linux-kernel - Re: [PATCH] pty: Fix input race when closing

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Date:	Thu, 9 Apr 2015 15:11:53 -0700
From:	"H.J. Lu" <hjl.tools@...il.com>
To:	Peter Hurley <peter@...leysoftware.com>
Cc:	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Jiri Slaby <jslaby@...e.cz>,
	LKML <linux-kernel@...r.kernel.org>,
	Alan Cox <alan@...ux.intel.com>,
	Andy Whitcroft <apw@...onical.com>,
	Josh Boyer <jwboyer@...hat.com>, stable@...r.kernel.org
Subject: Re: [PATCH] pty: Fix input race when closing

On Thu, Apr 9, 2015 at 10:53 AM, Peter Hurley <peter@...leysoftware.com> wrote:
> On 04/09/2015 01:43 PM, H.J. Lu wrote:
>> On Thu, Apr 9, 2015 at 7:54 AM, Peter Hurley <peter@...leysoftware.com> wrote:
>>> A read() from a pty master may mistakenly indicate EOF (errno == -EIO)
>>> after the pty slave has closed, even though input data remains to be read.
>>> For example,
>>>
>>>        pty slave       |        input worker        |    pty master
>>>                        |                            |
>>>                        |                            |   n_tty_read()
>>> pty_write()            |                            |     input avail? no
>>>   add data             |                            |     sleep
>>>   schedule worker  --->|                            |     .
>>>                        |---> flush_to_ldisc()       |     .
>>> pty_close()            |       fill read buffer     |     .
>>>   wait for worker      |       wakeup reader    --->|     .
>>>                        |       read buffer full?    |---> input avail ? yes
>>>                        |<---   yes - exit worker    |     copy 4096 bytes to user
>>>   TTY_OTHER_CLOSED <---|                            |<--- kick worker
>>>                        |                            |
>>>
>>>                                 **** New read() before worker starts ****
>>>
>>>                        |                            |   n_tty_read()
>>>                        |                            |     input avail? no
>>>                        |                            |     TTY_OTHER_CLOSED? yes
>>>                        |                            |     return -EIO
>>>
>>> Several conditions are required to trigger this race:
>>> 1. the ldisc read buffer must become full so the input worker exits
>>> 2. the read() count parameter must be >= 4096 so the ldisc read buffer
>>>    is empty
>>> 3. the subsequent read() occurs before the kicked worker has processed
>>>    more input
>>>
>>> However, the underlying cause of the race is that data is pipelined, while
>>> tty state is not; ie., data already written by the pty slave end is not
>>> yet visible to the pty master end, but state changes by the pty slave end
>>> are visible to the pty master end immediately.
>>>
>>> Pipeline the TTY_OTHER_CLOSED state through input worker to the reader.
>>> 1. Introduce TTY_OTHER_DONE which is set by the input worker when
>>>    TTY_OTHER_CLOSED is set and either the input buffers are flushed or
>>>    input processing has completed. Readers/polls are woken when
>>>    TTY_OTHER_DONE is set.
>>> 2. Reader/poll checks TTY_OTHER_DONE instead of TTY_OTHER_CLOSED.
>>> 3. A new input worker is started from pty_close() after setting
>>>    TTY_OTHER_CLOSED, which ensures the TTY_OTHER_DONE state will be
>>>    set if the last input worker is already finished (or just about to
>>>    exit).
>>>
>>> Remove tty_flush_to_ldisc(); no in-tree callers.
>>>
>>> Fixes: 52bce7f8d4fc ("pty, n_tty: Simplify input processing on final close")
>>> Bugzilla: https://bugzilla.kernel.org/show_bug.cgi?id=96311
>>> BugLink: http://bugs.launchpad.net/bugs/1429756
>>> Cc: <stable@...r.kernel.org> # 3.19+
>>> Reported-by: Andy Whitcroft <apw@...onical.com>
>>> Reported-by: H.J. Lu <hjl.tools@...il.com>
>>> Signed-off-by: Peter Hurley <peter@...leysoftware.com>
>>> ---
>>>  Documentation/serial/tty.txt |  3 +++
>>>  drivers/tty/n_hdlc.c         |  4 ++--
>>>  drivers/tty/n_tty.c          |  4 ++--
>>>  drivers/tty/pty.c            |  3 +--
>>>  drivers/tty/tty_buffer.c     | 25 +++++++++++--------------
>>>  include/linux/tty.h          |  2 +-
>>>  6 files changed, 20 insertions(+), 21 deletions(-)
>>>
>>
>> I tried it on 3.19.3 and it doesn't work with the testcase in
>>
>> https://bugzilla.kernel.org/show_bug.cgi?id=96311
>
> Can you test this patch on top of mainline? There's a couple of
> fixes in 4.0-rc that specifically address weakly-ordered CPUs.

I tried it on 4.0.0-rc7 and it doesn't work.

-- 
H.J.
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