On Tue, Jan 29, 2019 at 06:35:48AM -0800, Guenter Roeck wrote: > On Mon, Jan 28, 2019 at 10:58 PM Greg KH <gregkh@xxxxxxxxxxxxxxxxxxx> wrote: > > > > On Mon, Jan 28, 2019 at 09:31:30AM -0800, Zubin Mithra wrote: > > > From: Benjamin Herrenschmidt <benh@xxxxxxxxxxxxxxxxxxx> > > > > > > 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@xxxxxxxxxxxxxxxxxxx> > > > Acked-by: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx> > > > Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx> > > > Signed-off-by: Zubin Mithra <zsm@xxxxxxxxxxxx> > > > --- > > > drivers/base/core.c | 2 ++ > > > include/linux/kobject.h | 17 +++++++++++++++++ > > > 2 files changed, 19 insertions(+) > > > > Wait, why is this needed? > > > > We have syzcaller/syzbot hits, and there is a reproducer. Is this sufficient ? Nice, where? > > 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? > > > > Good point. Sorry, that got lost. We are, after all, oly human. I'd > ask Zubin to provide backports, or do it myself, but we'll have to > resolve the issue you bring up below first. > > > 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 think we may need updated rules for stable. Many bug fixes are > backported to stable releases without having been seen in the "real > world" (whatever that means). At the same time we do see many races in > the real world, many of them not fully understood. Our policy so far > is to fix as many problems as possible if they are understood, in the > hope that it fixes at least some of those problems seen in the field. I don't have a problem with backporting this, but it went in as a "fixes a theoritical issue, and let's WARN_ON if it really ever is hit". So I would like to see where we are hitting that WARN_ON, as it sounds like you found an easy-to-reproduce way to do it. > If there is a new rule that problems have to have been observed in the > real world (ie without debugging options enabled, and without specific > reproducer) before a patch is applied to stable, can we have that as > generic rule that is not only selectively applied ? It's not a specific rule, it's just that this specific patch went through a 30+ email chain of us arguing about it, so I really want to see why this is needed, and why Ben was right and I was wrong :) I'm not trying to be extra hard here, it's just that I have a lot of history with this patch... So if you all have a reproducer, I would love to see it. Oh, and the backports to the other kernel versions as well, you don't want to have to fix this again in a year when you all upgrade to a new release. thanks, greg k-h
