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

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Message-ID: <20120309223932.GB725@moon>
Date:	Sat, 10 Mar 2012 02:39:32 +0400
From:	Cyrill Gorcunov <gorcunov@...nvz.org>
To:	Matt Helsley <matthltc@...ibm.com>
Cc:	Oleg Nesterov <oleg@...hat.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 02:02:44PM -0800, Matt Helsley wrote:
...
> 
> Sorry about the other email -- hadn't full caught up on this thread.

No problem.

> This is even better, yes.
>

Well, in final version I switched back to

+	if (mm->num_exe_file_vmas)
+		return -EBUSY;

simply because it's more flexible than mm->exe_file.

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.

> Of course I'd prefer it if there was a way to keep num_exe_file_vmas
> correct and not special-case c/r. The first approximation of a solution

It remains correct actually. There is no way to map new VM_EXECUTABLE
from user-space after we've unmapped previous ones, so num_exe_file_vmas
will remain 0.

> might be to increment the count whenever a new mmap filp == mm->exe_file
> and decrement on unmap. I think there are a bunch of details needed to
> make that work but my feeling is it's do-able. Have you investigated this
> already and rejected it for some reason (did I miss that discussion
> somehow?)?

As far as I understand overall num_exe_file_vmas concept -- we track
a number of VM_EXECUTABLE with it, so setting new exe_file should not
change num_exe_file_vmas I think.

	Cyrill
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