From: Al Viro <viro@ZenIV.linux.org.uk>
To: Jack Wang <jinpu.wang@profitbricks.com>
Cc: linux-fsdevel@vger.kernel.org,
"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
Jens Axboe <axboe@kernel.dk>
Subject: Re: [BUG]NULL pointer dereference at 0000000000000008 __blkdev_put+0x17f/0x1d0
Date: Sat, 4 Jan 2014 06:09:25 +0000 [thread overview]
Message-ID: <20140104060925.GF10323@ZenIV.linux.org.uk> (raw)
In-Reply-To: <52C5331E.3050304@profitbricks.com>
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...
next prev parent reply other threads:[~2014-01-04 6:09 UTC|newest]
Thread overview: 4+ messages / expand[flat|nested] mbox.gz Atom feed top
2013-12-30 15:55 [BUG]NULL pointer dereference at 0000000000000008 __blkdev_put+0x17f/0x1d0 Jack Wang
2014-01-02 9:36 ` Jack Wang
2014-01-04 6:09 ` Al Viro [this message]
2014-01-06 8:45 ` Jack Wang
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