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

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Date:   Fri, 26 Jul 2019 09:13:49 +0200
From:   Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:     Sagi Grimberg <sagi@...mberg.me>
Cc:     Al Viro <viro@...iv.linux.org.uk>,
        Logan Gunthorpe <logang@...tatee.com>,
        Matthew Wilcox <willy@...radead.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()

On Thu, Jul 25, 2019 at 09:29:40PM -0700, Sagi Grimberg wrote:
> 
> > > > > > > > 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.

That is good, that solves the "/dev/random/" issue I was talking about
earlier as well.

Odds are you all can do the same for the blockdev interface as well.

thanks,

greg k-h