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

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Message-ID: <20120308192504.GH21812@moon>
Date:	Thu, 8 Mar 2012 23:25:04 +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 Thu, Mar 08, 2012 at 08:05:34PM +0100, Oleg Nesterov wrote:
...
> >
> > Yes, exactly, I need to remove old mappings first (because VMAs
> > we're about to restore may intersect with current map the host
> > program has). And yes, once they all are removed I don't have
> > /proc/pid/exe anymore. That's why I need num_exe_file_vmas == 0
> > case.
> 
> OK, in this case PR_SET_MM_EXE_FILE should probably fail if
> mm->num_exe_file_vmas != 0 ? This way it would be more or less
> consistent or at least understandable. Just we add the new
> special case: num_exe_file_vmas == 0 but exe_file != NULL
> because c/r people are crazy.
> 

Sure, I can drop num_exe_file_vmas != 0 case and refuse to
setup new exe symlink if there some VM_EXECUTABLE remains
unmapped. Sounds good?

> > > And I don't think the unconditional
> > >
> > > 	down_write(&mm->mmap_sem);
> > > 	set_mm_exe_file(mm, exe_file);
> > > 	up_write(&mm->mmap_sem);
> > >
> > > is 100% right, this clears ->num_exe_file_vmas. This means that
> > > (if you still have the old mapping) the new exe_file can go away
> > > after added_exe_file_vma() + removed_exe_file_vma(). Normally this
> > > should happen, but afaics this is possible. Note that even, say,
> > > mprotect() can trigger added_exe_file_vma().
> > >
> >
> > Wait, Oleg, I'm confused, in case if there *is* exitsting VM_EXECUTABLEs
> > then we jump into first banch and simply replace old exe_file.
> 
> Yes. And then later you remove the old mapping (which do not match
> the new file anyway) and the new exe_file goes away. Unlikely you
> want this.

Yes, unlikely ;)

	Cyrill
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