linux-kernel - Re: [PATCH 1/1] s390/dasd: remove ioctl_by

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Date:   Mon, 4 May 2020 10:45:33 +0200
From:   Stefan Haberland <sth@...ux.ibm.com>
To:     Christoph Hellwig <hch@....de>
Cc:     axboe@...nel.dk, linux-block@...r.kernel.org,
        hoeppner@...ux.ibm.com, linux-s390@...r.kernel.org,
        heiko.carstens@...ibm.com, gor@...ux.ibm.com,
        borntraeger@...ibm.com, linux-kernel@...r.kernel.org,
        Peter Oberparleiter <oberpar@...ux.ibm.com>
Subject: Re: [PATCH 1/1] s390/dasd: remove ioctl_by_bdev from DASD driver

Am 30.04.20 um 16:02 schrieb Stefan Haberland:
> Am 30.04.20 um 15:13 schrieb Christoph Hellwig:
>> On Thu, Apr 30, 2020 at 01:17:54PM +0200, Stefan Haberland wrote:
>>> Remove the calls to ioctl_by_bdev from the DASD partition detection code
>>> to enable the removal of the specific code.
>>>
>>> To do so reuse the gendisk private_data pointer and not only provide a
>>> pointer to the devmap but provide a new structure containing a pointer
>>> to the devmap as well as all required information for the partition
>>> detection. This makes it independent from the dasd_information2_t
>>> structure.
>> I think sharing the data structure in private data is pretty dangerous.
> Thought of this as well. This is why I check for the major number before I
> use the private pointer to reference the data structure. Thought this would
> be enough checking.
> Do you think this is not sufficient?
>
>> In the meantime I thought of another idea - the partition code could
>> do a symbol_get of a symbol exported by the dasd driver and use that
>> to query the information.
> Then I would need to export a lot of DASD internal structures to be
> available
> in thepartition detection code if I would like to walk down our device
> map to
> 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.

What do you think?