4.4-stable review patch. If anyone has any objections, please let me know.
------------------
From: Benjamin Herrenschmidt benh@kernel.crashing.org
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.
Signed-off-by: Benjamin Herrenschmidt benh@kernel.crashing.org Acked-by: Linus Torvalds torvalds@linux-foundation.org Signed-off-by: Zubin Mithra zsm@chromium.org Cc: Guenter Roeck groeck@google.com Signed-off-by: Greg Kroah-Hartman gregkh@linuxfoundation.org
--- drivers/base/core.c | 2 ++ include/linux/kobject.h | 17 +++++++++++++++++ 2 files changed, 19 insertions(+)
--- a/drivers/base/core.c +++ b/drivers/base/core.c @@ -862,6 +862,8 @@ static void cleanup_glue_dir(struct devi return;
mutex_lock(&gdp_mutex); + if (!kobject_has_children(glue_dir)) + kobject_del(glue_dir); kobject_put(glue_dir); mutex_unlock(&gdp_mutex); } --- a/include/linux/kobject.h +++ b/include/linux/kobject.h @@ -113,6 +113,23 @@ extern void kobject_put(struct kobject * 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(atomic_read(&kobj->kref.refcount) == 0); + + return kobj->sd && kobj->sd->dir.subdirs; +} + struct kobj_type { void (*release)(struct kobject *kobj); const struct sysfs_ops *sysfs_ops;