linux-kernel - Re: [patch 2/6] tty: export tty_open_by

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Message-ID: <CAOtcWM3zVhGyvvLm3f-7PuFr4p07e1CdWA0fXBZz38hGC=h5-Q@mail.gmail.com>
Date:   Mon, 15 May 2017 12:10:06 +0100
From:   Okash Khawaja <okash.khawaja@...il.com>
To:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Cc:     Jiri Slaby <jslaby@...e.com>,
        Samuel Thibault <samuel.thibault@...-lyon.org>,
        linux-kernel@...r.kernel.org, devel@...verdev.osuosl.org,
        Kirk Reiser <kirk@...sers.ca>,
        Chris Brannon <chris@...-brannons.com>,
        "Speakup is a screen review system for Linux." 
        <speakup@...ux-speakup.org>, Alan Cox <gnomes@...rguk.ukuu.org.uk>
Subject: Re: [patch 2/6] tty: export tty_open_by_driver

Hi,

On Mon, May 15, 2017 at 11:31 AM, Greg Kroah-Hartman
<gregkh@...uxfoundation.org> wrote:
> On Sat, Apr 29, 2017 at 08:52:59PM +0100, Okash Khawaja wrote:
>> This applies on top of the changes already in staging-next branch which allow
>> kernel access to TTY dev.
>>
>> Signe-doff-by: Okash Khawaja <okash.khawaja@...il.com>
>> Reviewed-by: Samuel Thibault <samuel.thibault@...-lyon.org>
>>
>> Index: linux-staging/drivers/tty/tty_io.c
>> ===================================================================
>> --- linux-staging.orig/drivers/tty/tty_io.c
>> +++ linux-staging/drivers/tty/tty_io.c
>> @@ -1369,7 +1369,10 @@ static struct tty_struct *tty_driver_loo
>>       struct tty_struct *tty;
>>
>>       if (driver->ops->lookup)
>> -             tty = driver->ops->lookup(driver, file, idx);
>> +             if (!file)
>> +                     tty = ERR_PTR(-EIO);
>> +             else
>> +                     tty = driver->ops->lookup(driver, file, idx);
>
> Why make this change?  Shouldn't the lookup function allow a NULL file
> pointer?  Or is the problem that they do not?
>
>>       else
>>               tty = driver->ttys[idx];
>>
>> @@ -2001,7 +2004,7 @@ static struct tty_driver *tty_lookup_dri
>>               struct tty_driver *console_driver = console_device(index);
>>               if (console_driver) {
>>                       driver = tty_driver_kref_get(console_driver);
>> -                     if (driver) {
>> +                     if (driver && filp) {
>
> Why change this too?
>
> Your changelog does not explain any of this, please do so.
>
> thanks,
>
> greg k-h


Sorry, I should have been more descriptive here. The changes which
check file pointer for null are basically from Alan Cox's patch here:
http://www.mail-archive.com/linux-kernel@vger.kernel.org/msg1215095.html.
The description from that patch is quoted below:

"[RFC] tty_port: allow a port to be opened with a tty that has no file handle

    Let us create tty objects entirely in kernel space. Untested proposal to
    show why all the ideas around rewriting half the uart stack are not needed.

    With this a kernel created non file backed tty object could be used to
handle
    data, and set terminal modes. Not all ldiscs can cope with this as N_TTY in
    particular has to work back to the fs/tty layer.

    The tty_port code is however otherwise clean of file handles as far as I can
    tell as is the low level tty port write path used by the ldisc, the
    configuration low level interfaces and most of the ldiscs.

    Currently you don't have any exposure to see tty hangups because those are
    built around the file layer. However a) it's a fixed port so you probably
    don't care about that b) if you do we can add a callback and c) you almost
    certainly don't want the userspace tear down/rebuild behaviour anyway.

    This should however be sufficient if we wanted for example to enumerate all
    the bluetooth bound fixed ports via ACPI and make them directly available.

    It doesn't deal with the case of a user opening a port that's also kernel
    opened and that would need some locking out (so it returned EBUSY if bound
    to a kernel device of some kind). That needs resolving along with how you
    "up" or "down" your new bluetooth device, or enumerate it while providing
    the existing tty API to avoid regressions (and to debug)."

With this patchset tty_open_by_driver is now called from inside kernel
with file pointer set to null. I can resend this patch with above
description.

Thanks,
Okash