Re: [PATCH bpf-next v3 2/7] bpf: enable detaching links of struct_ops objects.

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On 5/21/24 12:30 AM, Kui-Feng Lee wrote:


On 5/20/24 18:22, Martin KaFai Lau wrote:
On 5/9/24 5:29 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);
+
+    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 +1208,19 @@ 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.
+     */

It is not obvious in the patch how this (immediate detach by subsystem after reg) may work without race, so I think it is easier to ask.

[ I put back the err_out context at the end ]

+    RCU_INIT_POINTER(link->map, map);
+
      err = st_map->st_ops_desc->st_ops->reg(st_map->kvalue.data, &link->link);
      if (err) {
+        RCU_INIT_POINTER(link->map, NULL);

In the bpf_struct_ops_map_link_detach() above, the update to link->map is protected by the update_mutex. Could you explain how the link->map update to NULL is safe here without holding the update_mutex?

If err is not zero, it means the subsystem rejects the pair of the
object and the link passing in. So, it has no reasonable to call
bpf_struct_ops_map_link_detach() for this link.

Does it make sense to you?


          bpf_link_cleanup(&link_primer);
+        /* The link has been free by bpf_link_cleanup() */
          link = NULL;
          goto err_out;
      }

At this point, we don't change the content of the link anymore except
changing link->fd in bpf_link_settle(). So, it should be safe to call
bpf_struct_ops_map_link_detach() from the subsystem.

Should I explain it in a comment if you think it makes sense to you?

It makes sense and will be useful to get this details in the comment.


-    RCU_INIT_POINTER(link->map, map);
      return bpf_link_settle(&link_primer);


err_out:
     bpf_map_put(map);
     kfree(link);
     return err;
}







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