Re: [PATCH bpf-next v6 2/8] bpf: enable detaching links of struct_ops objects.

[Martin KaFai Lau <martin.lau@xxxxxxxxx>]

On 5/29/24 8:04 AM, Kuifeng Lee wrote:
> On Tue, May 28, 2024 at 11:17 PM Martin KaFai Lau <martin.lau@xxxxxxxxx> wrote:
> > On 5/24/24 3:30 PM, Kui-Feng Lee wrote:
> > > +static int bpf_struct_ops_map_link_detach(struct bpf_link *link)
> > > +{
> > > +     struct bpf_struct_ops_link *st_link = container_of(link, struct bpf_struct_ops_link, link);
> > > +     struct bpf_struct_ops_map *st_map;
> > > +     struct bpf_map *map;
> > > +
> > > +     mutex_lock(&update_mutex);
> >
> > update_mutex is needed to detach.
> >
> > > +
> > > +     map = rcu_dereference_protected(st_link->map, lockdep_is_held(&update_mutex));
> > > +     if (!map) {
> > > +             mutex_unlock(&update_mutex);
> > > +             return 0;
> > > +     }
> > > +     st_map = container_of(map, struct bpf_struct_ops_map, map);
> > > +
> > > +     st_map->st_ops_desc->st_ops->unreg(&st_map->kvalue.data, link);
> > > +
> > > +     rcu_assign_pointer(st_link->map, NULL);
> > > +     /* Pair with bpf_map_get() in bpf_struct_ops_link_create() or
> > > +      * bpf_map_inc() in bpf_struct_ops_map_link_update().
> > > +      */
> > > +     bpf_map_put(&st_map->map);
> > > +
> > > +     mutex_unlock(&update_mutex);
> > > +
> > > +     return 0;
> > > +}
> > > +
> > >    static const struct bpf_link_ops bpf_struct_ops_map_lops = {
> > >        .dealloc = bpf_struct_ops_map_link_dealloc,
> > > +     .detach = bpf_struct_ops_map_link_detach,
> > >        .show_fdinfo = bpf_struct_ops_map_link_show_fdinfo,
> > >        .fill_link_info = bpf_struct_ops_map_link_fill_link_info,
> > >        .update_map = bpf_struct_ops_map_link_update,
> > > @@ -1176,13 +1208,22 @@ int bpf_struct_ops_link_create(union bpf_attr *attr)
> > >        if (err)
> > >                goto err_out;
> > >
> > > +     /* Init link->map before calling reg() in case being detached
> > > +      * immediately.
> > > +      */
> >
> > With update_mutex held in link_create here, the parallel detach can still happen
> > before the link is fully initialized (the link->map pointer here in particular)?
> >
> > > +     RCU_INIT_POINTER(link->map, map);
> > > +
> > > +     mutex_lock(&update_mutex);
> > >        err = st_map->st_ops_desc->st_ops->reg(st_map->kvalue.data, &link->link);
> > >        if (err) {
> > > +             RCU_INIT_POINTER(link->map, NULL);
> >
> > I was hoping by holding the the update_mutex, it can avoid this link->map init
> > dance, like RCU_INIT_POINTER(link->map, map) above and then resetting here on
> > the error case.
> >
> > > +             mutex_unlock(&update_mutex);
> > >                bpf_link_cleanup(&link_primer);
> > > +             /* The link has been free by bpf_link_cleanup() */
> > >                link = NULL;
> > >                goto err_out;
> > >        }
> > > -     RCU_INIT_POINTER(link->map, map);
> >
> > If only init link->map once here like the existing code (and the init is
> > protected by the update_mutex), the subsystem should not be able to detach until
> > the link->map is fully initialized.
> >
> > or I am missing something obvious. Can you explain why this link->map init dance
> > is still needed?
>
> Ok, I get what you mean.
>
> I will move RCU_INIT_POINTER() back to its original place, and move the check
> on the value of "err" to the place after mutext_unlock().
The RCU_INIT_POINTER(link->map, map) needs to be done with update_mutex held and
it should be init after the err check, so the err check needs to be inside
update_mutex lock also.

Something like this (untested):

	mutex_lock(&update_mutex);

	err = st_map->st_ops_desc->st_ops->reg(st_map->kvalue.data, &link->link);
	if (err) {
		mutex_unlock(&update_mutex);
		bpf_link_cleanup(&link_primer);
		link = NULL;
		goto err_out;
	}
	RCU_INIT_POINTER(link->map, map);

	mutex_unlock(&update_mutex);


> > > +     mutex_unlock(&update_mutex);