linux-kernel - Re: [RFC] c/r: prctl: Add ability to set new mm_struct::exe

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Date:	Fri, 9 Mar 2012 18:13:49 +0400
From:	Cyrill Gorcunov <gorcunov@...nvz.org>
To:	Oleg Nesterov <oleg@...hat.com>
Cc:	Matt Helsley <matthltc@...ibm.com>,
	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	Pavel Emelyanov <xemul@...allels.com>,
	Kees Cook <keescook@...omium.org>, Tejun Heo <tj@...nel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [RFC] c/r: prctl: Add ability to set new mm_struct::exe_file v3

On Fri, Mar 09, 2012 at 02:47:32PM +0100, Oleg Nesterov wrote:
> On 03/09, Cyrill Gorcunov wrote:
> >
> > On Fri, Mar 09, 2012 at 04:57:35PM +0400, Cyrill Gorcunov wrote:
> > >
> > > yeah, thanks, will update.
> > >
> >
> > This one should fit all requirements I hope.
> 
> Oh, sorry Cyrill, I simply can't resist...

Hehe ;) No problem, please continue complaining,
I don't wanna miss something and try to merge a
patch with nit/error/or-whatever.

> > +	/*
> > +	 * Setting new mm::exe_file is only allowed
> > +	 * when no VM_EXECUTABLE vma's left. This is
> > +	 * a special C/R case when a restored program
> > +	 * need to change own /proc/$pid/exe symlink.
> > +	 * After this call mm::num_exe_file_vmas become
> > +	 * meaningless. If mm::num_exe_file_vmas will
> > +	 * ever increase back from zero -- this code
> > +	 * needs to be revised, thus WARN_ here, just
> > +	 * to be sure.
> 
> To be shure in what??

To be sure it's not increased somewhere else before
down_write taken.

> 
> > +	 */
> > +	down_write(&mm->mmap_sem);
> > +	WARN_ON_ONCE(mm->num_exe_file_vmas);
> 
> We already checked it is zero. Yes, it shouldn't grow. But why
> do we need another check here?
> 
> If it can grow, it can grow after we drop mmap_sem as well and
> this would be wrong. So may be we need another WARN_ON() at the
> end?
> 
> I'd understand if you add something like
> 
> 	WARN_ON(!mm->num_exe_file_vmas && !current->in_exec);
> 
> into added_exe_file_vma().
> 
> Or
> 	WARN_ON(mm->num_exe_file_vmas <= 0);
> 
> into removed_exe_file_vma().

This one looks like a good idea for me -- it's cheap and
not a hot path.

> 
> But imho your WARN looks like "OK, I checked it lockless but I
> am not sure this is correct".

Oleg, I bet if someone will be changing num_exe_file_vmas overall
idea -- this prctl code will be fixed at last moment (if ever) only
because it's very specific, so I wanted to not miss such moment
and add some check that the rest of the kernel is in a good state.
This test is cheap but may prevent potential problem if one day
mm::exe_file concept will be reworked.

Sure I can simply drop this WARN_ON ;)

	Cyrill
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