Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S932922AbbDIRxT (ORCPT ); Thu, 9 Apr 2015 13:53:19 -0400 Received: from mail-qc0-f171.google.com ([209.85.216.171]:35535 "EHLO mail-qc0-f171.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1754036AbbDIRxR (ORCPT ); Thu, 9 Apr 2015 13:53:17 -0400 Message-ID: <5526BC8B.202@hurleysoftware.com> Date: Thu, 09 Apr 2015 13:53:15 -0400 From: Peter Hurley User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:31.0) Gecko/20100101 Thunderbird/31.6.0 MIME-Version: 1.0 To: "H.J. Lu" CC: Greg Kroah-Hartman , Jiri Slaby , LKML , Alan Cox , Andy Whitcroft , Josh Boyer , stable@vger.kernel.org Subject: Re: [PATCH] pty: Fix input race when closing References: <55118928.1090203@hurleysoftware.com> <1428591293-25076-1-git-send-email-peter@hurleysoftware.com> In-Reply-To: Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 4080 Lines: 85 On 04/09/2015 01:43 PM, H.J. Lu wrote: > On Thu, Apr 9, 2015 at 7:54 AM, Peter Hurley 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: # 3.19+ >> Reported-by: Andy Whitcroft >> Reported-by: H.J. Lu >> Signed-off-by: Peter Hurley >> --- >> 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. Regards, Peter Hurley -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/