Re: [PATCH] kobject: Avoid premature parent object freeing in kobject_cleanup()

[Qu Wenruo]

On 2020/6/5 äå1:46, Rafael J. Wysocki wrote:
> From: Heikki Krogerus <heikki.krogerus@xxxxxxxxxxxxxxx>
> 
> If kobject_del() is invoked by kobject_cleanup() to delete the
> target kobject, it may cause its parent kobject to be freed
> before invoking the target kobject's ->release() method, which
> effectively means freeing the parent before dealing with the
> child entirely.
> 
> That is confusing at best and it may also lead to functional
> issues if the callers of kobject_cleanup() are not careful enough
> about the order in which these calls are made, so avoid the
> problem by making kobject_cleanup() drop the last reference to
> the target kobject's parent at the end, after invoking the target
> kobject's ->release() method.
> 
> [ rjw: Rewrite the subject and changelog, make kobject_cleanup()
>   drop the parent reference only when __kobject_del() 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>
> Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@xxxxxxxxx>
> ---
> 
> Hi Greg,
> 
> This is a replacement for commit 4ef12f719802 ("kobject: Make sure the parent
> does not get released before its children"), that you reverted, because it
> broke things and the reason why was that it was incorrect.
> 
> Namely, it called kobject_put() on the target kobject's parent in
> kobject_cleanup() unconditionally, but it should only call it after
> invoking __kobject_del() on the target kobject.
> 
> That problem is fixed in this patch and a functionally equivalent patch has
> been tested by Guenter without issues.
> 
> The underlying issue addressed by the reverted commit is still there and
> it may show up again even though the test that triggered it originally was
> fixed in the meantime.  IMO it is worth fixing even though it may not be
> readily visible in the current kernel, so please consider this one for
> applying.
> 
> Cheers!
> 
> ---
>  lib/kobject.c |   33 +++++++++++++++++++++++----------
>  1 file changed, 23 insertions(+), 10 deletions(-)
> 
> Index: linux-pm/lib/kobject.c
> ===================================================================
> --- linux-pm.orig/lib/kobject.c
> +++ linux-pm/lib/kobject.c
> @@ -599,14 +599,7 @@ out:
>  }
>  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);

Could you please add an extra check on kobj before accessing kobj->parent?

This patch in fact removes the ability to call kobject_del() on NULL
pointer while not cause anything wrong.

I know this is not a big deal, but such behavior change has already
caused some problem for the incoming btrfs code.
(Now I feels guilty just by looking into the old
kobject_del()/kobject_put() and utilize that feature in btrfs)

Since the old kobject_del() accepts NULL pointer intentionally, it would
be much better to keep such behavior.

Or at least mention we require a valid kobject pointer.

Thanks,
Qu

> +}
>  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 +692,10 @@ static void kobject_cleanup(struct kobje
>  	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);
> +	} else {
> +		/* avoid dropping the parent reference unnecessarily */
> +		parent = NULL;
>  	}
>  
>  	if (t && t->release) {
> @@ -698,6 +709,8 @@ static void kobject_cleanup(struct kobje
>  		pr_debug("kobject: '%s': free name\n", name);
>  		kfree_const(name);
>  	}
> +
> +	kobject_put(parent);
>  }
>  
>  #ifdef CONFIG_DEBUG_KOBJECT_RELEASE
> 
> 
> 
>

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