linux-kernel - Re: [BUG]NULL pointer dereference at 0000000000000008 __blkdev

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Message-ID: <20140104060925.GF10323@ZenIV.linux.org.uk>
Date:	Sat, 4 Jan 2014 06:09:25 +0000
From:	Al Viro <viro@...IV.linux.org.uk>
To:	Jack Wang <jinpu.wang@...fitbricks.com>
Cc:	linux-fsdevel@...r.kernel.org,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Jens Axboe <axboe@...nel.dk>
Subject: Re: [BUG]NULL pointer dereference at 0000000000000008
 __blkdev_put+0x17f/0x1d0

On Thu, Jan 02, 2014 at 10:36:30AM +0100, Jack Wang wrote:

> > Bug happened at line 1486, looks disk->fops is NULL here for some
> > reason, is it reasonable to add a check like:
> > 
> > if (disk->fops)
> > 	if (disk->fops->release)
> > 		ret = disk->fops->release(disk, mode);
> > 
> > 
> > Happy New Year and Best regards:)
> > Jack
> > 
> 
> Ping, could you share opnions on this, attached with patch I proposaled.

Sorry, had been sick since mid-December ;-/  The patch is not a good idea -
in the best case it's papering over a bug (and insufficiently so, at that,
since there are other places where disk->fops->some_method is checked).

gendisk->fops should never be assigned NULL; it starts life with NULL
->fops, but that should be assigned a non-NULL value (and never modified
afterwards) before anyone can see it.  Moreover, even if some driver has
fscked up and forgot to initialize the damn thing, get_gendisk() would've
refused to return such a thing to any callers (including __blkdev_get()).
Note that __blkdev_get() would oops on such a thing if get_gendisk()
somehow returned it.

Looks like something is shitting over bdev->bd_disk or bdev->bd_disk->fops.
The offsets in the disassembled code are all wrong (including that from
beginning of function to oopsing instruction), but the code match is good,
so I agree that we are hitting bdev->bd_disk->fops == NULL here.  The
question is how it has happened - that's where the real bug is...

How reproducible it is?  And which kernel, while we are at it?  This area
didn't get a lot of changes lately, but still...
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