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

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Message-ID: <77a3e81d-2542-6782-0fc1-1d25bcc75598@deltatee.com>
Date:   Thu, 25 Jul 2019 13:41:16 -0600
From:   Logan Gunthorpe <logang@...tatee.com>
To:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Sagi Grimberg <sagi@...mberg.me>
Cc:     linux-kernel@...r.kernel.org, linux-nvme@...ts.infradead.org,
        linux-block@...r.kernel.org, linux-fsdevel@...r.kernel.org,
        Christoph Hellwig <hch@....de>,
        Keith Busch <kbusch@...nel.org>, Jens Axboe <axboe@...com>,
        Chaitanya Kulkarni <Chaitanya.Kulkarni@....com>,
        Max Gurtovoy <maxg@...lanox.com>,
        Stephen Bates <sbates@...thlin.com>,
        Alexander Viro <viro@...iv.linux.org.uk>
Subject: Re: [PATCH v6 02/16] chardev: introduce cdev_get_by_path()



On 2019-07-25 1:34 p.m., Greg Kroah-Hartman wrote:
> On Thu, Jul 25, 2019 at 12:02:30PM -0700, Sagi Grimberg wrote:
>>
>>>>>> Why do you have a "string" within the kernel and are not using the
>>>>>> normal open() call from userspace on the character device node on the
>>>>>> filesystem in your namespace/mount/whatever?
>>>>>
>>>>> 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 wouldn't know the answer to this but I assume because once we decided
>>> to use configfs, there was no way for the user to pass the kernel an fd.
>>
>> That's definitely not changing. But this is not different than how we
>> use the block device or file configuration, this just happen to need the
>> nvme controller chardev now to issue I/O.
> 
> So, as was kind of alluded to in another part of the thread, what are
> you doing about permissions?  It seems that any user/group permissions
> are out the window when you have the kernel itself do the opening of the
> char device, right?  Why is that ok?  You can pass it _any_ character
> device node and away it goes?  What if you give it a "wrong" one?  Char
> devices are very different from block devices this way.

Well the permission question is no different from the block-device case
we already have. The user has to be root to configure a target so it has
access to the block/char device. Containers and NVMe-of are really not
designed to mix and would take a lot of work to make this make any sense
(And that's way out of scope of what I'm trying to do here and doesn't
change the need for a the cdev_get_by_path()).

If the user specifies a non-nvme char device, it is rejected by the code
in nvme_ctrl_get_by_path() when it compares the fops. See patch 4.

Logan