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

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Message-ID: <20160610200739.GA14789@redhat.com>
Date:	Fri, 10 Jun 2016 22:07:49 +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)

On 06/09, Andy Lutomirski wrote:
>
> On Jun 6, 2016 3:21 PM, "Oleg Nesterov" <oleg@...hat.com> wrote:
> >
> > On 06/01, Dmitry Safonov wrote:
> > >
> > > Note, that this will work only if application has changed it's CS.
> >
> > So, suppose it changes it's CS and crashes,
> >
> > > If the application does 32-bit syscall with __USER_CS, ptrace
> > > @@ -1355,7 +1355,7 @@ void update_regset_xstate_info(unsigned int size, u64 xstate_mask)
> > >  const struct user_regset_view *task_user_regset_view(struct task_struct *task)
> > >  {
> > >  #ifdef CONFIG_IA32_EMULATION
> > > -     if (test_tsk_thread_flag(task, TIF_IA32))
> > > +     if (!user_64bit_mode(task_pt_regs(task)))
> > >  #endif
> >
> > then coredump will do fill_elf_header(view->e_machine) and use EM_X86_64
> > instead of EM_386, or vice versa...
> >
> > I simply can't understand is this better or worse, I guess gdb or any
> > other tool which looks at this coredump will be confused anyway.
> >
>
> I think it's better.

and I tend to agree, I didn't try to argue with this change, but

> CRIU will change CS and someone will make the
> restored process crash afterwards.

I don't understand what do you mean... could you explain?

IIRC, CRIU can't c/r the 32-bit applications, or this is no longer true?

Oleg.