linux-kernel - Re: [PATCH 2/4] ipc,shm: prevent race with rmid in shmat(2)

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Date:	Fri, 27 Sep 2013 07:45:01 +0200
From:	Manfred Spraul <manfred@...orfullife.com>
To:	Davidlohr Bueso <davidlohr@...com>
CC:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Rik van Riel <riel@...hat.com>,
	Mike Galbraith <efault@....de>, sedat.dilek@...il.com,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 2/4] ipc,shm: prevent race with rmid in shmat(2)

Hi Davidlohr,

On 09/16/2013 05:04 AM, Davidlohr Bueso wrote:
> This fixes a race in shmat() between finding the msq and
> actually attaching the segment, as another thread can delete shmid
> underneath us if we are preempted before acquiring the kern_ipc_perm.lock.
According the the man page, Linux supports attaching to deleted shm 
segments:

http://linux.die.net/man/2/shmat
>
> On Linux, it is possible to attach a shared memory segment even if it 
> is already marked to be deleted. However, POSIX.1-2001 does not 
> specify this behavior and many other implementations do not support it.
>
Does your patch change that?
Unfortunately, I have neither any experience with ipc/shm nor any test 
cases.

And:
As far as I can see it's not a problem if we are attaching to a deleted 
segment: shm_close cleans up everything.

--
     Manfred
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