Re: [PATCH v4 5/6] bpf: add bpf_list_is_first/last/empty kfuncs

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

On 3/3/26 21:52, Chengkaitao wrote:
> From: Kaitao Cheng <chengkaitao@kylinos.cn>
> 
> Add three kfuncs for BPF linked list queries:
> - bpf_list_is_first(head, node): true if node is the first in the list.
> - bpf_list_is_last(head, node): true if node is the last in the list.
> - bpf_list_empty(head): true if the list has no entries.
> 
> In previous versions, to implement the above functionality, it was
> necessary to first call bpf_list_pop_front/back to retrieve the first
> or last node before checking whether the passed-in node was the first
> or last one. After the check, the node had to be pushed back into the
> list using bpf_list_push_front/back, which was very inefficient.
> 
> Now, with the bpf_list_is_first/last/empty kfuncs, we can directly
> check whether a node is the first, last, or whether the list is empty,
> without having to first retrieve the node.
> 
> Signed-off-by: Kaitao Cheng <chengkaitao@kylinos.cn>
> ---
>  kernel/bpf/helpers.c  | 41 +++++++++++++++++++++++++++++++++++++++++
>  kernel/bpf/verifier.c | 15 +++++++++++++--
>  2 files changed, 54 insertions(+), 2 deletions(-)
> 
> diff --git a/kernel/bpf/helpers.c b/kernel/bpf/helpers.c
> index 488810da8f30..3a64a9bab06b 100644
> --- a/kernel/bpf/helpers.c
> +++ b/kernel/bpf/helpers.c
> @@ -2523,6 +2523,44 @@ __bpf_kfunc int bpf_list_add_impl(struct bpf_list_head *head,
>  	return 0;
>  }
>  
> +__bpf_kfunc bool bpf_list_is_first(struct bpf_list_head *head, struct bpf_list_node *node)
> +{
> +	struct list_head *h = (struct list_head *)head;
> +	struct bpf_list_node_kern *n = (struct bpf_list_node_kern *)node;
> +
> +	if (unlikely(!h->next) || list_empty(h))
> +		return false;
> +
> +	if (READ_ONCE(n->owner) != head)
> +		return false;
> +
> +	return h->next == &n->list_head;
> +}
> +
> +__bpf_kfunc bool bpf_list_is_last(struct bpf_list_head *head, struct bpf_list_node *node)
> +{
> +	struct list_head *h = (struct list_head *)head;
> +	struct bpf_list_node_kern *n = (struct bpf_list_node_kern *)node;
> +
> +	if (unlikely(!h->next) || list_empty(h))
> +		return false;
> +
> +	if (READ_ONCE(n->owner) != head)
> +		return false;
> +
> +	return h->prev == &n->list_head;
> +}
> +

bpf_list_is_first() and bpf_list_is_last() are nearly identical. The
duplicated code should be avoided.

> +__bpf_kfunc bool bpf_list_empty(struct bpf_list_head *head)
> +{
> +	struct list_head *h = (struct list_head *)head;
> +
> +	if (unlikely(!h->next))
> +		return true;

NIT: A comment here explaining why !h->next returns true would be helpful.

Thanks,
Leon

> +
> +	return list_empty(h);
> +}
> +
>  __bpf_kfunc struct bpf_rb_node *bpf_rbtree_remove(struct bpf_rb_root *root,
>  						  struct bpf_rb_node *node)
>  {
> @@ -4593,6 +4631,9 @@ 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_list_add_impl)
> +BTF_ID_FLAGS(func, bpf_list_is_first)
> +BTF_ID_FLAGS(func, bpf_list_is_last)
> +BTF_ID_FLAGS(func, bpf_list_empty)
>  BTF_ID_FLAGS(func, bpf_task_acquire, KF_ACQUIRE | KF_RCU | KF_RET_NULL)
>  BTF_ID_FLAGS(func, bpf_task_release, KF_RELEASE)
>  BTF_ID_FLAGS(func, bpf_rbtree_remove, KF_ACQUIRE | KF_RET_NULL)
> diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
> index 6dfd4afff1cf..78652ccd2a89 100644
> --- a/kernel/bpf/verifier.c
> +++ b/kernel/bpf/verifier.c
> @@ -12465,6 +12465,9 @@ enum special_kfunc_type {
>  	KF_bpf_list_front,
>  	KF_bpf_list_back,
>  	KF_bpf_list_add_impl,
> +	KF_bpf_list_is_first,
> +	KF_bpf_list_is_last,
> +	KF_bpf_list_empty,
>  	KF_bpf_cast_to_kern_ctx,
>  	KF_bpf_rdonly_cast,
>  	KF_bpf_rcu_read_lock,
> @@ -12527,6 +12530,9 @@ BTF_ID(func, bpf_list_del)
>  BTF_ID(func, bpf_list_front)
>  BTF_ID(func, bpf_list_back)
>  BTF_ID(func, bpf_list_add_impl)
> +BTF_ID(func, bpf_list_is_first)
> +BTF_ID(func, bpf_list_is_last)
> +BTF_ID(func, bpf_list_empty)
>  BTF_ID(func, bpf_cast_to_kern_ctx)
>  BTF_ID(func, bpf_rdonly_cast)
>  BTF_ID(func, bpf_rcu_read_lock)
> @@ -13003,7 +13009,10 @@ static bool is_bpf_list_api_kfunc(u32 btf_id)
>  	       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] ||
> -	       btf_id == special_kfunc_list[KF_bpf_list_add_impl];
> +	       btf_id == special_kfunc_list[KF_bpf_list_add_impl] ||
> +	       btf_id == special_kfunc_list[KF_bpf_list_is_first] ||
> +	       btf_id == special_kfunc_list[KF_bpf_list_is_last] ||
> +	       btf_id == special_kfunc_list[KF_bpf_list_empty];
>  }
>  
>  static bool is_bpf_rbtree_api_kfunc(u32 btf_id)
> @@ -13126,7 +13135,9 @@ static bool check_kfunc_is_graph_node_api(struct bpf_verifier_env *env,
>  		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_del] ||
> -		       kfunc_btf_id == special_kfunc_list[KF_bpf_list_add_impl]);
> +		       kfunc_btf_id == special_kfunc_list[KF_bpf_list_add_impl] ||
> +		       kfunc_btf_id == special_kfunc_list[KF_bpf_list_is_first] ||
> +		       kfunc_btf_id == special_kfunc_list[KF_bpf_list_is_last]);
>  		break;
>  	case BPF_RB_NODE:
>  		ret = (kfunc_btf_id == special_kfunc_list[KF_bpf_rbtree_remove] ||