On Thu, May 02, 2019 at 07:56:16AM +1000, Tobin C. Harding wrote: > On Sat, Apr 27, 2019 at 09:28:09PM +0200, Greg Kroah-Hartman wrote: > > On Sat, Apr 27, 2019 at 06:13:30PM +1000, Tobin C. Harding wrote: > > > (Note at bottom on reasons for 'To' list 'Cc' list) > > > > > > Hi, > > > > > > kobject_init_and_add() seems to be routinely misused. A failed call to > > > this > > > function requires a call to kobject_put() otherwise we leak memory. > > > > > > Examples memleaks can be seen in: > > > > > > mm/slub.c > > > fs/btrfs/sysfs.c > > > fs/xfs/xfs_sysfs.h: xfs_sysfs_init() > > > > > > Question: Do we fix the misuse or fix the API? > > > > Fix the misuse. > > > > > $ git grep kobject_init_and_add | wc -l > > > 117 > > > > > > Either way, we will have to go through all 117 call sites and check them. > > > > Yes. Same for other functions like device_add(), that is the "pattern" > > those users must follow. > > > > > I > > > don't mind fixing them all but I don't want to do it twice because I > > > chose the > > > wrong option. Reaching out to those more experienced for a suggestion > > > please. > > > > > > Fix the API > > > ----------- > > > > > > Typically init functions do not require cleanup if they fail, this > > > argument > > > leads to this patch > > > > > > diff --git a/lib/kobject.c b/lib/kobject.c > > > index aa89edcd2b63..62328054bbd0 100644 > > > --- a/lib/kobject.c > > > +++ b/lib/kobject.c > > > @@ -453,6 +453,9 @@ int kobject_init_and_add(struct kobject *kobj, struct > > > kobj_type *ktype, > > > retval = kobject_add_varg(kobj, parent, fmt, args); > > > va_end(args); > > > > > > + if (retval) > > > + kobject_put(kobj); > > > + > > > return retval; > > > } > > > EXPORT_SYMBOL_GPL(kobject_init_and_add); > > > > I would _love_ to do this, but realize what a kobject really is. > > > > It's just a "base object" that is embedded inside of some other object. > > The kobject core has no idea what is going on outside of itself. If the > > kobject_init_and_add() function fails, it can NOT drop the last > > reference on itself, as that would cause the memory owned by the _WHOLE_ > > structure the kobject is embedded in, to be freed. > > > > And the kobject core can not "know" that something else needed to be > > done _before_ that memory could be freed. What if the larger structure > > needs to have some other destructor called on it first? What if > > some other api initialization needs to be torn down. > > > > As an example, consider this code: > > > > struct foo { > > struct kobject kobj; > > struct baz *baz; > > }; > > > > void foo_release(struct kobject *kobj) > > { > > struct foo *foo = container_of(kobj, struct foo, kobj); > > kfree(foo); > > } > > > > struct kobj_type foo_ktype = { > > .release = foo_release, > > }; > > > > struct foo *foo_create(struct foo *parent, char *name) > > { > > struct *foo; > > > > foo = kzalloc(sizeof(*foo), GFP_KERNEL); > > if (!foo) > > return NULL; > > > > foo->baz = baz_create(name); > > if (!foo->baz) > > return NULL; > > > > ret = kobject_init_and_add(&foo->kobj, foo_ktype, &parent->kobj, > > "foo-%s", name); > > if (ret) { > > baz_destroy(foo->baz); > > kobject_put(&foo->kobj); > > return NULL; > > } > > > > return foo; > > } > > > > void foo_destroy(struct foo *foo) > > { > > baz_destroy(foo->baz); > > kobject_del(&foo->kobj); > kojbect_put(&foo->kobj); > > } > > Does this need this extra call to kobject_put()? Then foo_create() > leaves foo with a refcount of 1 and foo_destroy drops that refcount.
Oops, no, I messed this up, it should _only_ be a call to kobject_put(), kobject_del() is not needed here. kobject_del() is for people who "really want to control the lifetime" of a kobject. All it does is remove the kobject from sysfs, and drop the parent reference of the kobject, allowing the kobject to be "free" on it's own. Later a kobject_put() call must be called on it to really clean it up. If you just call kobject_put(), and this is the last reference, kobject_del() will be correctly called for you by the kobject code, as it "knows" this is time to clean up the sysfs entities. A "normal" user should never have to call kobject_del(). thanks, greg k-h