linux-kernel - Re: Force mkiss to reset the line discipline when serial device is removed

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Message-ID: <560DBA47.3040907@hurleysoftware.com>
Date:	Thu, 1 Oct 2015 18:57:11 -0400
From:	Peter Hurley <peter@...leysoftware.com>
To:	Jean-Christian de Rivaz <jc@...is.ch>
Cc:	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Jiri Slaby <jslaby@...e.com>,
	Thomas Osterried <thomas@...erried.de>,
	David Ranch <dranch@...nnet.net>,
	Ralf Baechle DL5RB <ralf@...ux-mips.org>,
	linux-hams@...nnet.net, linux-hams@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: Force mkiss to reset the line discipline when serial device is
 removed

On 10/01/2015 12:56 PM, Jean-Christian de Rivaz wrote:
> Hi Greg and Jiri,
> 
> I try to fix a kernel panic bug related to the AX25 (and probably SLIP) line discipline when the corresponding serial device is removed [1]. I proposed some patches [2] [3] on the linux-hams mailing list but I think there raise more questions about how tty_ldisc_hangup() should work when a serial device is removed [4].
> 
> I actually see the following options:
> 
> a) Let the specific line discipline set the TTY_DRIVER_RESET_TERMIOS flag in tty->driver as in [2] but this is suspected bad practice [5].
> 
> b) Let the specific line discipline set the TTY_OTHER_CLOSED flag in tty and check it in tty_ldisc_hangup() as in [3].
> 
> c) Let the specific line discipline set the TTY_LDISC_HALTED flag in tty and check it in tty_ldisc_hangup().
> 
> d) Let the specific line discipline set a new flag for that purpose, for example TTY_LDISC_RESET, and check it in tty_ldisc_hangup().
> 
> e) Close the tty earlier so that tty_ldisc_reinit() is not even called. Need some advise on how this should be done.
> 
> f) That's all wrong, something other need to be changed.
> 
> I would appreciate some comments from tty subsystem experts about this issue.
> 
> [1] http://www.spinics.net/lists/linux-hams/msg03500.html

The crash reported here appears to be related to how mkiss handles its netdev;
maybe prematurely freeing the tx/rx buffers? I'd relook at how slip handles
netdev teardown.

I don't see a problem with the ACM tty/tty core side of this.

At the time the hangup occurs, there is actually still an ACM tty device.
The line discipline is reinited as a security precaution to prevent a previous
session's data from being visible in the new session. The tty core does not know
at the time the vhangup() occurs that the ACM driver plans to unregister the
tty device.

Don't do any of the things you suggest above.

Regards,
Peter Hurley
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