Re: [PATCH bpf-next v11 6/8] bpf: Add bpf_list_add to insert node after a given list node

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

在 2026/5/21 12:08, bot+bpf-ci@kernel.org 写道:
>> diff --git a/kernel/bpf/helpers.c b/kernel/bpf/helpers.c
>> index 1c69476c8a09..89579165ef4d 100644
>> --- a/kernel/bpf/helpers.c
>> +++ b/kernel/bpf/helpers.c
>> @@ -2577,6 +2577,16 @@ __bpf_kfunc int bpf_list_push_back_impl(struct bpf_list_head *head,
>>  	return bpf_list_push_back(head, node, meta__ign, off);
>>  }
>>
>> +__bpf_kfunc int bpf_list_add(struct bpf_list_head *head, struct bpf_list_node *new,
>> +			     struct bpf_list_node *prev__nonown_allowed,
>> +			     struct btf_struct_meta *meta, u64 off)
>> +{
>> +	struct bpf_list_node_kern *n = (void *)new, *p = (void *)prev__nonown_allowed;
>> +	struct list_head *prev_ptr = &p->list_head;
>> +
>> +	return __bpf_list_add(n, head, &prev_ptr, meta ? meta->record : NULL, off);
>> +}
> 
> If __bpf_list_add() fails at runtime (e.g., because prev is not in the
> list indicated by head), it drops the 'new' node via __bpf_obj_drop_impl
> in the fail path.
> 
> However, the verifier unconditionally converts 'new' into a non-owning
> reference during check_kfunc_call(). If __bpf_list_add() fails, the BPF
> program retains this non-owning reference to a freed node.
> 
> Could this allow the program to trigger a use-after-free within the lock
> critical section?
> 
> (sashiko-bot@kernel.org, v10 RESEND)
> https://lore.kernel.org/bpf/20260513204420.2BB22C19425@smtp.kernel.org/

This should be an acceptable case within the current design framework.

>>  static struct bpf_list_node *__bpf_list_del(struct bpf_list_head *head,
>>  					    struct list_head *n)
> 
> [ ... ]
> 
>> @@ -4756,6 +4766,7 @@ BTF_ID_FLAGS(func, bpf_list_push_front, KF_IMPLICIT_ARGS)
>>  BTF_ID_FLAGS(func, bpf_list_push_front_impl)
>>  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_add, KF_IMPLICIT_ARGS)
> 
> Since bpf_list_add is marked with KF_IMPLICIT_ARGS, won't the verifier
> look for a bpf_list_add_impl counterpart during program load?
> 
> Without defining and registering a bpf_list_add_impl function,
> find_kfunc_impl_proto() will fail to find the required _impl suffix in
> the vmlinux BTF.
> 
> Will this cause the verifier to unconditionally reject the kfunc?
> 
> (sashiko-bot@kernel.org, v10 RESEND)
> https://lore.kernel.org/bpf/20260513204420.2BB22C19425@smtp.kernel.org/

The target prototype with the _impl suffix has already been deprecated.

>>  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)
> 
> [ ... ]
> 
>> diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
>> index 35eebb5e7769..662ad7312697 100644
>> --- a/kernel/bpf/verifier.c
>> +++ b/kernel/bpf/verifier.c
> 
> [ ... ]
> 
>> @@ -19524,8 +19527,11 @@ int bpf_fixup_kfunc_call(struct bpf_verifier_env *env, struct bpf_insn *insn,
>>  		int struct_meta_reg = BPF_REG_3;
>>  		int node_offset_reg = BPF_REG_4;
>>
>> -		/* rbtree_add has extra 'less' arg, so args-to-fixup are in diff regs */
>> -		if (is_bpf_rbtree_add_kfunc(desc->func_id)) {
>> +		/* list_add/rbtree_add have an extra arg (prev/less),
>> +		 * so args-to-fixup are in diff regs.
>> +		 */
> 
> This isn't a bug, but the multi-line comment format doesn't follow the
> BPF subsystem style. The preferred form in kernel/bpf/ has the opening
> /* on its own line.
> 
> Additionally, consider adding an is_bpf_list_add_kfunc() helper instead
> of direct comparison with special_kfunc_list[KF_bpf_list_add].
> 
> (bot+bpf-ci@kernel.org and Eduard Zingerman, v10 RESEND)
> https://lore.kernel.org/bpf/f573f7e0a5140fe6254d2ddfd3f69add455fa25c67c020c0312e2c2ceeca7c69@mail.kernel.org/

I provided a more detailed explanation in the link below.
https://lore.kernel.org/all/9e300669-8793-41a2-84a1-98fb8195e263@linux.dev/

>> +		if (desc->func_id == special_kfunc_list[KF_bpf_list_add] ||
>> +		    is_bpf_rbtree_add_kfunc(desc->func_id)) {
>>  			struct_meta_reg = BPF_REG_4;
>>  			node_offset_reg = BPF_REG_5;
>>  		}
> 
> 
> ---
> 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/26204125015

-- 
Thanks
Kaitao Cheng