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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Message-ID: <20120419211216.GA2200@redhat.com>
Date:	Thu, 19 Apr 2012 23:12:16 +0200
From:	Oleg Nesterov <oleg@...hat.com>
To:	Cyrill Gorcunov <gorcunov@...nvz.org>
Cc:	akpm@...ux-foundation.org, khlebnikov@...nvz.org,
	keescook@...omium.org, kosaki.motohiro@...fujitsu.com,
	matthltc@...ibm.com, tj@...nel.org, xemul@...allels.com,
	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 04/20, Cyrill Gorcunov wrote:
>
> On Thu, Apr 19, 2012 at 09:20:05PM +0200, Oleg Nesterov wrote:
> > On 04/19, Andrew Morton wrote:
> > >
> > > From: Konstantin Khlebnikov <khlebnikov@...nvz.org>
> > > Subject: c/r: prctl: update prctl_set_mm_exe_file() after mm->num_exe_file_vmas removal
> > >
> > > [ fix for "c-r-prctl-add-ability-to-set-new-mm_struct-exe_file-v2" from mm tree ]
> > >
> > > After removing mm->num_exe_file_vmas kernel keeps mm->exe_file until final
> > > mmput(), it never becomes NULL while task is alive.
> > >
> > > We can check for other mapped files in mm instead of checking
> > > mm->num_exe_file_vmas, and mark mm with flag MMF_EXE_FILE_CHANGED in order
> > > to forbid second changing of mm->exe_file.
> >
> > I lost the track a long ago.
> >
> > Just one question, what does this "forbid second changing" actually mean?
>
> Heh :) Oleg, it was actually your idea to make this feature "one-shot".

Heh, no ;)

IIRC, I only asked you what do you actually want,

	Just one note for the record, prctl_set_mm_exe_file() does

		if (mm->num_exe_file_vmas)
			return -EBUSY;

	We could do

		if (mm->exe_file)
			return -EBUSY;

	This way "because this feature is a special to C/R" becomes
	really true. IOW, you can't do PR_SET_MM_EXE_FILE twice.

	I am fine either way, just I want to ensure you really want
	the current version.

and only because it was documented as "feature is a special to C/R".

> Once exe-file changed to a new value, it can't be changed again. The
> reason was to bring at least minimum disturbance in sysadmins life.

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.

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