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

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Date:   Tue, 15 Mar 2022 13:12:08 +0100
From:   Jakub Sitnicki <jakub@...udflare.com>
To:     wangyufen <wangyufen@...wei.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

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?