Re: [PATCH bpf-next v7 1/5] bpf: Introduce the bpf_list_del kfunc.

[Leon Hwang <leon.hwang@linux.dev>]

On 8/3/26 21:46, Chengkaitao wrote:
> From: Kaitao Cheng <chengkaitao@kylinos.cn>
> 
> If a user holds ownership of a node in the middle of a list, they
> can directly remove it from the list without strictly adhering to
> deletion rules from the head or tail.
> 
> We have added an additional parameter bpf_list_head *head to
> bpf_list_del, as the verifier requires the head parameter to
> check whether the lock is being held.
> 
> This is typically paired with bpf_refcount. After calling
> bpf_list_del, it is generally necessary to drop the reference to
> the list node twice to prevent reference count leaks.
> 
> Signed-off-by: Kaitao Cheng <chengkaitao@kylinos.cn>
> ---
>  kernel/bpf/helpers.c  | 30 +++++++++++++++++++++++-------
>  kernel/bpf/verifier.c |  6 +++++-
>  2 files changed, 28 insertions(+), 8 deletions(-)
> 
> diff --git a/kernel/bpf/helpers.c b/kernel/bpf/helpers.c
> index 6eb6c82ed2ee..01b74c4ac00d 100644
> --- a/kernel/bpf/helpers.c
> +++ b/kernel/bpf/helpers.c
> @@ -2426,20 +2426,23 @@ __bpf_kfunc int bpf_list_push_back_impl(struct bpf_list_head *head,
>  	return __bpf_list_add(n, head, true, meta ? meta->record : NULL, off);
>  }
>  
> -static struct bpf_list_node *__bpf_list_del(struct bpf_list_head *head, bool tail)
> +static struct bpf_list_node *__bpf_list_del(struct bpf_list_head *head,
> +					    struct list_head *n)
>  {
> -	struct list_head *n, *h = (void *)head;
> +	struct list_head *h = (void *)head;
>  	struct bpf_list_node_kern *node;
>  
>  	/* 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))
> +	if (unlikely(!h->next)) {
>  		INIT_LIST_HEAD(h);
> -	if (list_empty(h))
> +		return NULL;
> +	}
> +
> +	if (n == h)
>  		return NULL;
>  
> -	n = tail ? h->prev : h->next;
>  	node = container_of(n, struct bpf_list_node_kern, list_head);
>  	if (WARN_ON_ONCE(READ_ONCE(node->owner) != head))
>  		return NULL;

This refactoring is worth, because the "struct list_head *n" seems
better than "bool tail".

But, such refactoring should be a preparatory patch. Importantly,
refactoring should not introduce functional changes.

Thanks,
Leon

> @@ -2451,12 +2454,24 @@ static struct bpf_list_node *__bpf_list_del(struct bpf_list_head *head, bool tai
>  
>  __bpf_kfunc struct bpf_list_node *bpf_list_pop_front(struct bpf_list_head *head)
>  {
> -	return __bpf_list_del(head, false);
> +	struct list_head *h = (void *)head;
> +
> +	return __bpf_list_del(head, h->next);
>  }
>  
>  __bpf_kfunc struct bpf_list_node *bpf_list_pop_back(struct bpf_list_head *head)
>  {
> -	return __bpf_list_del(head, true);
> +	struct list_head *h = (void *)head;
> +
> +	return __bpf_list_del(head, h->prev);
> +}
> +
> +__bpf_kfunc struct bpf_list_node *bpf_list_del(struct bpf_list_head *head,
> +					       struct bpf_list_node *node)
> +{
> +	struct bpf_list_node_kern *kn = (void *)node;
> +
> +	return __bpf_list_del(head, &kn->list_head);
>  }
>  
>  __bpf_kfunc struct bpf_list_node *bpf_list_front(struct bpf_list_head *head)
> @@ -4545,6 +4560,7 @@ BTF_ID_FLAGS(func, bpf_list_push_front_impl)
>  BTF_ID_FLAGS(func, bpf_list_push_back_impl)
>  BTF_ID_FLAGS(func, bpf_list_pop_front, KF_ACQUIRE | KF_RET_NULL)
>  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_task_acquire, KF_ACQUIRE | KF_RCU | KF_RET_NULL)
> diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
> index 67c09b43a497..c9557d3fb8dd 100644
> --- a/kernel/bpf/verifier.c
> +++ b/kernel/bpf/verifier.c
> @@ -12461,6 +12461,7 @@ enum special_kfunc_type {
>  	KF_bpf_list_push_back_impl,
>  	KF_bpf_list_pop_front,
>  	KF_bpf_list_pop_back,
> +	KF_bpf_list_del,
>  	KF_bpf_list_front,
>  	KF_bpf_list_back,
>  	KF_bpf_cast_to_kern_ctx,
> @@ -12521,6 +12522,7 @@ BTF_ID(func, bpf_list_push_front_impl)
>  BTF_ID(func, bpf_list_push_back_impl)
>  BTF_ID(func, bpf_list_pop_front)
>  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_cast_to_kern_ctx)
> @@ -12996,6 +12998,7 @@ static bool is_bpf_list_api_kfunc(u32 btf_id)
>  	       btf_id == special_kfunc_list[KF_bpf_list_push_back_impl] ||
>  	       btf_id == special_kfunc_list[KF_bpf_list_pop_front] ||
>  	       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];
>  }
> @@ -13118,7 +13121,8 @@ static bool check_kfunc_is_graph_node_api(struct bpf_verifier_env *env,
>  	switch (node_field_type) {
>  	case BPF_LIST_NODE:
>  		ret = (kfunc_btf_id == special_kfunc_list[KF_bpf_list_push_front_impl] ||
> -		       kfunc_btf_id == special_kfunc_list[KF_bpf_list_push_back_impl]);
> +		       kfunc_btf_id == special_kfunc_list[KF_bpf_list_push_back_impl] ||
> +		       kfunc_btf_id == special_kfunc_list[KF_bpf_list_del]);
>  		break;
>  	case BPF_RB_NODE:
>  		ret = (kfunc_btf_id == special_kfunc_list[KF_bpf_rbtree_remove] ||