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

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Date:   Mon, 10 Jul 2017 13:33:07 +0100
From:   Okash Khawaja <okash.khawaja@...il.com>
To:     Alan Cox <gnomes@...rguk.ukuu.org.uk>
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Jiri Slaby <jslaby@...e.com>,
        Samuel Thibault <samuel.thibault@...-lyon.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,
        devel@...verdev.osuosl.org
Subject: Re: [patch 0/3] Re: tty contention resulting from tty_open_by_device
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On Mon, Jul 10, 2017 at 12:52:33PM +0100, Alan Cox wrote:
> On Sun, 09 Jul 2017 12:41:53 +0100
> Okash Khawaja <okash.khawaja@...il.com> wrote:
> 
> > On Sat, Jul 08, 2017 at 10:38:03AM +0200, Greg Kroah-Hartman wrote:
> > > Overall, the idea looks sane to me.  Keeping userspace from opening a
> > > tty that the kernel has opened internally makes sense, hopefully
> > > userspace doesn't get too confused when that happens.  I don't think we
> > > normally return -EBUSY from an open call, have you seen what happens
> > > with apps when you do this (like minicom?)
> > >  
> > I tested this wil minincom, picocom and commands like "echo foo >
> > /dev/ttyS0". They all correctly report "Device or resource busy".
> > 
> > I have addressed all the comments you made. I have also split the patch
> > into three. Following is summary of each.
> 
> If the tty counts are being misreported then it would be better to fix
> the code to actually manage the counts properly. The core tty code is
> telling you that the tty is not in a valid state. While this is of
> itself a good API to have, the underlying reference miscounting ought
> IMHO to be fixed as well.
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?

> 
> Also you don't need a new TTY_KOPENED flag as far as I can see. All tty's
> have a usage count and active bit flags already. Use those.
Ah may be I didn't notice the active bit. Is it one of the #defines in
tty.h? Can usage count and active bit be used to differentiate between
whether the tty was opened by kernel or user?

Thanks,
Okash