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

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Message-ID: <20200416122235.GA26982@lst.de>
Date:   Thu, 16 Apr 2020 14:22:35 +0200
From:   Christoph Hellwig <hch@....de>
To:     Jan Kara <jack@...e.cz>
Cc:     Yufen Yu <yuyufen@...wei.com>, Christoph Hellwig <hch@....de>,
        axboe@...nel.dk, tj@...nel.org, bvanassche@....org, tytso@....edu,
        gregkh@...uxfoundation.org, linux-block@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 3/8] bdi: add a ->dev_name field to struct
 backing_dev_info

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.