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

[Dan Williams]

On Fri, Feb 10, 2017 at 11:19 AM, Logan Gunthorpe <logang@xxxxxxxxxxxx> 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@xxxxxxxxxxxx>

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?