Messages in this thread | | | Date | Wed, 23 Jul 2014 17:21:48 +0800 | From | "xinhui.pan" <> | Subject | Re: [PATCH] tty/tty_io.c: make a check before reuse cdev |
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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 > -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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