Re: [PATCH] touchscreen: ads7846: please don't touch free'd memory

[Kevin Hilman]

Dmitry Torokhov <dmitry.torokhov@xxxxxxxxx> writes:

> On Tue, May 25, 2010 at 12:52:12PM -0700, Kevin Hilman wrote:
>> Dmitry Torokhov <dmitry.torokhov@xxxxxxxxx> writes:
>> 
>> > On Tue, May 18, 2010 at 04:46:53PM -0700, Kevin Hilman wrote:
>> >> If the _probe() method fails, the 'ts' struct is freed, yet it is
>> >> still used as the drvdata passed to suspend/resume/remove methods.
>> >> Even though the input device does not get registerd, the driver's
>> >> suspend/resume methods still get called as it's a registered SPI
>> >> device.  This patch adds sanity checks to these methods to ensure that
>> >> drvdata is valid before using it.
>> >> 
>> >
>> > This is a load of crap. If probe() fails that means that driver does not
>> > manage the device and thus ads7846_suspend() and ads7846_resume() should
>> > not be called _at all_. If SPI core manages to call random methods from
>> > the drivers that failed to bind to a device that should be fixed in SPI
>> > core.
>> 
>> Agreed, this is indeed a bug in the SPI driver core.
>> 
>> However, by fixing the SPI core to unregister the driver on failure
>> (patch below), we prevent the suspend & resume methods from being
>> called, but the driver's ->remove() method will still be called due to
>> the driver_unregister().  So at least the remove() method needs fixing
>> to prevent accessing free'd memory.
>> 
>> Unless there are objections, I'll post the below along with an updated
>> version of ads7846 patch.
>> 
>> Kevin
>> 
>> diff --git a/drivers/spi/spi.c b/drivers/spi/spi.c
>> index b3a1f92..42d4d26 100644
>> --- a/drivers/spi/spi.c
>> +++ b/drivers/spi/spi.c
>> @@ -175,6 +175,8 @@ static void spi_drv_shutdown(struct device *dev)
>>   */
>>  int spi_register_driver(struct spi_driver *sdrv)
>>  {
>> +	int ret;
>> +
>>  	sdrv->driver.bus = &spi_bus_type;
>>  	if (sdrv->probe)
>>  		sdrv->driver.probe = spi_drv_probe;
>> @@ -182,7 +184,12 @@ int spi_register_driver(struct spi_driver *sdrv)
>>  		sdrv->driver.remove = spi_drv_remove;
>>  	if (sdrv->shutdown)
>>  		sdrv->driver.shutdown = spi_drv_shutdown;
>> -	return driver_register(&sdrv->driver);
>> +
>> +	ret = driver_register(&sdrv->driver);
>> +	if (!ret)
>> +		driver_unregister(&sdrv->driver);
>
> This is still wrong. driver_register() should properly clean up after
> itself and not require calls to driver_unregister() (and I believe it
> does).
>
> Besides, I do not see how this patch changes anything with regard to
> binding devices and drivers. Even if driver_register() succeeds, binding
> may still fail and we should not be calling neither ->remove(), nor
> ->suspend()/->resume() for the devices that failed to be bound.

Hmm, good point.

After digging into the driver core and realizing that it seemed to
have sane error handling itself, I took a closer look at
ads7846_probe() and discovered it doesn't actually return an error
code for certain failure cases!  That was the root cause.

The patch below fixes the problem.

Thanks,

Kevin