Re: [PATCH 3/8] bdi: add a ->dev_name field to struct backing_dev_info

From: Jan Kara <jack@suse.cz>
To: Christoph Hellwig <hch@lst.de>
Cc: Jan Kara <jack@suse.cz>, Yufen Yu <yuyufen@huawei.com>,
	axboe@kernel.dk, tj@kernel.org, bvanassche@acm.org,
	tytso@mit.edu, gregkh@linuxfoundation.org,
	linux-block@vger.kernel.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH 3/8] bdi: add a ->dev_name field to struct backing_dev_info
Date: Thu, 16 Apr 2020 14:31:40 +0200	[thread overview]
Message-ID: <20200416123140.GN23739@quack2.suse.cz> (raw)
In-Reply-To: <20200416122235.GA26982@lst.de>

On Thu 16-04-20 14:22:35, Christoph Hellwig wrote:
> On Thu, Apr 16, 2020 at 02:19:01PM +0200, Christoph Hellwig wrote:
> > On Thu, Apr 16, 2020 at 02:02:23PM +0200, Jan Kara wrote:
> > > Yes, that can indeed happen. E.g. I remember that drivers/scsi/sd.c calls
> > > device_add_disk() + del_gendisk() repeatedly for one request_queue and that
> > > would result in leaking the name (and possibly cause use-after-free
> > > issues).
> > 
> > Sd calls device_add_disk once in ->probe, and del_gendisk once in
> > sd_remove.  Note that sd_probe allocates a new scsi_disk structure and
> > a new gendisk everytime, but it does indeed reuse the request_queue
> > and thus bdi.
> > 
> > > I think dev_name has to be just a static array inside
> > > backing_dev_info which gets overwritten on reregistration. The question is
> > > how big should be this array... Some grepping shows that 40 bytes should be
> > > enough for everybody except fs/vboxsf/super.c which puts 'fc->source' into
> > > the name which can be presumably rather large. Anyway, I'd make it 40 and
> > > just truncate it case in case it does not fit. bdi_dev_name() is used for
> > > informational purposes anyway...
> > 
> > We could just make it a variable sized array at the end of the structure
> > and size it based on the len.
> 
> Which doesn't always work as the size might not always be the same.
> But I think the fundamental problem is that we are trying to re-register
> previous unregistered bdis.  We really should not have bdi_alloc
> separate from bdi_register and solve this properly.

Yes, that would be easier then but it seems like a much larger change
because currently bdi is disassociated from request_queue only in
__blk_release_queue() (blk_exit_queue()). I guess the separate bdi
registration / deregistration is partially a leftover from times when bdi
was still embedded in request_queue but now it's difficult to undo it.

								Honza
-- 
Jan Kara <jack@suse.com>
SUSE Labs, CR