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

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Date:   Thu, 25 Jul 2019 13:24:22 -0600
From:   Logan Gunthorpe <logang@...tatee.com>
To:     Matthew Wilcox <willy@...radead.org>,
        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>,
        Christoph Hellwig <hch@....de>
Subject: Re: [PATCH v6 02/16] chardev: introduce cdev_get_by_path()



On 2019-07-25 1:11 p.m., Matthew Wilcox wrote:
> 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...

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.

> Well, that's how we'd know that the application echoing /dev/nvme3 into
> configfs actually has permission to access /dev/nvme3.  

It's the kernel that's accessing the device so it has permission. root
permission is required to configure the kernel.

> 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?

I don't think it really makes any sense to talk about NVMe-of and
containers. Though, if we did it would be solely on the configuration
interface so that users inside a container might be able to configure a
new target for resources they can see and they'd have to have their own
view into configfs....

Logan