Re: [PATCH v4 1/6] bpf: Introduce the bpf_list_del kfunc.

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

On 3/3/26 21:52, 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.
> 
> 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  | 19 +++++++++++++++++++
>  kernel/bpf/verifier.c |  6 +++++-
>  2 files changed, 24 insertions(+), 1 deletion(-)
> 
> diff --git a/kernel/bpf/helpers.c b/kernel/bpf/helpers.c
> index 6eb6c82ed2ee..19d88da8e694 100644
> --- a/kernel/bpf/helpers.c
> +++ b/kernel/bpf/helpers.c
> @@ -2459,6 +2459,24 @@ __bpf_kfunc struct bpf_list_node *bpf_list_pop_back(struct bpf_list_head *head)
>  	return __bpf_list_del(head, true);
>  }
>  
> +__bpf_kfunc struct bpf_list_node *bpf_list_del(struct bpf_list_head *head,

NIT: The commit log should explain why the head parameter is needed,
similar to patch #3.

> +					       struct bpf_list_node *node)
> +{
> +	struct bpf_list_node_kern *knode = (struct bpf_list_node_kern *)node;
> +	struct list_head *h = (void *)head;
> +
> +	if (unlikely(!knode))
> +		return NULL;

Similar to __bpf_list_del(), what if the list head was 0-initialized?

> +
> +	if (WARN_ON_ONCE(READ_ONCE(knode->owner) != h))
> +		return NULL;
> +
> +	list_del_init(&knode->list_head);
> +	WRITE_ONCE(knode->owner, NULL);
> +
> +	return node;
> +}
> +
>  __bpf_kfunc struct bpf_list_node *bpf_list_front(struct bpf_list_head *head)
>  {
>  	struct list_head *h = (struct list_head *)head;
> @@ -4545,6 +4563,7 @@ 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] ||

NIT: This series adds 5 list kfuncs, growing the is_bpf_list_api_kfunc()
chain from 5 to 10. It is worth considering a table-driven approach,
which could also apply to check_kfunc_is_graph_node_api().

Thanks,
Leon

>  	       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] ||