Re: 2.6.30-rc6: Reported regressions from 2.6.29

[Linus Torvalds]

On Sat, 16 May 2009, Rafael J. Wysocki wrote:
> 
> Bug-Entry	: http://bugzilla.kernel.org/show_bug.cgi?id=13326
> Subject		: Null pointer dereference in rtc-cmos driver
> Submitter	: Ozan ÃaÄlayan <ozan@xxxxxxxxxxxxx>
> Date		: 2009-05-14 16:16 (3 days old)
> References	: http://marc.info/?l=linux-kernel&m=124231783704696&w=4

Ok, I'm of two minds on this one.

The thing that triggers it is a module that is both a module _and_ 
compiled in (ie the same kernel compiled twice, and the stale module kept 
around). I can see how it happens, and we should react more gracefully to 
it, but at the same time I can't really bring myself to care deeply.

What is going on is that the cmos-rtc.c driver does this:

	#ifdef  CONFIG_PNP
	        pnp_register_driver(&cmos_pnp_driver);
	#endif

	        if (!cmos_rtc.dev)
	                retval = platform_driver_probe(&cmos_platform_driver,
                                               cmos_platform_probe);

	        if (retval == 0)
	                return 0;

	#ifdef  CONFIG_PNP
	        pnp_unregister_driver(&cmos_pnp_driver);
	#endif
	        return retval;

and what happens is that the pnp_register_driver fails when the module 
calls it (because the built-in driver already exists under the same name):

	[   10.428691] Error: Driver 'rtc_cmos' is already registered, aborting...

but the driver doesn't really care whether that succeeded or not, but then 
the platform_driver_probe fails (because the thing is already in use), so 
then it ends up unregistering something that never got registered in the 
first place.

I think this is strictly speaking a bug in driver_unregister(), which is 
too fragile. If you unregister a drivert that wasn't registered, we 
shouldn't oops. 

But we could certainly do it at the rtc-cmos.c level too, and just not 
unregister it if the registration failed. My gut feel is that we should 
aim for the core driver helpers to be less fragile, though - we'll always 
have driver bugs.

Greg?

			Linus
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