linux-kernel - Re: [patch 0/3] Re: tty contention resulting from tty_open_by

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Date:   Mon, 17 Jul 2017 14:23:46 +0100
From:   Okash Khawaja <okash.khawaja@...il.com>
To:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Cc:     Alan Cox <gnomes@...rguk.ukuu.org.uk>,
        Jiri Slaby <jslaby@...e.com>,
        Samuel Thibault <samuel.thibault@...-lyon.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        William Hubbs <w.d.hubbs@...il.com>,
        Chris Brannon <chris@...-brannons.com>,
        Kirk Reiser <kirk@...sers.ca>,
        "speakup@...ux-speakup.org" <speakup@...ux-speakup.org>,
        "devel@...verdev.osuosl.org" <devel@...verdev.osuosl.org>
Subject: Re: [patch 0/3] Re: tty contention resulting from tty_open_by_device export



> On 17 Jul 2017, at 13:31, Greg Kroah-Hartman <gregkh@...uxfoundation.org> wrote:
> 
>> On Thu, Jul 13, 2017 at 12:29:54PM +0100, Okash Khawaja wrote:
>>> On Wed, Jul 12, 2017 at 07:20:28PM +0100, Alan Cox wrote:
>>> 
>>>> When opening from kernel, we don't use file pointer. The count mismatch
>>>> is between tty->count and #fd's. So opening from kernel leads to #fd's
>>>> being less than tty->count. I thought this difference is relevant to
>>>> user-space opening of tty, and not to kernel opening of tty. Can you
>>>> suggest how to address this mismatch?
>>> 
>>> Your kernel reference is the same as having a file open reference so I
>>> think this actually needs addressing in the maths. In other words count
>>> the number of kernel references and also add that into the test for
>>> check_tty_count (kernel references + #fds == count).
>>> 
>>> I'd really like to keep this right because that check has a long history
>>> of catching really nasty race conditions in the tty code. The
>>> open/close/hangup code is really fragile so worth the debugability.
>> 
>> I see. Okay based this, check_tty_count can be easily updated to take
>> into account kernel references.
> 
> Ok, I'll drop this series from my "to-apply" queue and wait for you to
> redo it.

Sure. I can fix the tty->count mismatch based on Alan's suggestion. However I don't understand why the exclusivity flag should belong to tty_port and not tty_struct. It will be good to know why. 

Thanks,
Okash