linux-kernel - Re: [PATCH] btrfs: Don't check for file->private

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Date:	Wed, 12 Nov 2014 12:59:33 -0500
From:	Chris Mason <clm@...com>
To:	Martin Kepplinger <martink@...teo.de>
CC:	<linux-btrfs@...r.kernel.org>, <gregkh@...uxfoundation.org>,
	<linux-kernel@...r.kernel.org>,
	Martin Kepplinger <martink@...teo.de>
Subject: Re: [PATCH] btrfs: Don't check for file->private_data on open(). It
 is set by the core.

On Wed, Nov 12, 2014 at 11:38 AM, Martin Kepplinger <martink@...teo.de> 
wrote:
> The miscdevice core now sets file->private_data to the struct 
> miscdevice
> so don't fail when this is not NULL.
> 
> Signed-off-by: Martin Kepplinger <martink@...teo.de>
> ---
> This is a question: what does this check provide and does overwriting
> file->private_data make any difference?
> 
> Is miscdevice's open() by the user not allowed here, if 
> file->private_data
> is set?
> 
> thanks!!

Btrfs uses this in the transaction start ioctl to record the 
transaction handle being started.  Ceph is the main user of the ioctl, 
and we could setup a hash table if needed.  But which call path in 
miscdevice is doing this?

With your patch in place, btrfs would end up overwriting the miscdevice 
private_data field, which would probably cause problems.

-chris

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