[PATCH bpf-next v10 4/8] bpf: refactor __bpf_list_add to take insertion point via **prev_ptr

Kaitao cheng
2026-04-27T16:59:39Z

From: Kaitao Cheng <chengkaitao@kylinos.cn>

Refactor __bpf_list_add to accept (node, 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(head, new, prev, ..) to insert after a given
list node.

Signed-off-by: Kaitao Cheng <chengkaitao@kylinos.cn>
---
kernel/bpf/helpers.c | 36 ++++++++++++++++++++++++++----------
1 file changed, 26 insertions(+), 10 deletions(-)

diff --git a/kernel/bpf/helpers.c b/kernel/bpf/helpers.c
index 51b6ea4bb8cb..5388078f3171 100644
--- a/kernel/bpf/helpers.c
+++ b/kernel/bpf/helpers.c
@@ -2471,9 +2471,11 @@ __bpf_kfunc void *bpf_refcount_acquire_impl(void *p__refcounted_kptr, void *meta

static int __bpf_list_add(struct bpf_list_node_kern *node,
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 *prev;

/* If list_head was 0-initialized by map, bpf_obj_init_field wasn't
* called on its fields, so init here
@@ -2481,19 +2483,31 @@ static int __bpf_list_add(struct bpf_list_node_kern *node,
if (unlikely(!h->next))
INIT_LIST_HEAD(h);

+ prev = *prev_ptr;
+
+ /* When prev is not the list head, it must be a node in this list. */
+ if (prev != h) {
+ struct bpf_list_node_kern *prev_kn =
+ container_of(prev, struct bpf_list_node_kern, list_head);
+
+ if (unlikely(READ_ONCE(prev_kn->owner) != head))
+ goto fail;
+ }
+
/* node->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;
- }
+ if (cmpxchg(&node->owner, NULL, BPF_PTR_POISON))
+ goto fail;

- tail ? list_add_tail(n, h) : list_add(n, h);
+ list_add(n, prev);
WRITE_ONCE(node->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;
}

/**
@@ -2514,8 +2528,9 @@ __bpf_kfunc int bpf_list_push_front(struct bpf_list_head *head,
u64 off)
{
struct bpf_list_node_kern *n = (void *)node;
+ struct list_head *h = (void *)head;

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

__bpf_kfunc int bpf_list_push_front_impl(struct bpf_list_head *head,
@@ -2543,8 +2558,9 @@ __bpf_kfunc int bpf_list_push_back(struct bpf_list_head *head,
u64 off)
{
struct bpf_list_node_kern *n = (void *)node;
+ struct list_head *h = (void *)head;

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

__bpf_kfunc int bpf_list_push_back_impl(struct bpf_list_head *head,
--
2.50.1 (Apple Git-155)