linux-kernel - Re: [PATCH, RESEND] ipc/shm: handle removed segments gracefully in shm

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Date:	Sat, 2 Jan 2016 12:45:07 +0100
From:	Manfred Spraul <manfred@...orfullife.com>
To:	"Kirill A. Shutemov" <kirill@...temov.name>,
	"Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org,
	Dmitry Vyukov <dvyukov@...gle.com>
Subject: Re: [PATCH, RESEND] ipc/shm: handle removed segments gracefully in
 shm_mmap()

On 11/13/2015 08:23 PM, Davidlohr Bueso wrote:
>
> So considering EINVAL, even your approach to bumping up nattach by 
> calling
> _shm_open earlier isn't enough. Races exposed to user called rmid can 
> still
> occur between dropping the lock and doing ->mmap(). Ultimately this 
> leads to
> all ipc_valid_object() checks, as we totally ignore SHM_DEST segments 
> nowadays
> since we forbid mapping previously removed segments.
>
> I think this is the first thing we must decide before going forward 
> with this
> mess. ipc currently defines invalid objects by merely checking the 
> deleted flag.
>
> Manfred, any thoughts?
>
With regards to locking: Sorry, shm is too different to msg/sem/mqueue.

With regards to EIDRM / EINVAL:
When all kernel memory was released, then the kernel cannot find out if 
the ID was valid at one time or not.
Thus EIDRM can only be a hint, the OS (kernel/libc) cannot guarantee 
that user space will never see something else.
(trivial example: user space sleeps just before the syscall)

So I would not create special code to optimize EIDRM handling for races. 
If we sometimes report EINVAL, it would be probably ok as well.

--
     Manfred
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