Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1755996AbbDIRnR (ORCPT ); Thu, 9 Apr 2015 13:43:17 -0400 Received: from mail-ob0-f178.google.com ([209.85.214.178]:33900 "EHLO mail-ob0-f178.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753847AbbDIRnO (ORCPT ); Thu, 9 Apr 2015 13:43:14 -0400 MIME-Version: 1.0 In-Reply-To: <1428591293-25076-1-git-send-email-peter@hurleysoftware.com> References: <55118928.1090203@hurleysoftware.com> <1428591293-25076-1-git-send-email-peter@hurleysoftware.com> Date: Thu, 9 Apr 2015 10:43:13 -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: 3823 Lines: 80 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 -- 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/