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

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Message-ID: <53CF7EAC.8010201@intel.com>
Date:	Wed, 23 Jul 2014 17:21:48 +0800
From:	"xinhui.pan" <xinhuix.pan@...el.com>
To:	Peter Hurley <peter@...leysoftware.com>,
	Greg KH <gregkh@...uxfoundation.org>, mnipxh <mnipxh@...il.com>
CC:	jslaby@...e.cz, linux-kernel@...r.kernel.org,
	yanmin_zhang@...ux.intel.com
Subject: Re: [PATCH] tty/tty_io.c: make a check before reuse cdev



于 2014年07月23日 00:40, Peter Hurley 写道:
> 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.
> 

hi, Peter
	Agree with you. Thanks for your nice comments.


>>>> 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.
> 
> The GSM driver uses a global table, gsm_mux[], to allocate device indexes
> but prematurely clears the table entry in gsm_mux_cleanup(). If instead,
> clearing the gsm_mux table entry were deferred to gsm_mux_free(), then
> device indexes would not be getting reused until after the last tty
> associated with the last gsm attach was closed.
> 

Very nice solution. We will check if this can cause any risk, both to kernel and user space.
Using a new tty base to register with new cdevs may give us more chance to wait PROCESS quit/close.
when total 256 tty used up, what we should do is still in discuss.
thanks, I even want to have a cup of coffee with you :)

thanks

xinhui

> Regards,
> Peter Hurley
> 
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