netdev - Re: [PATCH bpf-next] bpf, sockmap: Manual deletion of sockmap elements in user mode is not allowed

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Date:   Wed, 16 Mar 2022 11:25:01 +0800
From:   wangyufen <wangyufen@...wei.com>
To:     Jakub Sitnicki <jakub@...udflare.com>
CC:     <ast@...nel.org>, <john.fastabend@...il.com>,
        <daniel@...earbox.net>, <lmb@...udflare.com>,
        <davem@...emloft.net>, <kafai@...com>, <dsahern@...nel.org>,
        <kuba@...nel.org>, <songliubraving@...com>, <yhs@...com>,
        <kpsingh@...nel.org>, <netdev@...r.kernel.org>,
        <bpf@...r.kernel.org>
Subject: Re: [PATCH bpf-next] bpf, sockmap: Manual deletion of sockmap
 elements in user mode is not allowed


在 2022/3/15 20:12, Jakub Sitnicki 写道:
> On Tue, Mar 15, 2022 at 03:24 PM +08, wangyufen wrote:
>> 在 2022/3/14 23:30, Jakub Sitnicki 写道:
>>> On Mon, Mar 14, 2022 at 08:44 PM +08, Wang Yufen wrote:
>>>> A tcp socket in a sockmap. If user invokes bpf_map_delete_elem to delete
>>>> the sockmap element, the tcp socket will switch to use the TCP protocol
>>>> stack to send and receive packets. The switching process may cause some
>>>> issues, such as if some msgs exist in the ingress queue and are cleared
>>>> by sk_psock_drop(), the packets are lost, and the tcp data is abnormal.
>>>>
>>>> Signed-off-by: Wang Yufen <wangyufen@...wei.com>
>>>> ---
>>> Can you please tell us a bit more about the life-cycle of the socket in
>>> your workload? Questions that come to mind:
>>>
>>> 1) What triggers the removal of the socket from sockmap in your case?
>> We use sk_msg to redirect with sock hash, like this:
>>
>>   skA   redirect    skB
>>   Tx <-----------> skB,Rx
>>
>> And construct a scenario where the packet sending speed is high, the
>> packet receiving speed is slow, so the packets are stacked in the ingress
>> queue on the receiving side. In this case, if run bpf_map_delete_elem() to
>> delete the sockmap entry, will trigger the following procedure:
>>
>> sock_hash_delete_elem()
>>    sock_map_unref()
>>      sk_psock_put()
>>        sk_psock_drop()
>>          sk_psock_stop()
>>            __sk_psock_zap_ingress()
>>              __sk_psock_purge_ingress_msg()
>>
>>> 2) Would it still be a problem if removal from sockmap did not cause any
>>> packets to get dropped?
>> Yes, it still be a problem. If removal from sockmap  did not cause any
>> packets to get dropped, packet receiving process switches to use TCP
>> protocol stack. The packets in the psock ingress queue cannot be received
>>
>> by the user.
> Thanks for the context. So, if I understand correctly, you want to avoid
> breaking the network pipe by updating the sockmap from user-space.
>
> This sounds awfully similar to BPF_MAP_FREEZE. Have you considered that?
> .
Sorry, I didn't notice this. I used BPF_MAP_FREEZE to verify, can solve 
my problem, thanks.