Re: [PATCH] thermal: add thermal_zone_remove_device_groups()

[Yasuaki Ishimatsu]

On 12/19/2016 11:21 PM, Eduardo Valentin wrote:
> Hey Yasuaki San,
>
> On Thu, Dec 15, 2016 at 04:47:08PM -0500, Yasuaki Ishimatsu wrote:
> > When offlining all cores on a CPU, the following system panic
> > occurs:
> >
> > BUG: unable to handle kernel NULL pointer dereference at (null)
> > IP: strlen+0x0/0x20
> > <snip>
> > Call Trace:
> >  ? kernfs_name_hash+0x17/0x80
> >  kernfs_find_ns+0x3f/0xd0
> >  kernfs_remove_by_name_ns+0x36/0xa0
> >  remove_files.isra.1+0x36/0x70
> >  sysfs_remove_group+0x44/0x90
> >  sysfs_remove_groups+0x2e/0x50
> >  device_remove_attrs+0x5e/0x90
> >  device_del+0x1ea/0x350
> >  device_unregister+0x1a/0x60
> >  thermal_zone_device_unregister+0x1f2/0x210
> >  pkg_thermal_cpu_offline+0x14f/0x1a0 [x86_pkg_temp_thermal]
> >  ? kzalloc.constprop.2+0x10/0x10 [x86_pkg_temp_thermal]
> >  cpuhp_invoke_callback+0x8d/0x3f0
> >  cpuhp_down_callbacks+0x42/0x80
> >  cpuhp_thread_fun+0x8b/0xf0
> >  smpboot_thread_fn+0x110/0x160
> >  kthread+0x101/0x140
> >  ? sort_range+0x30/0x30
> >  ? kthread_park+0x90/0x90
> >  ret_from_fork+0x25/0x30
> >
> > thermal_zone_create_device_group() sets attribute_groups in
> > thermal_zone_attribute_groups[] to tz->device.groups. But these
> > attributes_groups do not have name argument.
> >
> > So when offlining all cores on CPU and executing
> > thermal_zone_device_unregister(), the panic occurs in strlen()
> > called from kernfs_name_hash() because name argument is NULL.
> >
> > The patch adds thermal_zone_remove_device_groups() to free
> > tz->device.groups and set NULL pointer.
>
> Thanks for finding this and sending a fix. Overall I am OK with the
> patch, but I have some questions, as follows.
>
> > Signed-off-by: Yasuaki Ishimatsu <isimatu.yasuaki@xxxxxxxxxxxxxx>
> > CC: Zhang Rui <rui.zhang@xxxxxxxxx>
> > CC: Eduardo Valentin <edubezval@xxxxxxxxx>
> > ---
> >  drivers/thermal/thermal_core.c  | 3 ++-
> >  drivers/thermal/thermal_core.h  | 1 +
> >  drivers/thermal/thermal_sysfs.c | 6 ++++++

> First question is, are you seeing same problem with cooling devices ?

No, I'm not.
I tested physical cpu hotplug on my server and found only the issue.

> They follow same strategy, of setting the groups property, and the
> cooling device groups also have no name.
>
> >  3 files changed, 9 insertions(+), 1 deletion(-)
> >
> > diff --git a/drivers/thermal/thermal_core.c b/drivers/thermal/thermal_core.c
> > index 641faab..926e385 100644
> > --- a/drivers/thermal/thermal_core.c
> > +++ b/drivers/thermal/thermal_core.c
> > @@ -1251,6 +1251,7 @@ struct thermal_zone_device *
> >
> >  unregister:
> >  	release_idr(&thermal_tz_idr, &thermal_idr_lock, tz->id);
> > +	thermal_zone_remove_device_groups(tz);
> >  	device_unregister(&tz->device);
> >  	return ERR_PTR(result);
> >  }
> > @@ -1315,8 +1316,8 @@ void thermal_zone_device_unregister(struct thermal_zone_device *tz)
> >  	release_idr(&thermal_tz_idr, &thermal_idr_lock, tz->id);
> >  	idr_destroy(&tz->idr);
> >  	mutex_destroy(&tz->lock);
> > +	thermal_zone_remove_device_groups(tz);
> >  	device_unregister(&tz->device);
> > -	kfree(tz->device.groups);
> >  }
> >  EXPORT_SYMBOL_GPL(thermal_zone_device_unregister);
> >
> > diff --git a/drivers/thermal/thermal_core.h b/drivers/thermal/thermal_core.h
> > index 2412b37..e3a60db 100644
> > --- a/drivers/thermal/thermal_core.h
> > +++ b/drivers/thermal/thermal_core.h
> > @@ -70,6 +70,7 @@ void thermal_zone_device_unbind_exception(struct thermal_zone_device *,
> >  int thermal_build_list_of_policies(char *buf);
> >
> >  /* sysfs I/F */
> > +void thermal_zone_remove_device_groups(struct thermal_zone_device *tz);
> >  int thermal_zone_create_device_groups(struct thermal_zone_device *, int);
> >  void thermal_cooling_device_setup_sysfs(struct thermal_cooling_device *);
> >  /* used only at binding time */
> > diff --git a/drivers/thermal/thermal_sysfs.c b/drivers/thermal/thermal_sysfs.c
> > index a694de9..3dfd29b 100644
> > --- a/drivers/thermal/thermal_sysfs.c
> > +++ b/drivers/thermal/thermal_sysfs.c
> > @@ -605,6 +605,12 @@ static int create_trip_attrs(struct thermal_zone_device *tz, int mask)
> >  	return 0;
> >  }
> >
> > +void thermal_zone_remove_device_groups(struct thermal_zone_device *tz)
> > +{
> > +	kfree(tz->device.groups);
>
> OK. This part needs to be done regardless
>
> > +	tz->device.groups = NULL;
>
> This almost defeats the purpose of the effort to put all properties to
> device.groups, as we need to care about releasing them.

tz->device.groups has been released by kfree(). So if we don't set NULL
to tz->device.groups, it is possible to touch the released memory. Therefore
tz->device.groups should be set to NULL.

Thanks,
Yasuaki Ishimatsu

> I guess the problem is we cannot keep the same sysfs entries if we set a
> name on the thermal device groups. That would break userspace. And if we
> do not set, we need to care about releasing them.
>
> Almost feel like we might consider patching sysfs core.
> Or maybe find another way to setup the device groups for thermal
> devices.
>
> > +}
> > +
> >  int thermal_zone_create_device_groups(struct thermal_zone_device *tz,
> >  				      int mask)
> >  {
> > --
> > 1.8.3.1