linux-kernel - Re: Slab corruption in floppy driver module

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Message-ID: <4F223F11.4050307@suse.com>
Date:	Fri, 27 Jan 2012 11:37:13 +0530
From:	Suresh Jayaraman <sjayaraman@...e.com>
To:	Dirk Gouders <gouders@...bocholt.fh-gelsenkirchen.de>
CC:	Vivek Goyal <vgoyal@...hat.com>, Tejun Heo <tj@...nel.org>,
	LKML <linux-kernel@...r.kernel.org>, Jens Axboe <axboe@...nel.dk>
Subject: Re: Slab corruption in floppy driver module

On 01/27/2012 03:18 AM, Dirk Gouders wrote:
> Vivek Goyal <vgoyal@...hat.com> writes:
> 
>> On Thu, Jan 26, 2012 at 10:05:32AM -0800, Tejun Heo wrote:
>>> Hello,
>>>
>>> On Thu, Jan 26, 2012 at 10:04:20AM -0500, Vivek Goyal wrote:
>>>>  out_put_disk:
>>>>  	while (dr--) {
>>>>  		del_timer_sync(&motor_off_timer[dr]);
>>>> -		if (disks[dr]->queue)
>>>> +		if (disks[dr]->queue) {
>>>>  			blk_cleanup_queue(disks[dr]->queue);
>>>> +			/*
>>>> +			 * The request queue reference we took at device
>>>> +			 * creation time has been put by above
>>>> +			 * blk_cleanup_queue(). We have not called add_disk()
>>>> +			 * yet and due to failure calling put_disk(). Put disk
>>>> +			 * will try to put a reference to disk->queue which is
>>>> +			 * taken in add_disk(). As we have not taken that
>>>> +			 * extra reference, putting extra reference down
>>>> +			 * will try to access already freed queue. Clear
>>>> +			 * disk->queue before calling put_disk().> 

>>>> +			 */
>>>> +			disks[dr]->queue = NULL;
>>>
>>> Yeah, this looks correct to me.  It might be better to tone down the
>>> comment a bit tho.  Wouldn't it be sufficient to say put_disk() isn't
>>> paired with add_disk() and will put one extra time?
>>
>> Sure. Toned down the comment as suggested. Here is the new patch.
>>
>> floppy: Cleanup disk->queue before caling put_disk() if add_disk() was never called
>>
>> add_disk() takes gendisk reference on request queue. If driver failed during
>> initialization and never called add_disk() then that extra reference is not
>> taken. That reference is put in put_disk(). floppy driver allocates the
>> disk, allocates queue, sets disk->queue and then relizes that floppy
>> controller is not present. It tries to tear down everything and tries to
>> put a reference down in put_disk() which was never taken.
>>
>> In such error cases cleanup disk->queue before calling put_disk() so that
>> we never try to put down a reference which was never taken in first place.
>>
>> Reported-by: Suresh Jayaraman <sjayaraman@...e.com>
>> Tested-by: Dirk Gouders <gouders@...bocholt.fh-gelsenkirchen.de>
>> Signed-off-by: Vivek Goyal <vgoyal@...hat.com>
>> ---
>>  drivers/block/floppy.c |    8 +++++++-
>>  1 file changed, 7 insertions(+), 1 deletion(-)
>>
>> Index: linux-2.6/drivers/block/floppy.c
>> ===================================================================
>> --- linux-2.6.orig/drivers/block/floppy.c	2012-01-15 09:49:14.000000000 -0500
>> +++ linux-2.6/drivers/block/floppy.c	2012-01-26 14:35:14.662374464 -0500
>> @@ -4368,8 +4368,14 @@ out_unreg_blkdev:
>>  out_put_disk:
>>  	while (dr--) {
>>  		del_timer_sync(&motor_off_timer[dr]);
>> -		if (disks[dr]->queue)
>> +		if (disks[dr]->queue) {
>>  			blk_cleanup_queue(disks[dr]->queue);
>> +			/*
>> +			 * put_disk() is not paired with add_disk() and
>> +			 * will put queue reference one extra time. fix it.
>> +			 */
>> +			disks[dr]->queue = NULL;
>> +		}
>>  		put_disk(disks[dr]);
>>  	}
>>  	return err;
> 
> 
> Probably a rare and uncommon one but it seems that the reloading case on
> a machine that has a floppy controller is a different problem.  To be
> sure I tested the patch on a machine that has a floppy controller and
> when unloading and reloading the floppy module the log messages that I
> attached to a mail earlier in this thread are still generated.
> 

Yeah, this seems like a different problem. Could you please try enabling
CONFIG_DEBUG_PAGEALLOC and see whether is it pointing to the problem
code while loading/unloading the module?


Suresh


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