[PATCH bpf-next v10 7/8] bpf: allow non-owning list-node args via __nonown_allowed

Kaitao cheng
2026-04-27T16:59:53Z

From: Kaitao Cheng <chengkaitao@kylinos.cn>

KF_ARG_PTR_TO_LIST_NODE normally requires an owning reference
(PTR_TO_BTF_ID | MEM_ALLOC with ref_obj_id). Introduce and use
the __nonown_allowed annotation on selected list-node arguments
so non-owning references with ref_obj_id==0 are accepted as well.

This enables passing bpf_list_front() / bpf_list_back() results to:

bpf_list_add() as insertion point (prev)
bpf_list_del() as deletion target (node)
bpf_list_is_first/last() as query target (node)

Verifier keeps existing owning-ref checks by default; only arguments
annotated with __nonown_allowed bypass MEM_ALLOC/ref_obj_id checks
and then follow the same list-node validation path.

Signed-off-by: Kaitao Cheng <chengkaitao@kylinos.cn>
---
Documentation/bpf/kfuncs.rst | 22 ++++++++++++++++++++--
kernel/bpf/helpers.c | 20 +++++++++++---------
kernel/bpf/verifier.c | 13 +++++++++++++
3 files changed, 44 insertions(+), 11 deletions(-)

diff --git a/Documentation/bpf/kfuncs.rst b/Documentation/bpf/kfuncs.rst
index 75e6c078e0e7..3a9db1108b95 100644
--- a/Documentation/bpf/kfuncs.rst
+++ b/Documentation/bpf/kfuncs.rst
@@ -207,8 +207,26 @@ Here, the buffer may be NULL. If the buffer is not NULL, it must be at least
buffer__szk bytes in size. The kfunc is responsible for checking if the buffer
is NULL before using it.

-2.3.5 __str Annotation
-----------------------------
+2.3.5 __nonown_allowed Annotation
+---------------------------------
+
+This annotation is used to indicate that the parameter may be a non-owning reference.
+
+An example is given below::
+
+ __bpf_kfunc int bpf_list_add(..., struct bpf_list_node
+ *prev__nonown_allowed, ...)
+ {
+ ...
+ }
+
+For the ``prev__nonown_allowed`` parameter (resolved as ``KF_ARG_PTR_TO_LIST_NODE``),
+suffix ``__nonown_allowed`` retains the usual owning-pointer rules and also
+permits a non-owning reference with no ref_obj_id (e.g. the return value of
+bpf_list_front() / bpf_list_back()).
+
+2.3.6 __str Annotation
+----------------------
This annotation is used to indicate that the argument is a constant string.

An example is given below::
diff --git a/kernel/bpf/helpers.c b/kernel/bpf/helpers.c
index dfd465badd9d..f2f8705f0e9a 100644
--- a/kernel/bpf/helpers.c
+++ b/kernel/bpf/helpers.c
@@ -2571,10 +2571,10 @@ __bpf_kfunc int bpf_list_push_back_impl(struct bpf_list_head *head,
}

__bpf_kfunc int bpf_list_add(struct bpf_list_head *head, struct bpf_list_node *new,
- struct bpf_list_node *prev, struct btf_struct_meta *meta,
- u64 off)
+ struct bpf_list_node *prev__nonown_allowed,
+ struct btf_struct_meta *meta, u64 off)
{
- struct bpf_list_node_kern *n = (void *)new, *p = (void *)prev;
+ struct bpf_list_node_kern *n = (void *)new, *p = (void *)prev__nonown_allowed;
struct list_head *prev_ptr = &p->list_head;

return __bpf_list_add(n, head, &prev_ptr, meta ? meta->record : NULL, off);
@@ -2620,9 +2620,9 @@ __bpf_kfunc struct bpf_list_node *bpf_list_pop_back(struct bpf_list_head *head)
}

__bpf_kfunc struct bpf_list_node *bpf_list_del(struct bpf_list_head *head,
- struct bpf_list_node *node)
+ struct bpf_list_node *node__nonown_allowed)
{
- struct bpf_list_node_kern *kn = (void *)node;
+ struct bpf_list_node_kern *kn = (void *)node__nonown_allowed;

/* verifier guarantees node is a list node rather than list head */
return __bpf_list_del(head, &kn->list_head);
@@ -2648,10 +2648,11 @@ __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)
+__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;
+ struct bpf_list_node_kern *kn = (struct bpf_list_node_kern *)node__nonown_allowed;

if (READ_ONCE(kn->owner) != head)
return false;
@@ -2659,10 +2660,11 @@ __bpf_kfunc bool bpf_list_is_first(struct bpf_list_head *head, struct bpf_list_n
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)
+__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;
+ struct bpf_list_node_kern *kn = (struct bpf_list_node_kern *)node__nonown_allowed;

if (READ_ONCE(kn->owner) != head)
return false;
diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index ca33f35bc3eb..08ab337866bf 100644
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -10502,6 +10502,11 @@ static bool is_kfunc_arg_nullable(const struct btf *btf, const struct btf_param
return btf_param_match_suffix(btf, arg, "__nullable");
}

+static bool is_kfunc_arg_nonown_allowed(const struct btf *btf, const struct btf_param *arg)
+{
+ return btf_param_match_suffix(btf, arg, "__nonown_allowed");
+}
+
static bool is_kfunc_arg_const_str(const struct btf *btf, const struct btf_param *arg)
{
return btf_param_match_suffix(btf, arg, "__str");
@@ -12017,6 +12022,13 @@ static int check_kfunc_args(struct bpf_verifier_env *env, struct bpf_kfunc_call_
return ret;
break;
case KF_ARG_PTR_TO_LIST_NODE:
+ if (is_kfunc_arg_nonown_allowed(btf, &args[i]) &&
+ type_is_non_owning_ref(reg->type) && !reg->ref_obj_id) {
+ /* Allow bpf_list_front/back return value for
+ * __nonown_allowed list-node arguments.
+ */
+ goto check_ok;
+ }
if (reg->type != (PTR_TO_BTF_ID | MEM_ALLOC)) {
verbose(env, "%s expected pointer to allocated object\n",
reg_arg_name(env, argno));
@@ -12026,6 +12038,7 @@ static int check_kfunc_args(struct bpf_verifier_env *env, struct bpf_kfunc_call_
verbose(env, "allocated object must be referenced\n");
return -EINVAL;
}
+check_ok:
ret = process_kf_arg_ptr_to_list_node(env, reg, argno, meta);
if (ret < 0)
return ret;
--
2.50.1 (Apple Git-155)