Re: [PATCH v4.4.y] drivers: core: Remove glue dirs from sysfs earlier

[Greg KH <gregkh@linuxfoundation.org>]

On Mon, Jan 28, 2019 at 09:31:30AM -0800, Zubin Mithra wrote:
> From: Benjamin Herrenschmidt <benh@kernel.crashing.org>
> 
> commit 726e41097920a73e4c7c33385dcc0debb1281e18 upstream
> 
> For devices with a class, we create a "glue" directory between
> the parent device and the new device with the class name.
> 
> This directory is never "explicitely" removed when empty however,
> this is left to the implicit sysfs removal done by kobject_release()
> when the object loses its last reference via kobject_put().
> 
> This is problematic because as long as it's not been removed from
> sysfs, it is still present in the class kset and in sysfs directory
> structure.
> 
> The presence in the class kset exposes a use after free bug fixed
> by the previous patch, but the presence in sysfs means that until
> the kobject is released, which can take a while (especially with
> kobject debugging), any attempt at re-creating such as binding a
> new device for that class/parent pair, will result in a sysfs
> duplicate file name error.
> 
> This fixes it by instead doing an explicit kobject_del() when
> the glue dir is empty, by keeping track of the number of
> child devices of the gluedir.
> 
> This is made easy by the fact that all glue dir operations are
> done with a global mutex, and there's already a function
> (cleanup_glue_dir) called in all the right places taking that
> mutex that can be enhanced for this. It appears that this was
> in fact the intent of the function, but the implementation was
> wrong.
> 
> Backport Note: kref_read() is not present in 4.4. Hence,
> use atomic_read(&kref.refcount) instead of kref_read(&kref).
> 
> Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
> Acked-by: Linus Torvalds <torvalds@linux-foundation.org>
> Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
> Signed-off-by: Zubin Mithra <zsm@chromium.org>
> ---
>  drivers/base/core.c     |  2 ++
>  include/linux/kobject.h | 17 +++++++++++++++++
>  2 files changed, 19 insertions(+)

Wait, why is this needed?

And why only for 4.4?  What about 4.9 and 4.14?  Do you want to upgrade
and suddenly hit the same "bug" that you fixed before?

There was a reason that I did not backport this to the stable tree when
it was submitted, and that was because this was an odd race to ever hit.
Are you hitting this in the real world without kobject deferred
release enabled?  And if so, are you hitting the WARN_ON that is added
here?

I would like to hold off on this until some more information is
provided, and at the least, if it is accepted, we add it to all of the
stable trees, not just 4.4.y.  So I'm going to go drop it from the 4.4.y
queue for now.

thanks,

greg k-h