linux-kernel - Re: Bug in BCMA: device_unregister causing "NULL pointer dereference at"

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Date:	Thu, 21 Jul 2011 12:33:41 -0400
From:	Pavel Roskin <proski@....org>
To:	Rafał Miłecki <zajec5@...il.com>
CC:	linux-wireless@...r.kernel.org,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: Bug in BCMA: device_unregister causing "NULL pointer dereference
 at"

On 07/21/2011 03:18 AM, Rafał Miłecki wrote:

>> So when I unload bcma after I got driver (b43) for 0x812 core, I get
>> NULL pointer dereference.
>>
>> Any tip, why does it happen?
>
> I've tracked where does crash really happen (kobject_del does not
> really say much). The real forwardtrace is:
> device_unregister → device_del → kobject_del → kobj_kset_leave →
> kobj_kset_leave → list_del_init
>
> If you take a look at list_del_init, it touches "prev" and "next". So
> I've added some debugging:
> pr_info("core->dev.kobj.entry.prev: 0x%p\n", core->dev.kobj.entry.prev);
> pr_info("core->dev.kobj.entry.next: 0x%p\n", core->dev.kobj.entry.next);

There are options for debugging that you may want to enable:

CONFIG_DEBUG_LIST
CONFIG_DEBUG_OBJECTS
CONFIG_DEBUG_KOBJECT

Actually, consider enabling most debug options as possible, except 
perhaps the most time consuming (such as CONFIG_DEBUG_KMEMLEAK).  Maybe 
you are passing a freed pointer or something.

Print the pointers you are passing to device_register() and 
device_unregister().

> [  612.819320] bcma: core->dev.kobj.entry.prev: 0x  (null)

You may want to make it a macro and print it in most bcma functions.

-- 
Regards,
Pavel Roskin
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