[PATCH bpf-next v8 4/8] bpf: refactor __bpf_list_add to take insertion point via **prev_ptr

Chengkaitao
2026-03-16T11:29:24Z

From: Kaitao Cheng <chengkaitao@kylinos.cn>

Refactor __bpf_list_add to accept (new, head, struct list_head **prev_ptr,
..) instead of (node, head, bool tail, ..). Load prev from *prev_ptr after
INIT_LIST_HEAD(h), so we never dereference an uninitialized h->prev when
head was 0-initialized (e.g. push_back passes &h->prev).

When prev is not the list head, validate that prev is in the list via
its owner.

Prepares for bpf_list_add_impl(head, new, prev, ..) to insert after a
given list node.

Signed-off-by: Kaitao Cheng <chengkaitao@kylinos.cn>
---
kernel/bpf/helpers.c | 44 ++++++++++++++++++++++++++++----------------
1 file changed, 28 insertions(+), 16 deletions(-)

diff --git a/kernel/bpf/helpers.c b/kernel/bpf/helpers.c
index dac346eb1e2f..a9665f97b3bc 100644
--- a/kernel/bpf/helpers.c
+++ b/kernel/bpf/helpers.c
@@ -2379,11 +2379,13 @@ __bpf_kfunc void *bpf_refcount_acquire_impl(void *p__refcounted_kptr, void *meta
return (void *)p__refcounted_kptr;
}

-static int __bpf_list_add(struct bpf_list_node_kern *node,
+static int __bpf_list_add(struct bpf_list_node_kern *new,
struct bpf_list_head *head,
- bool tail, struct btf_record *rec, u64 off)
+ struct list_head **prev_ptr,
+ struct btf_record *rec, u64 off)
{
- struct list_head *n = &node->list_head, *h = (void *)head;
+ struct list_head *n = &new->list_head, *h = (void *)head;
+ struct list_head *prev;

/* If list_head was 0-initialized by map, bpf_obj_init_field wasn't
* called on its fields, so init here
@@ -2391,39 +2393,49 @@ static int __bpf_list_add(struct bpf_list_node_kern *node,
if (unlikely(!h->next))
INIT_LIST_HEAD(h);

- /* node->owner != NULL implies !list_empty(n), no need to separately
+ prev = *prev_ptr;
+
+ /* When prev is not the list head, it must be a node in this list. */
+ if (prev != h && WARN_ON_ONCE(READ_ONCE(container_of(
+ prev, struct bpf_list_node_kern, list_head)->owner) != head))
+ goto fail;
+
+ /* new->owner != NULL implies !list_empty(n), no need to separately
* check the latter
*/
- if (cmpxchg(&node->owner, NULL, BPF_PTR_POISON)) {
- /* Only called from BPF prog, no need to migrate_disable */
- __bpf_obj_drop_impl((void *)n - off, rec, false);
- return -EINVAL;
- }
-
- tail ? list_add_tail(n, h) : list_add(n, h);
- WRITE_ONCE(node->owner, head);
+ if (cmpxchg(&new->owner, NULL, BPF_PTR_POISON))
+ goto fail;

+ list_add(n, prev);
+ WRITE_ONCE(new->owner, head);
return 0;
+
+fail:
+ /* Only called from BPF prog, no need to migrate_disable */
+ __bpf_obj_drop_impl((void *)n - off, rec, false);
+ return -EINVAL;
}

__bpf_kfunc int bpf_list_push_front_impl(struct bpf_list_head *head,
struct bpf_list_node *node,
void *meta__ign, u64 off)
{
- struct bpf_list_node_kern *n = (void *)node;
+ struct bpf_list_node_kern *new = (void *)node;
struct btf_struct_meta *meta = meta__ign;
+ struct list_head *h = (void *)head;

- return __bpf_list_add(n, head, false, meta ? meta->record : NULL, off);
+ return __bpf_list_add(new, head, &h, meta ? meta->record : NULL, off);
}

__bpf_kfunc int bpf_list_push_back_impl(struct bpf_list_head *head,
struct bpf_list_node *node,
void *meta__ign, u64 off)
{
- struct bpf_list_node_kern *n = (void *)node;
+ struct bpf_list_node_kern *new = (void *)node;
struct btf_struct_meta *meta = meta__ign;
+ struct list_head *h = (void *)head;

- return __bpf_list_add(n, head, true, meta ? meta->record : NULL, off);
+ return __bpf_list_add(new, head, &h->prev, meta ? meta->record : NULL, off);
}

static struct bpf_list_node *__bpf_list_del(struct bpf_list_head *head,
--
2.50.1 (Apple Git-155)