Re: [PATCH RESEND bpf-next v10 3/8] bpf: Introduce the bpf_list_del kfunc.

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

I have clarified this issue in the [PATCH bpf-next v10 3/8] patch,
with the link provided below:

https://lore.kernel.org/bpf/0637c1ce-0e73-4505-8a0c-e77339f4aefb@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 1e8754877dd1..51b6ea4bb8cb 100644
>> --- a/kernel/bpf/helpers.c
>> +++ b/kernel/bpf/helpers.c
>> @@ -2593,6 +2593,15 @@ __bpf_kfunc struct bpf_list_node *bpf_list_pop_back(struct bpf_list_head *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;
>> +
>> +	/* verifier guarantees node is a list node rather than list head */
>> +	return __bpf_list_del(head, &kn->list_head);
>> +}
>> +
> 
> Looking at bpf_list_pop_front() and bpf_list_pop_back() in the same
> file, both functions read from the list_head (h->next or h->prev)
> before passing it to __bpf_list_del(). If the list is uninitialized,
> they end up passing potentially invalid pointer values to __bpf_list_del(),
> which then has to handle this by initializing the list and returning NULL.
> 
> While __bpf_list_del() handles this correctly, would it be clearer to
> move the initialization check into the top-level kfuncs (like
> bpf_list_pop_front/back) so that __bpf_list_del() always receives a
> valid list_head pointer rather than potentially garbage values that
> get ignored later?
> 
> Reference: https://lore.kernel.org/bpf/af50d188f6bc2de81fe3c321b26a645a9425ba9f2644cb6f8525d5bb637385f9@mail.kernel.org/
> 
>> @@ -4725,6 +4734,7 @@ BTF_ID_FLAGS(func, bpf_list_push_back, KF_IMPLICIT_ARGS)
>>  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 11054ad89c14..a6bcbe68cd72 100644
>> --- a/kernel/bpf/verifier.c
>> +++ b/kernel/bpf/verifier.c
>> @@ -10744,6 +10744,7 @@ enum special_kfunc_type {
>>  	KF_bpf_list_push_back,
>>  	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,
> 
> [ ... ]
> 
>> @@ -11456,7 +11459,8 @@ static bool check_kfunc_is_graph_node_api(struct bpf_verifier_env *env,
>>
>>  	switch (node_field_type) {
>>  	case BPF_LIST_NODE:
>> -		ret = is_bpf_list_push_kfunc(kfunc_btf_id);
>> +		ret = is_bpf_list_push_kfunc(kfunc_btf_id) ||
>> +		      kfunc_btf_id == special_kfunc_list[KF_bpf_list_del];
>>  		break;
>>  	case BPF_RB_NODE:
>>  		ret = (is_bpf_rbtree_add_kfunc(kfunc_btf_id) ||
> 
> Can bpf_list_del actually be called from BPF programs? Looking at the
> verifier's handling of KF_ARG_PTR_TO_LIST_NODE arguments in
> check_kfunc_args():
> 
> 	case KF_ARG_PTR_TO_LIST_NODE:
> 		...
> 		if (!reg->ref_obj_id) {
> 			verbose(env, "allocated object must be referenced\n");
> 			return -EINVAL;
> 		}
> 
> This code path unconditionally requires an owning reference (ref_obj_id
> must be non-zero). However, once a node is added to the list via
> bpf_list_push_front/back, the owning reference is converted to a
> non-owning reference (ref_obj_id becomes 0). This means bpf_list_del
> cannot be called on nodes that are actually in the list.
> 
> The sister kfunc bpf_rbtree_remove handles this by having a special
> branch in check_kfunc_args() that accepts non-owning references for
> that specific kfunc. Does bpf_list_del need similar handling to allow
> removing nodes that are actually in the list?
> 
> Reference: https://lore.kernel.org/bpf/294242698d576e4fedf243ab055c6f9a984e55033933a9bd09808f62a7018e9d@mail.kernel.org/
> 
> 
> ---
> 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