Messages in this thread | | | Date | Tue, 22 Jul 2014 12:40:59 -0400 | From | Peter Hurley <> | Subject | Re: [PATCH] tty/tty_io.c: make a check before reuse cdev |
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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 -- 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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