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

[Peter Hurley <peter@hurleysoftware.com>]

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.

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.

Regards,
Peter Hurley