bpf: add bpf_list_is_first/last/empty kfuncs

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>
Link: https://lore.kernel.org/r/20260521032306.97118-8-kaitao.cheng@linux.dev
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

diff --git a/kernel/bpf/helpers.c b/kernel/bpf/helpers.c
index 89579165ef4dc7de604318b2b3404b683264b497..b6c3d02d5593c95eb689ec6e172267cb363ef202 100644 (file)
--- a/kernel/bpf/helpers.c
+++ b/kernel/bpf/helpers.c
@@ -2656,6 +2656,43 @@ __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__nonown_allowed)
+{
+       struct list_head *h = (struct list_head *)head;
+       struct bpf_list_node_kern *kn = (struct bpf_list_node_kern *)node__nonown_allowed;
+
+       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__nonown_allowed)
+{
+       struct list_head *h = (struct list_head *)head;
+       struct bpf_list_node_kern *kn = (struct bpf_list_node_kern *)node__nonown_allowed;
+
+       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)
 {
@@ -4772,6 +4809,9 @@ 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 662ad7312697094f1deed3a199668d83ba8728cf..d9bdc3b32c05006302c9539b2b9b22b09ec1473f 100644 (file)
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -10965,6 +10965,9 @@ 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,
@@ -11035,6 +11038,9 @@ 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)
@@ -11556,7 +11562,10 @@ 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)
@@ -11678,7 +11687,9 @@ 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) ||