Re: [PATCH bpf-next v8 7/8] bpf: add bpf_list_is_first/last/empty kfuncs

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

On Sat Mar 21, 2026 at 9:01 PM EDT, Emil Tsalapatis wrote:
> On Mon Mar 16, 2026 at 7:28 AM EDT, 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.
>>
>> Currently, without these kfuncs, to implement the above functionality
>> it is 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.
>
> Do you need to expand the exceptions you added in patch 5 for non-owning
> reference arguments to also hold for this? Checking the selftests, we
> don't have one where either bpf_list_is_last or bpf_list_is_first
> return true, can you add one?
>

I misread the selftests, my bad. The tests pass fine, and we just have
to keep an owning reference of the nodes around to pass to the helpers.

Reviewed-by: Emil Tsalapatis <emil@etsalapatis.com>

>>
>> Signed-off-by: Kaitao Cheng <chengkaitao@kylinos.cn>
>> ---
>>  kernel/bpf/helpers.c  | 38 ++++++++++++++++++++++++++++++++++++++
>>  kernel/bpf/verifier.c | 11 +++++++++++
>>  2 files changed, 49 insertions(+)
>>
>> diff --git a/kernel/bpf/helpers.c b/kernel/bpf/helpers.c
>> index dc4f8b4eec01..da70dcff4ba8 100644
>> --- a/kernel/bpf/helpers.c
>> +++ b/kernel/bpf/helpers.c
>> @@ -2518,6 +2518,41 @@ __bpf_kfunc struct bpf_list_node *bpf_list_back(struct bpf_list_head *head)
>>  	return (struct bpf_list_node *)h->prev;
>>  }
>>  
>> +__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 *kn = (struct bpf_list_node_kern *)node;
>> +
>> +	if (READ_ONCE(kn->owner) != head)
>> +		return false;
>> +
>> +	return list_is_first(&kn->list_head, h);
>> +}
>> +
>> +__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 *kn = (struct bpf_list_node_kern *)node;
>> +
>> +	if (READ_ONCE(kn->owner) != head)
>> +		return false;
>> +
>> +	return list_is_last(&kn->list_head, h);
>> +}
>> +
>> +__bpf_kfunc bool bpf_list_empty(struct bpf_list_head *head)
>> +{
>> +	struct list_head *h = (struct list_head *)head;
>> +
>> +	/* 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))
>> +		INIT_LIST_HEAD(h);
>> +
>> +	return list_empty(h);
>> +}
>> +
>>  __bpf_kfunc struct bpf_rb_node *bpf_rbtree_remove(struct bpf_rb_root *root,
>>  						  struct bpf_rb_node *node)
>>  {
>> @@ -4588,6 +4623,9 @@ 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 0a6f2e6a5d28..c2cc622bb3a1 100644
>> --- a/kernel/bpf/verifier.c
>> +++ b/kernel/bpf/verifier.c
>> @@ -12512,6 +12512,9 @@ enum special_kfunc_type {
>>  	KF_bpf_list_del,
>>  	KF_bpf_list_front,
>>  	KF_bpf_list_back,
>> +	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,
>> @@ -12574,6 +12577,9 @@ 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_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)
>> @@ -12652,6 +12658,9 @@ static const enum special_kfunc_type bpf_list_api_kfuncs[] = {
>>  	KF_bpf_list_del,
>>  	KF_bpf_list_front,
>>  	KF_bpf_list_back,
>> +	KF_bpf_list_is_first,
>> +	KF_bpf_list_is_last,
>> +	KF_bpf_list_empty,
>>  };
>>  
>>  /* Kfuncs that take a list node argument (bpf_list_node *). */
>> @@ -12660,6 +12669,8 @@ static const enum special_kfunc_type bpf_list_node_api_kfuncs[] = {
>>  	KF_bpf_list_push_back_impl,
>>  	KF_bpf_list_add_impl,
>>  	KF_bpf_list_del,
>> +	KF_bpf_list_is_first,
>> +	KF_bpf_list_is_last,
>>  };
>>  
>>  /* Kfuncs that take an rbtree node argument (bpf_rb_node *). */