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

["Emil Tsalapatis" <emil@etsalapatis.com>]

On Mon Mar 16, 2026 at 7:28 AM EDT, Chengkaitao wrote:
> 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;
> +

This is pretty difficult to read, can you clean this up?

> +	/* 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;

I don't think this rename or the one in __bpf_list_add are useful, they
also kind of obfuscate the point of the patch by accident imo.

>  	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,