Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S932966AbbDIRzT (ORCPT ); Thu, 9 Apr 2015 13:55:19 -0400 Received: from mail-ob0-f177.google.com ([209.85.214.177]:32771 "EHLO mail-ob0-f177.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S932528AbbDIRzR (ORCPT ); Thu, 9 Apr 2015 13:55:17 -0400 MIME-Version: 1.0 In-Reply-To: <5526BC8B.202@hurleysoftware.com> References: <55118928.1090203@hurleysoftware.com> <1428591293-25076-1-git-send-email-peter@hurleysoftware.com> <5526BC8B.202@hurleysoftware.com> Date: Thu, 9 Apr 2015 10:55:16 -0700 Message-ID: Subject: Re: [PATCH] pty: Fix input race when closing From: "H.J. Lu" To: Peter Hurley Cc: Greg Kroah-Hartman , Jiri Slaby , LKML , Alan Cox , Andy Whitcroft , Josh Boyer , stable@vger.kernel.org Content-Type: text/plain; charset=UTF-8 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 4247 Lines: 88 On Thu, Apr 9, 2015 at 10:53 AM, Peter Hurley wrote: > 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. > I will give it a try. -- H.J. -- 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/