On Fri, Feb 10, 2017 at 12:17 PM, Greg Kroah-Hartman <[email protected]> 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 <[email protected]> >> 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 <[email protected]> >> >> 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.
