lkml.org 
[lkml]   [2015]   [Apr]   [13]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
Date
From
SubjectRe: [PATCH v3] pty: Fix input race when closing
On Mon, Apr 13, 2015 at 01:24:34PM -0400, 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: <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>
> ---

Thanks for this, it's a bit late at the moment for 4.1-rc1, but I'll get
it queued up after that.

thanks,

greg k-h


\
 
 \ /
  Last update: 2015-04-13 20:41    [W:0.054 / U:1.428 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site