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Date:   Tue, 5 May 2020 14:44:23 +0200
From:   Christoph Hellwig <hch@lst.de>
To:     Stefan Haberland <sth@linux.ibm.com>
Cc:     Christoph Hellwig <hch@lst.de>, axboe@kernel.dk,
        linux-block@vger.kernel.org, hoeppner@linux.ibm.com,
        linux-s390@vger.kernel.org, heiko.carstens@de.ibm.com,
        gor@linux.ibm.com, borntraeger@de.ibm.com,
        linux-kernel@vger.kernel.org,
        Peter Oberparleiter <oberpar@linux.ibm.com>
Subject: Re: [PATCH 1/1] s390/dasd: remove ioctl_by_bdev from DASD driver
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On Mon, May 04, 2020 at 10:45:33AM +0200, Stefan Haberland wrote:
> > findthe corresponding device for example. Not sure if this is that easy.
> 
> I did some additional research on this.
> What I could imagine:
> 
> The gendisk->private_data pointer currently contains a pointer to
> the dasd_devmap structure. This one is also reachable by iterating
> over an exported dasd_hashlist.
> So I could export the dasd_hashlist symbol, iterate over it and try
> to find the dasd_devmap pointer I have from the gendisk->private_data
> pointer.
> This would ensure that the gendisk belongs to the DASD driver and I
> could use the additional information that is somehow reachable through
> the gendisk->private_data pointer.
> 
> But again, I am not sure if this additional code and effort is needed.
> From my point of view checking the gendisk->major for DASD_MAJOR is
> OK to ensure that the device belongs to the DASD driver.

With CONFIG_DEBUG_BLOCK_EXT_DEVT you can't rely on major numbers.

And compared to all the complications I think the biodasdinfo method
is the least of all those evils.  Jens, any opinion?