Messages in this thread | | | Date | Thu, 14 Apr 2022 16:50:55 +0530 | From | Mukesh Ojha <> | Subject | Re: Possible race in dev_coredumpm()-del_timer() path |
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On Thu, Apr 14, 2022 at 12:38:13PM +0200, Thomas Gleixner wrote: > On Wed, Apr 13 2022 at 12:58, Greg KH wrote: > > On Wed, Apr 13, 2022 at 03:46:39PM +0530, Mukesh Ojha wrote: > >> p1 p2(X) > >> > >> dev_coredump() uevent sent to userspace > >> device_add() =========================> userspace process X reads the uevents > >> writes to devcd fd which > >> results into writes to > >> > >> devcd_data_write() > >> mod_delayed_work() > >> try_to_grab_pending() > >> del_timer() > >> debug_assert_init() > >> INIT_DELAYED_WORK > >> schedule_delayed_work > >> debug_object_fixup() > > > > Why do you have object debugging enabled? That's going to take a LONG > > time, and will find bugs in your code. Perhaps like this one? > > It's not finding bugs in his code. It finds bug in the upstream > dev_coredump code. > > > And if you turn object debugging off, what happens? > > The debugobject splat goes away, but the problem persists. > > device_add() -> uevent > > Preemption or concurrency: > > devcd_data_write() > mod_delayed_work(..., w, 0); <- Uninitialized. > > The dev_coredump code exposes the device before it is fully initialized > and a write ending up in devcd_data_write() touches uninitialized work. > > It does not help to move the initialization before device_add() as that > creates another problem: > > INIT_DELAYED_WORK(w) > ... > device_add() -> uevent > > Preemption or concurrency: > > devcd_data_write() > mod_delayed_work(..., w, 0); <- Schedules work immediately > > work_queue_runs() > devcd_del(w) > device_del() > put_device() <- Drops the last reference > > initialization continues... > > So, yes this needs serialization of some sort.
Hi Thomas,
Thanks for understanding the problem. Can the patch mentioned at below link helps with the first problem ?
https://lore.kernel.org/lkml/57a04278-0a60-cc7d-7ce8-a75c2befd568@quicinc.com/
> > Same problem vs. disabled_store().
you mean, while userspace is reading the data and suddenly disable_store() done from sysfs.
-Mukesh > > Thanks, > > tglx
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