun 2016 15:50:20 +0200
From:	Oleg Nesterov <oleg@...hat.com>
To:	Andy Lutomirski <luto@...capital.net>
Cc:	Dmitry Safonov <dsafonov@...tuozzo.com>,
	"H. Peter Anvin" <hpa@...or.com>,
	Dmitry Safonov <0x7f454c46@...il.com>, khorenko@...tuozzo.com,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Cyrill Gorcunov <gorcunov@...nvz.org>, xemul@...tuozzo.com,
	X86 ML <x86@...nel.org>, Ingo Molnar <mingo@...hat.com>
Subject: Re: [PATCH 5/6] x86/ptrace: down with test_thread_flag(TIF_IA32)

To avoid the confusion, let me first say that I am not going to argue
with these changes, I simply do not understand the problem space enough.

On 06/10, Andy Lutomirski wrote:
>
> On Fri, Jun 10, 2016 at 1:07 PM, Oleg Nesterov <oleg@...hat.com> wrote:
> >
> > IIRC, CRIU can't c/r the 32-bit applications, or this is no longer true?
> >
>
> CRIU has a horrible, nasty, brilliant idea: it will start restoring
> 32-bit processes by treating them mostly like 64-bit processes.  The
> restorer will start out 64-bit, set everything up, and long
> jump/return/sigreturn/whatever back to 32-bit mode.

OK, I see,

> My proposal was
> that, rather than coming up with nasty hacks to switch the kernel's
> idea of the task bitness,

Well, I can't resist but to me SA_IA32_ABI/SA_X32_ABI looks like a hack
too.  We actually shift TIF_*32 into k_sigaction->flags, and the fact
that we do this per-signal looks, well, interesting ;)

And at first glance it would be very simple to change the task bitness,
CRIU can simply exec a dummy 32-bit application before anything else.
In this case (I think) we also do not need do_map_vdso/ARCH_MAP_VDSO_*
at least right now.

Yes, I guess this will complicate CRIU significantly.

> we instead teach the kernel to respect that
> actual bitness as indicated by CS and the syscalls used to the extent
> possible.

I am still not sure the idea to remove TIF_IA32/TIF_X32 is really good.
But again, I won't argue, I do not feel I understand pro/cons enough.

> So, yes, a restored 32-bit process that crashes should dump core as
> though it's 32-bit even though it was 64-bit when execve was last
> called :)

OK, thanks for you explanation Andy.

Oleg.