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, [email protected] 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/[email protected]/
>>
>> 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
