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Subject: Re: [PATCH v6 02/16] chardev: introduce cdev_get_by_path()
To:     Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc:     Logan Gunthorpe <logang@deltatee.com>,
        linux-kernel@vger.kernel.org, linux-nvme@lists.infradead.org,
        linux-block@vger.kernel.org, linux-fsdevel@vger.kernel.org,
        Christoph Hellwig <hch@lst.de>,
        Keith Busch <kbusch@kernel.org>, Jens Axboe <axboe@fb.com>,
        Chaitanya Kulkarni <Chaitanya.Kulkarni@wdc.com>,
        Max Gurtovoy <maxg@mellanox.com>,
        Stephen Bates <sbates@raithlin.com>,
        Alexander Viro <viro@zeniv.linux.org.uk>
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 <20190725174032.GA27818@kroah.com>
 <682ff89f-04e0-7a94-5aeb-895ac65ee7c9@deltatee.com>
 <20190725180816.GA32305@kroah.com>
 <da0eacb7-3738-ddf3-8c61-7ffc61aa41f4@deltatee.com>
 <20190725182701.GA11547@kroah.com>
 <a3262a7f-b78e-05ba-cda3-a7587946bd91@deltatee.com>
 <5951e0f5-cc90-f3de-0083-088fecfd43bb@grimberg.me>
 <20190725193415.GA12117@kroah.com>
From:   Sagi Grimberg <sagi@grimberg.me>
Message-ID: <966fa988-de56-effe-dd52-3515ee83629c@grimberg.me>
Date:   Thu, 25 Jul 2019 12:37:11 -0700
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>>>>>> 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.

We could condition any configfs operation on capable(CAP_NET_ADMIN) to
close that hole for now..