linux-kernel - Re: [PATCH v2 2/2] drivers: core: Remove glue dirs from sysfs earlier

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Date:   Tue, 10 Jul 2018 16:55:19 +0200
From:   Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:     Benjamin Herrenschmidt <benh@...nel.crashing.org>
Cc:     Linus Torvalds <torvalds@...ux-foundation.org>,
        Tejun Heo <tj@...nel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2 2/2] drivers: core: Remove glue dirs from sysfs earlier

On Tue, Jul 10, 2018 at 10:29:10AM +1000, Benjamin Herrenschmidt wrote:
> 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.
> 
> Signed-off-by: Benjamin Herrenschmidt <benh@...nel.crashing.org>
> ---
>  drivers/base/core.c     |  2 ++
>  include/linux/kobject.h | 17 +++++++++++++++++
>  2 files changed, 19 insertions(+)
> 
> diff --git a/drivers/base/core.c b/drivers/base/core.c
> index e9eff2099896..93c0f8d1a447 100644
> --- a/drivers/base/core.c
> +++ b/drivers/base/core.c
> @@ -1572,6 +1572,8 @@ static void cleanup_glue_dir(struct device *dev, struct kobject *glue_dir)
>  		return;
>  
>  	mutex_lock(&gdp_mutex);
> +	if (!kobject_has_children(glue_dir))
> +		kobject_del(glue_dir);
>  	kobject_put(glue_dir);
>  	mutex_unlock(&gdp_mutex);
>  }
> diff --git a/include/linux/kobject.h b/include/linux/kobject.h
> index 7f6f93c3df9c..270b40515e79 100644
> --- a/include/linux/kobject.h
> +++ b/include/linux/kobject.h
> @@ -116,6 +116,23 @@ extern void kobject_put(struct kobject *kobj);
>  extern const void *kobject_namespace(struct kobject *kobj);
>  extern char *kobject_get_path(struct kobject *kobj, gfp_t flag);
>  
> +/**
> + * kobject_has_children - Returns whether a kobject has children.
> + * @kobj: the object to test
> + *
> + * This will return whether a kobject has other kobjects as children.
> + *
> + * It does NOT account for the presence of attribute files, only sub
> + * directories. It also assumes there is no concurrent addition or
> + * removal of such children, and thus relies on external locking.
> + */
> +static inline bool kobject_has_children(struct kobject *kobj)
> +{
> +	WARN_ON_ONCE(kref_read(&kobj->kref) == 0);

Why warn on?  Who is going to hit this and how are you going to fix up
the syzbot reports?  :)

Anyway, this looks good, I can just take this and not the 1/2 patch now,
right?  I really didn't like that patch.

thanks,

greg k-h