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SubjectRe: [PATCH] pty: Fix input race when closing
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On Thu, Apr 9, 2015 at 10:53 AM, Peter Hurley <peter@hurleysoftware.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@hurleysoftware.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@vger.kernel.org> # 3.19+
>>> Reported-by: Andy Whitcroft <apw@canonical.com>
>>> Reported-by: H.J. Lu <hjl.tools@gmail.com>
>>> Signed-off-by: Peter Hurley <peter@hurleysoftware.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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