Re: [PATCH RESEND bpf-next v10 2/8] bpf: clear list node owner and unlink before drop

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

在 2026/5/16 02:24, Eduard Zingerman 写道:
> On Fri, 2026-05-15 at 12:34 +0800, Kaitao Cheng wrote:
>>
>> 在 2026/5/14 09:50, Alexei Starovoitov 写道:
>>> On Wed May 13, 2026 at 3:53 PM PDT, Eduard Zingerman wrote:
>>>> On Tue, 2026-05-12 at 06:41 +0000, bot+bpf-ci@kernel.org wrote:
>>>>
>>>> [...]
>>>>
>>>>> When a BPF program holds an owning or refcount-acquired reference to
>>>>> one of these nodes (node X), which is structurally supported because
>>>>> __bpf_obj_drop_impl() uses refcount_dec_and_test() and only frees at
>>>>> refcount 0, a concurrent push to a DIFFERENT bpf_list_head becomes a
>>>>> corruption:
>>>>>
>>>>> CPU 0 (bpf_list_head_free, lock released)  CPU 1 (BPF prog, refcount X)
>>>>> -----------------------------------------   ----------------------------
>>>>> (owner of X == NULL, X linked in drain)
>>>>>                                             bpf_list_push_back(other, X)
>>>>>                                               __bpf_list_add: spin_lock()
>>>>>                                               cmpxchg(X->owner, NULL,
>>>>>                                                       POISON) -> OK
>>>>>                                               list_add_tail(&X->list_head,
>>>>>                                                             other_head)
>>>>>                                                 -> overwrites X->next,
>>>>>                                                    X->prev, corrupts
>>>>>                                                    other_head's chain
>>>>>                                                    because X is still
>>>>>                                                    stitched into drain
>>>>> pos = drain.next;      (may be X or neighbor using X's stale next)
>>>>> list_del_init(pos);    reads X->next/prev now pointing into other_head,
>>>>>                        corrupts other_head's list and/or drain
>>>>
>>>>
>>>> Kaitao, this scenario seem plausible, could you please comment on it?
>>>
>>> I think bot is correct.
>>> This patch looks buggy.
>>> It seems to me an optimization that breaks the concurrent logic.
>>> May be just drop this patch and reorder the other one, so that bot
>>> sees nonown suffix logic first.
>>
>> This patch is still necessary because it addresses the problem discussed
>> in this thread:
>> https://lore.kernel.org/all/DH846C0P88QU.16YT12I1LXBZM@etsalapatis.com/
>>
>> The patch does have a bug, however. To fix the issues we are seeing now,
>> I propose the additional changes below and would appreciate feedback.
>>
>> --- a/kernel/bpf/helpers.c
>> +++ b/kernel/bpf/helpers.c
>> @@ -2263,8 +2263,10 @@ void bpf_list_head_free(const struct btf_field *field, void *list_head,
>>         if (!head->next || list_empty(head))
>>                 goto unlock;
>>         list_for_each_safe(pos, n, head) {
>> -               WRITE_ONCE(container_of(pos,
>> -                       struct bpf_list_node_kern, list_head)->owner, NULL);
>> +               struct bpf_list_node_kern *node;
>> +
>> +               node = container_of(pos, struct bpf_list_node_kern, list_head);
>> +               WRITE_ONCE(node->owner, BPF_PTR_POISON);
>>                 list_move_tail(pos, &drain);
>>         }
>>  unlock:
>> @@ -2272,8 +2274,12 @@ void bpf_list_head_free(const struct btf_field *field, void *list_head,
>>         __bpf_spin_unlock_irqrestore(spin_lock);
>>
>>         while (!list_empty(&drain)) {
>> +               struct bpf_list_node_kern *node;
>> +
>>                 pos = drain.next;
>> +               node = container_of(pos, struct bpf_list_node_kern, list_head);
>>                 list_del_init(pos);
>> +               WRITE_ONCE(node->owner, NULL);
> 
> I think this still leaves a short race window open.
> Why does the .owner has field to be NULL?
> Can the logic that implies for it to be NULL be extended to accept
> POISON as well?

Here, before setting owner to NULL, list_del_init() has already been
executed, which means the node no longer belongs to any list. This
should match the semantic meaning of owner == NULL.

Do you mean deleting WRITE_ONCE(node->owner, NULL) and preventing
all subsequent __bpf_list_add() operations on this node?

> 
>>                 /* The contained type can also have resources, including a
>>                  * bpf_list_head which needs to be freed.
>>                  */


>> @@ -2481,6 +2487,14 @@ static int __bpf_list_add(struct bpf_list_node_kern *node,
>>         if (unlikely(!h->next))
>>                 INIT_LIST_HEAD(h);
>> 
>> +       /* bpf_list_head_free() marks nodes being detached with BPF_PTR_POISON
>> +        * before list_del_init().  cmpxchg(NULL, POISON) below would fail with
>> +        * that old value and fall into the generic error path, which wrongly
>> +        * calls __bpf_obj_drop_impl().  Reject POISON up front instead.
>> +        */
>> +       if (READ_ONCE(node->owner) == BPF_PTR_POISON)
>> +               return -EINVAL;
>> +

This code block is not needed; I will remove it.

-- 
Thanks
Kaitao Cheng