linux-kernel - Re: [PATCH 6/7] mm: kill vma flag VM

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Date:	Fri, 06 Apr 2012 01:04:43 +0400
From:	Konstantin Khlebnikov <khlebnikov@...nvz.org>
To:	Matt Helsley <matthltc@...ibm.com>
CC:	Cyrill Gorcunov <gorcunov@...nvz.org>,
	Oleg Nesterov <oleg@...hat.com>,
	"linux-mm@...ck.org" <linux-mm@...ck.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Eric Paris <eparis@...hat.com>,
	"linux-security-module@...r.kernel.org" 
	<linux-security-module@...r.kernel.org>,
	"oprofile-list@...ts.sf.net" <oprofile-list@...ts.sf.net>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Al Viro <viro@...iv.linux.org.uk>
Subject: Re: [PATCH 6/7] mm: kill vma flag VM_EXECUTABLE

Matt Helsley wrote:
> On Tue, Apr 03, 2012 at 11:32:04PM +0400, Cyrill Gorcunov wrote:
>> On Tue, Apr 03, 2012 at 11:16:31AM -0700, Matt Helsley wrote:
>>> On Tue, Apr 03, 2012 at 09:10:20AM +0400, Konstantin Khlebnikov wrote:
>>>> Matt Helsley wrote:
>>>>> On Sat, Mar 31, 2012 at 10:13:24PM +0200, Oleg Nesterov wrote:
>>>>>> On 03/31, Konstantin Khlebnikov wrote:
>>>>>>>
>>>>>>> comment from v2.6.25-6245-g925d1c4 ("procfs task exe symlink"),
>>>>>>> where all this stuff was introduced:
>>>>>>>
>>>>>>>> ...
>>>>>>>> This avoids pinning the mounted filesystem.
>>>>>>>
>>>>>>> So, this logic is hooked into every file mmap/unmmap and vma split/merge just to
>>>>>>> fix some hypothetical pinning fs from umounting by mm which already unmapped all
>>>>>>> its executable files, but still alive. Does anyone know any real world example?
>>>>>>
>>>>>> This is the question to Matt.
>>>>>
>>>>> This is where I got the scenario:
>>>>>
>>>>> https://lkml.org/lkml/2007/7/12/398
>>>>
>>>> Cyrill Gogcunov's patch "c/r: prctl: add ability to set new mm_struct::exe_file"
>>>> gives userspace ability to unpin vfsmount explicitly.
>>>
>>> Doesn't that break the semantics of the kernel ABI?
>>
>> Which one? exe_file can be changed iif there is no MAP_EXECUTABLE left.
>> Still, once assigned (via this prctl) the mm_struct::exe_file can't be changed
>> again, until program exit.
>
> The prctl() interface itself is fine as it stands now.
>
> As far as I can tell Konstantin is proposing that we remove the unusual
> counter that tracks the number of mappings of the exe_file and require
> userspace use the prctl() to drop the last reference. That's what I think
> will break the ABI because after that change you *must* change userspace
> code to use the prctl(). It's an ABI change because the same sequence of
> system calls with the same input bits produces different behavior.

But common software does not require this at all. I did not found real examples,
only hypothesis by Al Viro: https://lkml.org/lkml/2007/7/12/398
libhugetlbfs isn't good example too, the man proc says: /proc/[pid]/exe is alive until
main thread is alive, but in case libhugetlbfs /proc/[pid]/exe disappears too early.
Also I would not call it ABI, this corner-case isn't documented, I'm afraid only few
people in the world knows about it =)
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