Re: [PATCH 5.6 041/161] kobject: Make sure the parent does not get released before its children

[Greg Kroah-Hartman]

On Tue, Jun 16, 2020 at 07:05:44PM +0200, Rafael J. Wysocki wrote:
> On Tue, Jun 16, 2020 at 5:50 PM Greg Kroah-Hartman
> <gregkh@xxxxxxxxxxxxxxxxxxx> wrote:
> >
> > From: Heikki Krogerus <heikki.krogerus@xxxxxxxxxxxxxxx>
> >
> > [ Upstream commit 4ef12f7198023c09ad6d25b652bd8748c965c7fa ]
> >
> > In the function kobject_cleanup(), kobject_del(kobj) is
> > called before the kobj->release(). That makes it possible to
> > release the parent of the kobject before the kobject itself.
> >
> > To fix that, adding function __kboject_del() that does
> > everything that kobject_del() does except release the parent
> > reference. kobject_cleanup() then calls __kobject_del()
> > instead of kobject_del(), and separately decrements the
> > reference count of the parent kobject after kobj->release()
> > has been called.
> >
> > Reported-by: Naresh Kamboju <naresh.kamboju@xxxxxxxxxx>
> > Reported-by: kernel test robot <rong.a.chen@xxxxxxxxx>
> > Fixes: 7589238a8cf3 ("Revert "software node: Simplify software_node_release() function"")
> > Suggested-by: "Rafael J. Wysocki" <rafael@xxxxxxxxxx>
> > Signed-off-by: Heikki Krogerus <heikki.krogerus@xxxxxxxxxxxxxxx>
> > Reviewed-by: Rafael J. Wysocki <rafael.j.wysocki@xxxxxxxxx>
> > Reviewed-by: Brendan Higgins <brendanhiggins@xxxxxxxxxx>
> > Tested-by: Brendan Higgins <brendanhiggins@xxxxxxxxxx>
> > Acked-by: Randy Dunlap <rdunlap@xxxxxxxxxxxxx>
> > Link: https://lore.kernel.org/r/20200513151840.36400-1-heikki.krogerus@xxxxxxxxxxxxxxx
> > Cc: stable <stable@xxxxxxxxxxxxxxx>
> > Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
> > Signed-off-by: Sasha Levin <sashal@xxxxxxxxxx>
> > ---
> >  lib/kobject.c | 30 ++++++++++++++++++++----------
> >  1 file changed, 20 insertions(+), 10 deletions(-)
> >
> > diff --git a/lib/kobject.c b/lib/kobject.c
> > index 83198cb37d8d..2bd631460e18 100644
> > --- a/lib/kobject.c
> > +++ b/lib/kobject.c
> > @@ -599,14 +599,7 @@ int kobject_move(struct kobject *kobj, struct kobject *new_parent)
> >  }
> >  EXPORT_SYMBOL_GPL(kobject_move);
> >
> > -/**
> > - * kobject_del() - Unlink kobject from hierarchy.
> > - * @kobj: object.
> > - *
> > - * This is the function that should be called to delete an object
> > - * successfully added via kobject_add().
> > - */
> > -void kobject_del(struct kobject *kobj)
> > +static void __kobject_del(struct kobject *kobj)
> >  {
> >         struct kernfs_node *sd;
> >         const struct kobj_type *ktype;
> > @@ -625,9 +618,23 @@ void kobject_del(struct kobject *kobj)
> >
> >         kobj->state_in_sysfs = 0;
> >         kobj_kset_leave(kobj);
> > -       kobject_put(kobj->parent);
> >         kobj->parent = NULL;
> >  }
> > +
> > +/**
> > + * kobject_del() - Unlink kobject from hierarchy.
> > + * @kobj: object.
> > + *
> > + * This is the function that should be called to delete an object
> > + * successfully added via kobject_add().
> > + */
> > +void kobject_del(struct kobject *kobj)
> > +{
> > +       struct kobject *parent = kobj->parent;
> > +
> > +       __kobject_del(kobj);
> > +       kobject_put(parent);
> > +}
> >  EXPORT_SYMBOL(kobject_del);
> >
> >  /**
> > @@ -663,6 +670,7 @@ EXPORT_SYMBOL(kobject_get_unless_zero);
> >   */
> >  static void kobject_cleanup(struct kobject *kobj)
> >  {
> > +       struct kobject *parent = kobj->parent;
> >         struct kobj_type *t = get_ktype(kobj);
> >         const char *name = kobj->name;
> >
> > @@ -684,7 +692,7 @@ static void kobject_cleanup(struct kobject *kobj)
> >         if (kobj->state_in_sysfs) {
> >                 pr_debug("kobject: '%s' (%p): auto cleanup kobject_del\n",
> >                          kobject_name(kobj), kobj);
> > -               kobject_del(kobj);
> > +               __kobject_del(kobj);
> >         }
> >
> >         if (t && t->release) {
> > @@ -698,6 +706,8 @@ static void kobject_cleanup(struct kobject *kobj)
> >                 pr_debug("kobject: '%s': free name\n", name);
> >                 kfree_const(name);
> >         }
> > +
> > +       kobject_put(parent);
> 
> This is known incorrect, because that should only be done if the
> __kobject_del() above has run.
> 
> Also this commit has been reverted from the mainline.

Argh, I should have caught this, my fault, sorry, I'll go drop it.

> I have posted a fixed replacement for it with no response whatever so far:
> 
> https://lore.kernel.org/lkml/1908555.IiAGLGrh1Z@kreacher/

It's been the merge window, I couldn't do anything until Monday :)

It's in my queue, give me a chance to catch up...

thanks,

greg k-h