[PATCH v6 3/5] bpf: Add bpf_list_is_edge/empty kfuncs

[Chengkaitao]

From: Kaitao Cheng <chengkaitao@xxxxxxxxxx>

Add three kfuncs for BPF linked list queries:
- bpf_list_node_is_edge(head, node, is_first): true if node is the first
  (when is_first is true) or the last (when is_first is false) in the list.
- bpf_list_empty(head): true if the list has no entries.

In previous versions, to implement the above functionality, it was
necessary to first call bpf_list_pop_front/back to retrieve the first
or last node before checking whether the passed-in node was the first
or last one. After the check, the node had to be pushed back into the
list using bpf_list_push_front/back, which was very inefficient.

Now, with the bpf_list_is_edge/empty kfuncs, we can directly
check whether a node is the first, last, or whether the list is empty,
without having to first retrieve the node.

Signed-off-by: Kaitao Cheng <chengkaitao@xxxxxxxxxx>
---
 kernel/bpf/helpers.c  | 37 +++++++++++++++++++++++++++++++++++++
 kernel/bpf/verifier.c | 11 +++++++++--
 2 files changed, 46 insertions(+), 2 deletions(-)

diff --git a/kernel/bpf/helpers.c b/kernel/bpf/helpers.c
index 740b53024283..3d22b6080185 100644
--- a/kernel/bpf/helpers.c
+++ b/kernel/bpf/helpers.c
@@ -2538,6 +2538,41 @@ __bpf_kfunc int bpf_list_add_impl(struct bpf_list_head *head,
 	return -EINVAL;
 }
 
+__bpf_kfunc bool bpf_list_node_is_edge(struct bpf_list_head *head,
+				       struct bpf_list_node *node, bool is_first)
+{
+	struct list_head *h = (struct list_head *)head, *edge;
+	struct bpf_list_node_kern *n = (struct bpf_list_node_kern *)node;
+
+	/* If list_head was 0-initialized by map, bpf_obj_init_field wasn't
+	 * called on its fields, so init here
+	 */
+	if (unlikely(!h->next))
+		INIT_LIST_HEAD(h);
+
+	if (list_empty(h))
+		return false;
+
+	if (READ_ONCE(n->owner) != head)
+		return false;
+
+	edge = is_first ? h->next : h->prev;
+	return edge == &n->list_head;
+}
+
+__bpf_kfunc bool bpf_list_empty(struct bpf_list_head *head)
+{
+	struct list_head *h = (struct list_head *)head;
+
+	/* If list_head was 0-initialized by map, bpf_obj_init_field wasn't
+	 * called on its fields, so init here
+	 */
+	if (unlikely(!h->next))
+		INIT_LIST_HEAD(h);
+
+	return list_empty(h);
+}
+
 __bpf_kfunc struct bpf_rb_node *bpf_rbtree_remove(struct bpf_rb_root *root,
 						  struct bpf_rb_node *node)
 {
@@ -4608,6 +4643,8 @@ BTF_ID_FLAGS(func, bpf_list_del, KF_ACQUIRE | KF_RET_NULL)
 BTF_ID_FLAGS(func, bpf_list_front, KF_RET_NULL)
 BTF_ID_FLAGS(func, bpf_list_back, KF_RET_NULL)
 BTF_ID_FLAGS(func, bpf_list_add_impl)
+BTF_ID_FLAGS(func, bpf_list_node_is_edge)
+BTF_ID_FLAGS(func, bpf_list_empty)
 BTF_ID_FLAGS(func, bpf_task_acquire, KF_ACQUIRE | KF_RCU | KF_RET_NULL)
 BTF_ID_FLAGS(func, bpf_task_release, KF_RELEASE)
 BTF_ID_FLAGS(func, bpf_rbtree_remove, KF_ACQUIRE | KF_RET_NULL)
diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index e458cf3b1dd1..d133d18aa0cc 100644
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -12465,6 +12465,8 @@ enum special_kfunc_type {
 	KF_bpf_list_front,
 	KF_bpf_list_back,
 	KF_bpf_list_add_impl,
+	KF_bpf_list_node_is_edge,
+	KF_bpf_list_empty,
 	KF_bpf_cast_to_kern_ctx,
 	KF_bpf_rdonly_cast,
 	KF_bpf_rcu_read_lock,
@@ -12527,6 +12529,8 @@ BTF_ID(func, bpf_list_del)
 BTF_ID(func, bpf_list_front)
 BTF_ID(func, bpf_list_back)
 BTF_ID(func, bpf_list_add_impl)
+BTF_ID(func, bpf_list_node_is_edge)
+BTF_ID(func, bpf_list_empty)
 BTF_ID(func, bpf_cast_to_kern_ctx)
 BTF_ID(func, bpf_rdonly_cast)
 BTF_ID(func, bpf_rcu_read_lock)
@@ -13003,7 +13007,9 @@ static bool is_bpf_list_api_kfunc(u32 btf_id)
 	       btf_id == special_kfunc_list[KF_bpf_list_del] ||
 	       btf_id == special_kfunc_list[KF_bpf_list_front] ||
 	       btf_id == special_kfunc_list[KF_bpf_list_back] ||
-	       btf_id == special_kfunc_list[KF_bpf_list_add_impl];
+	       btf_id == special_kfunc_list[KF_bpf_list_add_impl] ||
+	       btf_id == special_kfunc_list[KF_bpf_list_node_is_edge] ||
+	       btf_id == special_kfunc_list[KF_bpf_list_empty];
 }
 
 static bool is_bpf_rbtree_api_kfunc(u32 btf_id)
@@ -13126,7 +13132,8 @@ static bool check_kfunc_is_graph_node_api(struct bpf_verifier_env *env,
 		ret = (kfunc_btf_id == special_kfunc_list[KF_bpf_list_push_front_impl] ||
 		       kfunc_btf_id == special_kfunc_list[KF_bpf_list_push_back_impl] ||
 		       kfunc_btf_id == special_kfunc_list[KF_bpf_list_del] ||
-		       kfunc_btf_id == special_kfunc_list[KF_bpf_list_add_impl]);
+		       kfunc_btf_id == special_kfunc_list[KF_bpf_list_add_impl] ||
+		       kfunc_btf_id == special_kfunc_list[KF_bpf_list_node_is_edge]);
 		break;
 	case BPF_RB_NODE:
 		ret = (kfunc_btf_id == special_kfunc_list[KF_bpf_rbtree_remove] ||
-- 
2.50.1 (Apple Git-155)