[PATCH bpf-next v4 2/7] bpf: enable detaching links of struct_ops objects.

Kui-Feng Lee
2024-05-21T22:53:04Z

Implement the detach callback in bpf_link_ops for struct_ops so that user
programs can detach a struct_ops link. The subsystems that struct_ops
objects are registered to can also use this callback to detach the links
being passed to them.

Signed-off-by: Kui-Feng Lee <thinker.li@gmail.com>
---
kernel/bpf/bpf_struct_ops.c | 63 +++++++++++++++++++++++++++++++++----
1 file changed, 57 insertions(+), 6 deletions(-)

diff --git a/kernel/bpf/bpf_struct_ops.c b/kernel/bpf/bpf_struct_ops.c
index 1542dded7489..fb6e8a3190ef 100644
--- a/kernel/bpf/bpf_struct_ops.c
+++ b/kernel/bpf/bpf_struct_ops.c
@@ -1057,9 +1057,6 @@ static void bpf_struct_ops_map_link_dealloc(struct bpf_link *link)
st_map = (struct bpf_struct_ops_map *)
rcu_dereference_protected(st_link->map, true);
if (st_map) {
- /* st_link->map can be NULL if
- * bpf_struct_ops_link_create() fails to register.
- */
st_map->st_ops_desc->st_ops->unreg(&st_map->kvalue.data, link);
bpf_map_put(&st_map->map);
}
@@ -1075,7 +1072,8 @@ static void bpf_struct_ops_map_link_show_fdinfo(const struct bpf_link *link,
st_link = container_of(link, struct bpf_struct_ops_link, link);
rcu_read_lock();
map = rcu_dereference(st_link->map);
- seq_printf(seq, "map_id:\t%d\n", map->id);
+ if (map)
+ seq_printf(seq, "map_id:\t%d\n", map->id);
rcu_read_unlock();
}

@@ -1088,7 +1086,8 @@ static int bpf_struct_ops_map_link_fill_link_info(const struct bpf_link *link,
st_link = container_of(link, struct bpf_struct_ops_link, link);
rcu_read_lock();
map = rcu_dereference(st_link->map);
- info->struct_ops.map_id = map->id;
+ if (map)
+ info->struct_ops.map_id = map->id;
rcu_read_unlock();
return 0;
}
@@ -1113,6 +1112,10 @@ static int bpf_struct_ops_map_link_update(struct bpf_link *link, struct bpf_map
mutex_lock(&update_mutex);

old_map = rcu_dereference_protected(st_link->map, lockdep_is_held(&update_mutex));
+ if (!old_map) {
+ err = -EINVAL;
+ goto err_out;
+ }
if (expected_old_map && old_map != expected_old_map) {
err = -EPERM;
goto err_out;
@@ -1139,8 +1142,37 @@ static int bpf_struct_ops_map_link_update(struct bpf_link *link, struct bpf_map
return err;
}

+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);
+
+ map = rcu_dereference_protected(st_link->map, lockdep_is_held(&update_mutex));
+ if (!map) {
+ mutex_unlock(&update_mutex);
+ return -EINVAL;
+ }
+ 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 +1208,32 @@ 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.
+ */
+ RCU_INIT_POINTER(link->map, map);
+
+ /* Once reg() is called, the object and link is already available
+ * to the subsystem, and it can call
+ * bpf_struct_ops_map_link_detach() to unreg() it. However, it is
+ * sfae not holding update_mutex here.
+ *
+ * In the case of failure in reg(), the subsystem has no reason to
+ * call bpf_struct_ops_map_link_detach() since the object is not
+ * accepted by it. In the case of success, the subsystem may call
+ * bpf_struct_ops_map_link_detach() to unreg() it, but we don't
+ * change the content of the link anymore except changing link->id
+ * in bpf_link_settle(). So, it is safe to not hold update_mutex
+ * here.
+ */
err = st_map->st_ops_desc->st_ops->reg(st_map->kvalue.data, &link->link);
if (err) {
+ RCU_INIT_POINTER(link->map, NULL);
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);

return bpf_link_settle(&link_primer);

--
2.34.1