linux-kernel - Re: Q: x86: add context tag to mark mm when running a task in 32-bit compatibility mode

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Message-ID: <20110928155539.GA10550@redhat.com>
Date:	Wed, 28 Sep 2011 17:55:39 +0200
From:	Oleg Nesterov <oleg@...hat.com>
To:	Al Viro <viro@...IV.linux.org.uk>
Cc:	Stephen Wilson <wilsons@...rt.ca>, linux-kernel@...r.kernel.org
Subject: Re: Q: x86: add context tag to mark mm when running a task in
	32-bit compatibility mode

On 09/27, Al Viro wrote:
>
> On Tue, Sep 27, 2011 at 07:04:48PM +0200, Oleg Nesterov wrote:
> > --- x/arch/x86/kernel/process_64.c
> > +++ x/arch/x86/kernel/process_64.c
> > @@ -502,10 +502,6 @@ void set_personality_64bit(void)
> >  	/* Make sure to be in 64bit mode */
> >  	clear_thread_flag(TIF_IA32);
> >
> > -	/* Ensure the corresponding mm is not marked. */
> > -	if (current->mm)
> > -		current->mm->context.ia32_compat = 0;
>
> What happens when 32bit task does exec on 64bit binary?

Nothing. This bit is always zero after init_mm(), it can't copy
MMF_COMPAT.

But. This is wrong anyway. I forgot about fork(). We do not want to
uglify copy_mm(), MMF_INIT_MASK should include MMF_COMPAT and then
set_personality_64bit() needs to clear this bit.

Thanks!

> > -	if (current->mm)
> > -		current->mm->context.ia32_compat = 1;
> > +	set_bit(MMF_COMPAT, &current->mm->flags);
>
> ... assuming current->mm is never NULL here.

Yes, but it can't be NULL?

SET_PERSONALITY() can only be called during exec, and it must be called
after exec_mmap() has already installed the new mm != NULL, otherwise
the current code is buggy anyway.

Oleg.

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