Re: [PATCH RESEND bpf-next v10 1/8] bpf: refactor __bpf_list_del to take list node pointer

[Kaitao Cheng <kaitao.cheng@linux.dev>]

I have clarified this issue in the [PATCH bpf-next v10 1/8] patch,
with the link provided below:

https://lore.kernel.org/bpf/e908cd1a-a0ee-49b1-8528-c455bbd4c432@linux.dev/

在 2026/5/12 14:41, bot+bpf-ci@kernel.org 写道:
>> diff --git a/kernel/bpf/helpers.c b/kernel/bpf/helpers.c
>> index baa12b24bb64..9cd7b028592c 100644
>> --- a/kernel/bpf/helpers.c
>> +++ b/kernel/bpf/helpers.c
>> @@ -2550,22 +2550,24 @@ __bpf_kfunc int bpf_list_push_back_impl(struct bpf_list_head *head,
>>  	return bpf_list_push_back(head, node, meta__ign, 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);
>> +		return NULL;
>> +	}
>>  	if (list_empty(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))
>> +	if (unlikely(READ_ONCE(node->owner) != head))
>>  		return NULL;
>>
>>  	list_del_init(n);
>> @@ -2575,12 +2577,16 @@ 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);
>>  }
> 
> The new code reads h->next before __bpf_list_del checks if the list is
> initialized. If the list is uninitialized (h->next is NULL), a NULL pointer
> is passed to __bpf_list_del.
> 
> While __bpf_list_del eventually initializes the list and returns NULL without
> dereferencing the NULL pointer, this is counterintuitive - passing a
> potentially garbage value that is later ignored.
> 
> Can you move the initialization check into the top-level kfuncs
> (bpf_list_pop_front and bpf_list_pop_back) to ensure the list_head passed to
> __bpf_list_del is always valid? You could initialize the list and return NULL
> in that case - we know it's empty.
> 
> This concern was raised by Emil Tsalapatis in v8:
> https://lore.kernel.org/bpf/DH6W7K2SLPYB.PCO7FQ6CR1YZ@etsalapatis.com/
> 
>>
>>  __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);
>>  }
> 
> 
> ---
> AI reviewed your patch. Please fix the bug or email reply why it's not a bug.
> See: https://github.com/kernel-patches/vmtest/blob/master/ci/claude/README.md
> 
> CI run summary: https://github.com/kernel-patches/bpf/actions/runs/25716874656

-- 
Thanks
Kaitao Cheng