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Re: [PATCH 5/6] x86/ptrace: down with test_thread_flag(TIF_IA32)

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  Re: [PATCH 5/6] x86/ptrace: down with test_thread_flag(TIF_IA32) Oleg Nesterov <oleg@redhat.com> - 2016-06-13 16:00 +0200
    Re: [PATCH 5/6] x86/ptrace: down with test_thread_flag(TIF_IA32) Andy Lutomirski <luto@amacapital.net> - 2016-06-13 22:50 +0200
      Re: [PATCH 5/6] x86/ptrace: down with test_thread_flag(TIF_IA32) Oleg Nesterov <oleg@redhat.com> - 2016-06-14 16:40 +0200

#1420871 — 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@redhat.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.

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#1421298

On Mon, Jun 13, 2016 at 6:50 AM, Oleg Nesterov <oleg@redhat.com> wrote:
> 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@redhat.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 ;)

Is anything actually wrong with this, though?

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#1421960

On 06/13, Andy Lutomirski wrote:
>
> On Mon, Jun 13, 2016 at 6:50 AM, Oleg Nesterov <oleg@redhat.com> wrote:
> >
> > 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 ;)
>
> Is anything actually wrong with this, though?

No, I think the patch is correct... It adds the user-visible change, but
I do not think it can break something.

Well, perhaps we should also remove SA_IA32_ABI|SA_X32_ABI from
oact->sa.sa_flags in do_sigaction() to ensure that these non-uapi flags won't
leak to user-space, but probably we do not really care.

Oleg.

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