linux-kernel - Re: + c-r-prctl-add-ability-to-set-new-mm_struct-exe_file-update-after-mm- num_exe_file

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Date:	Fri, 20 Apr 2012 02:02:01 +0400
From:	Cyrill Gorcunov <gorcunov@...nvz.org>
To:	Oleg Nesterov <oleg@...hat.com>
Cc:	Konstantin Khlebnikov <khlebnikov@...nvz.org>,
	"akpm@...ux-foundation.org" <akpm@...ux-foundation.org>,
	"keescook@...omium.org" <keescook@...omium.org>,
	"kosaki.motohiro@...fujitsu.com" <kosaki.motohiro@...fujitsu.com>,
	"matthltc@...ibm.com" <matthltc@...ibm.com>,
	"tj@...nel.org" <tj@...nel.org>,
	Pavel Emelianov <xemul@...allels.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: +
 c-r-prctl-add-ability-to-set-new-mm_struct-exe_file-update-after-mm-
 num_exe_file_vmas-removal.patch added to -mm tree

On Thu, Apr 19, 2012 at 11:51:09PM +0200, Oleg Nesterov wrote:
> On 04/20, Konstantin Khlebnikov wrote:
> >
> > Oleg Nesterov wrote:
> >>
> >> You misunderstood. I am not arguing with "one-shot", I do not really
> >> care.
> >>
> >> My question is: unless I missed something "it can't be changed again"
> >> is not actually true. A task does PR_SET_MM_EXE_FILE, then it forks
> >> the new child. The child can do PR_SET_MM_EXE_FILE again. Is this
> >> by design?
> >>
> >> Oleg.
> >>
> >
> > I found more weird case: child thread (with CLONE_THREAD and without CLONE_VM)
> > changes its exe_file...
> 
> No. copy_process() does:
> 
> 	if ((clone_flags & CLONE_THREAD) && !(clone_flags & CLONE_SIGHAND))
> 		return ERR_PTR(-EINVAL);
> 
> 	if ((clone_flags & CLONE_SIGHAND) && !(clone_flags & CLONE_VM))
> 		return ERR_PTR(-EINVAL);
> 
> IOW, CLONE_THREAD => CLONE_SIGHAND => CLONE_VM

Guys, while I more-less agree with Matt about single-shot behaviour

[ let me copy my and his email

  >> With mm->exe_file this prctl option become a one-shot
  >> only, and while at moment our user-space tool can perfectly
  >> live with that I thought that there is no strict need to
  >> limit the option this way from the very beginning.
  >>
  > As far as backward compatibility, isn't it better to lift that restriction
  > later rather than add it? I think the latter would very likely "break"
  > things whereas the former would not.
  >
  > I also prefer that restriction because it establishes a bound on how
  > frequently the symlink can change. Keeping it a one-shot deal makes the
  > values that show up in tools like top more reliable for admins.
]

I guess maybe it's time to drop one-shot requirement and as result
we can drop MMF_EXE_FILE_CHANGED bit completely, making overall code
simplier?

Our tool can live with any behaviour (and one shot and multishot,
doesn't matter).

	Cyrill
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