linux-kernel - Re: [PATCH] tty/tty_io.c: make a check before reuse cdev

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Date:	Tue, 22 Jul 2014 10:38:44 -0700
From:	Greg KH <gregkh@...uxfoundation.org>
To:	"xinhui.pan" <xinhuix.pan@...el.com>, mnipxh <mnipxh@...il.com>,
	jslaby@...e.cz, linux-kernel@...r.kernel.org,
	yanmin_zhang@...ux.intel.com
Cc:	Peter Hurley <peter@...leysoftware.com>
Subject: Re: [PATCH] tty/tty_io.c: make a check before reuse cdev

On Tue, Jul 22, 2014 at 12:40:59PM -0400, Peter Hurley wrote:
> On 07/22/2014 07:52 AM, xinhui.pan wrote:
> > 
> > 于 2014年07月21日 23:38, Greg KH 写道:
> >> On Mon, Jul 21, 2014 at 08:47:16PM +0800, pp wrote:
> >>> As reuse the cdev may cause panic. After we unregister the tty device, we may use tty_hangup() o
> >>> other similar function to send a signal(SIGHUP) to process which has opend our device. But that
> >>> not succeed if the process couldn't get the signal. for example, a process forked
> >>> but his parent quited never get SIGHUP.
> >>>
> >>> Here is our scence.
> >>> tty driver register its device and init the cdevs, then process "A" open one cdev.
> >>> tty driver unregister its device and cdev_del the cdevs, call tty_hangup to (S)send signal SIGHUP to process A.
> >>> But that step(S) fails.
> >>
> >> How can that fail?  What driver does this fail for?
> > 
> > hi, Greg
> > 	Thanks for your nice comments. :)
> > 	It's gsm driver that want to unregister/register tty device. We are working on our intel mobile phone,
> > When the phone goes into airplane-mode, the modem will disconnect from system, then gsmld_close() -> gsmld_detach_gsm() -> tty_unregister_device().
> > When the phone leaves airplane-mode, the modem will connect to system, then gsmld_open() -> gsmld_attach_gsm() -> tty_register_device()
> > In this way how gsm driver works.
> > It seems very normal and can work well. :)
> > 
> > But there is always something bad for us to deal with. 
> > If a process(A, its name) opens the /dev/gsmttyXX, and the process(A) is, for example, running with command "A &".
> > The process(A) is not able to receive the signal SIGHUP from __tty_hangup() -> tty_signal_session_leader(). 
> > There are several reasons that can stop process(A) from receiving signal SIGHUP. 
> > another example, B is running, and he makes a fork(), A is the child of B, then B quit, leave A running.
> > in such scenario, A is not able to receive signal SIGHUP, either. 
> > Anyway, we cannot guarantee process(A) will close /dev/gsmttyXX in time. That means we don't know when we can reuse the tty_driver->cdevs[XX].
> > one second, one minute? We don't know. We just don't trust user space. :)
> 
> Or a process could simply ignore SIGHUP, in which case /dev/gsmttyXX
> will not be closed until process termination.
> 
> >>> tty driver register its device and (D)init the cdevs again.
> >>
> >> What driver does this with an "old" device, it should have created a new
> >> one, otherwise, as you have pointed out, it's a bug.
> >>
> > 
> > I can't agree more with you. we should not use "old" device.
> 
> This is a gsm driver problem. The GSM driver is reusing device indexes
> for still-open ttys.

I agree, it should not be doing that at all, please fix that up, and all
should be fine.

greg k-h
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