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

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Date:	Fri, 02 Oct 2015 10:30:25 +0200
From:	Jean-Christian de Rivaz <jc@...is.ch>
To:	Peter Hurley <peter@...leysoftware.com>
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

Le 02. 10. 15 00:57, Peter Hurley a écrit :
> 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

Hi Peter, thanks for your time,

> 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.

Yes but this is a consequence of the fact that the ax0 interface was 
re-opened uninitialized while the corresponding serial device is no 
longer connected to the system. I don't see any rational to create this 
bogus interface: the serial device is gone.

> 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.

Not physically, sorry. The physical serial device was unplugged front 
the system (or in hardware forced reset in the case of my test), causing 
a USB disconnect. It's important to understand that the USB disconnect 
has already occurred seconds before the crash. The fact that there is 
still an ACM tty structure in the kernel corresponding to nothing real 
is the cause of the problem.

> The line discipline is reinited as a security precaution to prevent a previous
> session's data from being visible in the new session.

Pragmatically reinited to N_TTY is ok, this is in fact how my proposed 
patches work. But reinited to N_AX25 while the serial device is no more 
have no sense at all and cause the crash when the new uninitialized 
parasitic interface try to send a packet.

>   The tty core does not know
> at the time the vhangup() occurs that the ACM driver plans to unregister the
> tty device.

That's the root problem: It must a least known that it must not call 
mkiss_open(). That's the bug that must be fixed. Or maybe the option e) 
fix must be developed.

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

Can I ask what did you suggest to solve the problem ? The bug is real, 
causing a kernel panic and complete crash of the system, requiring a 
hardware reset to reboot.

Best Regards,
Jean-Christian de Rivaz

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