linux-kernel - Re: [PATCH v6 02/16] chardev: introduce cdev_get_by

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Date:   Thu, 25 Jul 2019 21:29:40 -0700
From:   Sagi Grimberg <sagi@...mberg.me>
To:     Al Viro <viro@...iv.linux.org.uk>,
        Logan Gunthorpe <logang@...tatee.com>
Cc:     Matthew Wilcox <willy@...radead.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Jens Axboe <axboe@...com>,
        Chaitanya Kulkarni <Chaitanya.Kulkarni@....com>,
        linux-kernel@...r.kernel.org, linux-nvme@...ts.infradead.org,
        Stephen Bates <sbates@...thlin.com>,
        linux-block@...r.kernel.org, Keith Busch <kbusch@...nel.org>,
        linux-fsdevel@...r.kernel.org, Max Gurtovoy <maxg@...lanox.com>,
        Christoph Hellwig <hch@....de>
Subject: Re: [PATCH v6 02/16] chardev: introduce cdev_get_by_path()


>>>>>>> NVMe-OF is configured using configfs. The target is specified by the
>>>>>>> user writing a path to a configfs attribute. This is the way it works
>>>>>>> today but with blkdev_get_by_path()[1]. For the passthru code, we need
>>>>>>> to get a nvme_ctrl instead of a block_device, but the principal is the same.
>>>>>>
>>>>>> Why isn't a fd being passed in there instead of a random string?
>>>>>
>>>>> I suppose we could echo a string of the file descriptor number there,
>>>>> and look up the fd in the process' file descriptor table ...
>>>>
>>>> Assuming that there is a open handle somewhere out there...
>>
>> Yes, that would be a step backwards from an interface. The user would
>> then need a special process to open the fd and pass it through configfs.
>> They couldn't just do it with basic bash commands.
> 
> First of all, they can, but... WTF not have filp_open() done right there?
> Yes, by name.  With permission checks done.  And pick your object from the
> sodding struct file you'll get.
> 
> What's the problem?  Why do you need cdev lookups, etc., when you are
> dealing with files under your full control?  Just open them and use
> ->private_data or whatever you set in ->open() to access the damn thing.
> All there is to it...
Oh this is so much simpler. There is really no point in using anything
else. Just need to remember to compare f->f_op to what we expect to make
sure that it is indeed the same device class.