linux-kernel - Re: [PATCH] device-dax: don't set kobj parent during cdev init

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Message-ID: <CAPcyv4gz0C+rhR=Xzk-G58yM=qpEBEuYbkushn5dByFD38KQbg@mail.gmail.com>
Date:   Fri, 10 Feb 2017 14:25:35 -0800
From:   Dan Williams <dan.j.williams@...el.com>
To:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Cc:     Logan Gunthorpe <logang@...tatee.com>,
        Johannes Thumshirn <jthumshirn@...e.de>,
        "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
        Sajjan Vikas C <vikas.cha.sajjan@....com>,
        Arnd Bergmann <arnd@...db.de>, Jan Kara <jack@...e.cz>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-nvdimm@...ts.01.org" <linux-nvdimm@...ts.01.org>
Subject: Re: [PATCH] device-dax: don't set kobj parent during cdev init

On Fri, Feb 10, 2017 at 12:17 PM, Greg Kroah-Hartman
<gregkh@...uxfoundation.org> wrote:
> On Fri, Feb 10, 2017 at 11:41:20AM -0800, Dan Williams wrote:
>> On Fri, Feb 10, 2017 at 11:19 AM, Logan Gunthorpe <logang@...tatee.com> wrote:
>> > I copied this code and per feedback from Greg Kroah-Hartman [1] the
>> > cdev's kobject's parent should not be set to the related device.
>> > This should have minor consequences but isn't doing what anyone
>> > expects it to.
>> >
>> > This patch then fixes device-dax so it doesn't make the same mistake.
>> >
>> > [1] https://lkml.org/lkml/2017/2/10/370
>> >
>> > Signed-off-by: Logan Gunthorpe <logang@...tatee.com>
>>
>> Thanks for following up with this fix, but this causes a
>> use-after-free regression:
>>
>>  general protection fault: 0000 [#1] SMP DEBUG_PAGEALLOC
>>  [..]
>>  Call Trace:
>>   vsnprintf+0x2d7/0x500
>>   snprintf+0x49/0x60
>>   dev_vprintk_emit+0x68/0x230
>>   ? debug_lockdep_rcu_enabled+0x1d/0x20
>>   ? trace_hardirqs_off+0xd/0x10
>>   ? cmpxchg_double_slab.isra.70+0x15a/0x1c0
>>   ? __slab_free+0x134/0x290
>>   dev_printk_emit+0x4e/0x70
>>   __dynamic_dev_dbg+0xc8/0x110
>>   ? __lock_acquire+0x33d/0x1290
>>   dax_dev_huge_fault+0xee/0x570 [dax]
>>   __handle_mm_fault+0x5aa/0x10a0
>>   handle_mm_fault+0x154/0x350
>>   ? handle_mm_fault+0x3c/0x350
>>   __do_page_fault+0x26b/0x4c0
>>   trace_do_page_fault+0x58/0x270
>>   do_async_page_fault+0x1a/0xa0
>>   async_page_fault+0x28/0x30
>>
>> I added this reference explicitly so the parent struct device has the
>> correct lifetime after this feedback from Al.
>>
>>    https://lists.01.org/pipermail/linux-nvdimm/2016-August/006563.html
>>
>> ...so I'm wondering what the actual problem is with setting cdev->parent?
>
> It shouldn't do anything at all.  The kobject in a cdev isn't a "normal"
> kobject, it doesn't show up in sysfs, or anywhere else.  It's used for
> an internal representation to the cdev code (a kmap) to look up the
> object to call when userspace opens the device node in a quick manner.
>
> Now changing from initialize/add to just register, does do different
> things, perhaps that is the issue here.  Just try removing the
> cdev->kobject parent stuff and see if that causes a problem or not.
>

 That doesn't help.  I rely on the "kobject_get(p->kobj.parent);" in
cdev_add() to pin my device and cdev_default_release() to free it.