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

[Rafael J. Wysocki]

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.

I have posted a fixed replacement for it with no response whatever so far:

https://lore.kernel.org/lkml/1908555.IiAGLGrh1Z@kreacher/

>  }
>
>  #ifdef CONFIG_DEBUG_KOBJECT_RELEASE
> --