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

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Message-ID: <a37e947d-c49a-837e-e97d-647ca9d378c3@acm.org>
Date:   Sun, 19 Apr 2020 08:29:21 -0700
From:   Bart Van Assche <bvanassche@....org>
To:     Christoph Hellwig <hch@....de>
Cc:     Jan Kara <jack@...e.cz>, axboe@...nel.dk, yuyufen@...wei.com,
        tj@...nel.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 4/19/20 12:58 AM, Christoph Hellwig wrote:
> On Sat, Apr 18, 2020 at 08:40:20AM -0700, Bart Van Assche wrote:
>>> This can have a sideeffect not only bdi->dev_name will be truncated to 64
>>> chars (which generally doesn't matter) but possibly also kobject name will
>>> be truncated in the same way.  Which may have user visible effects. E.g.
>>> for fs/vboxsf 64 chars need not be enough. So shouldn't we rather do it the
>>> other way around - i.e., let device_create_vargs() create the device name
>>> and then copy to bdi->dev_name whatever fits?
>>
>> How about using kvasprintf() instead of vsnprintf()?
> 
> That is what v1 did, see the thread in response to that on why it isn't
> a good idea.

Are you perhaps referring to patch "[PATCH 3/8] bdi: add a ->dev_name 
field to struct backing_dev_info" 
(https://lore.kernel.org/linux-block/20200416071519.807660-4-hch@lst.de/) 
and also to the replies to that patch? This is what I found in the 
replies: "When driver try to to re-register bdi but without 
release_bdi(), the old dev_name will be cover directly by the newer in 
bdi_register_va(). So, I am not sure whether it can cause memory leak 
for bdi->dev_name."

Has it been considered to avoid that leak by freeing bdi->dev_name from 
unregister_bdi(), e.g. as follows?

void bdi_unregister(struct backing_dev_info *bdi)
{
+	char *dev_name;

	/* make sure nobody finds us on the bdi_list anymore */
	bdi_remove_from_list(bdi);
	wb_shutdown(&bdi->wb);
	cgwb_bdi_unregister(bdi);

	if (bdi->dev) {
		bdi_debug_unregister(bdi);
		device_unregister(bdi->dev);
		bdi->dev = NULL;
+		dev_name = bdi->dev_name;
+		bdi->dev_name = "(unregistered)";
+		kfree(dev_name);
	}

	if (bdi->owner) {
		put_device(bdi->owner);
		bdi->owner = NULL;
	}
}

Thanks,

Bart.