[PATCH bpf-next v10 6/8] bpf: add bpf_list_is_first/last/empty kfuncs

Kaitao cheng
2026-04-27T16:59:48Z

From: Kaitao Cheng <chengkaitao@kylinos.cn>

Add three kfuncs for BPF linked list queries:
- bpf_list_is_first(head, node): true if node is the first in the list.
- bpf_list_is_last(head, node): true if node is the last in the list.
- bpf_list_empty(head): true if the list has no entries.

Currently, without these kfuncs, to implement the above functionality
it is 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_first/last/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@kylinos.cn>
Reviewed-by: Emil Tsalapatis <emil@etsalapatis.com>
---
kernel/bpf/helpers.c | 38 ++++++++++++++++++++++++++++++++++++++
kernel/bpf/verifier.c | 15 +++++++++++++--
2 files changed, 51 insertions(+), 2 deletions(-)

diff --git a/kernel/bpf/helpers.c b/kernel/bpf/helpers.c
index 2b8e8d4284a5..dfd465badd9d 100644
--- a/kernel/bpf/helpers.c
+++ b/kernel/bpf/helpers.c
@@ -2648,6 +2648,41 @@ __bpf_kfunc struct bpf_list_node *bpf_list_back(struct bpf_list_head *head)
return (struct bpf_list_node *)h->prev;
}

+__bpf_kfunc bool bpf_list_is_first(struct bpf_list_head *head, struct bpf_list_node *node)
+{
+ struct list_head *h = (struct list_head *)head;
+ struct bpf_list_node_kern *kn = (struct bpf_list_node_kern *)node;
+
+ if (READ_ONCE(kn->owner) != head)
+ return false;
+
+ return list_is_first(&kn->list_head, h);
+}
+
+__bpf_kfunc bool bpf_list_is_last(struct bpf_list_head *head, struct bpf_list_node *node)
+{
+ struct list_head *h = (struct list_head *)head;
+ struct bpf_list_node_kern *kn = (struct bpf_list_node_kern *)node;
+
+ if (READ_ONCE(kn->owner) != head)
+ return false;
+
+ return list_is_last(&kn->list_head, h);
+}
+
+__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)
{
@@ -4764,6 +4799,9 @@ BTF_ID_FLAGS(func, bpf_list_pop_back, KF_ACQUIRE | KF_RET_NULL)
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_is_first)
+BTF_ID_FLAGS(func, bpf_list_is_last)
+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 50f8732aa065..ca33f35bc3eb 100644
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -10748,6 +10748,9 @@ enum special_kfunc_type {
KF_bpf_list_del,
KF_bpf_list_front,
KF_bpf_list_back,
+ KF_bpf_list_is_first,
+ KF_bpf_list_is_last,
+ KF_bpf_list_empty,
KF_bpf_cast_to_kern_ctx,
KF_bpf_rdonly_cast,
KF_bpf_rcu_read_lock,
@@ -10818,6 +10821,9 @@ BTF_ID(func, bpf_list_pop_back)
BTF_ID(func, bpf_list_del)
BTF_ID(func, bpf_list_front)
BTF_ID(func, bpf_list_back)
+BTF_ID(func, bpf_list_is_first)
+BTF_ID(func, bpf_list_is_last)
+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)
@@ -11341,7 +11347,10 @@ static bool is_bpf_list_api_kfunc(u32 btf_id)
btf_id == special_kfunc_list[KF_bpf_list_pop_back] ||
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_back] ||
+ btf_id == special_kfunc_list[KF_bpf_list_is_first] ||
+ btf_id == special_kfunc_list[KF_bpf_list_is_last] ||
+ btf_id == special_kfunc_list[KF_bpf_list_empty];
}

static bool is_bpf_rbtree_api_kfunc(u32 btf_id)
@@ -11463,7 +11472,9 @@ static bool check_kfunc_is_graph_node_api(struct bpf_verifier_env *env,
switch (node_field_type) {
case BPF_LIST_NODE:
ret = is_bpf_list_push_kfunc(kfunc_btf_id) ||
- kfunc_btf_id == special_kfunc_list[KF_bpf_list_del];
+ kfunc_btf_id == special_kfunc_list[KF_bpf_list_del] ||
+ kfunc_btf_id == special_kfunc_list[KF_bpf_list_is_first] ||
+ kfunc_btf_id == special_kfunc_list[KF_bpf_list_is_last];
break;
case BPF_RB_NODE:
ret = (is_bpf_rbtree_add_kfunc(kfunc_btf_id) ||
--
2.50.1 (Apple Git-155)