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

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Message-ID: <20190725191124.GE30641@bombadil.infradead.org>
Date:   Thu, 25 Jul 2019 12:11:25 -0700
From:   Matthew Wilcox <willy@...radead.org>
To:     Sagi Grimberg <sagi@...mberg.me>
Cc:     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>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        linux-fsdevel@...r.kernel.org, Max Gurtovoy <maxg@...lanox.com>,
        Logan Gunthorpe <logang@...tatee.com>,
        Christoph Hellwig <hch@....de>
Subject: Re: [PATCH v6 02/16] chardev: introduce cdev_get_by_path()

On Thu, Jul 25, 2019 at 12:05:29PM -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...

Well, that's how we'd know that the application echoing /dev/nvme3 into
configfs actually has permission to access /dev/nvme3.  Think about
containers, for example.  It's not exactly safe to mount configfs in a
non-root container since it can access any NVMe device in the system,
not just ones which it's been given permission to access.  Right?