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

From: John Fastabend <[email-masked]>
Date: Tue, 15 Mar 2022 22:23:58 -0700

wangyufen wrote:
> 
> 在 2022/3/16 0:25, Daniel Borkmann 写道:
> > On 3/15/22 1:12 PM, Jakub Sitnicki wrote:
> >> 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 <[email-masked]>
> >>>>> ---
> >>>> 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?
> >
> > +1
> >
> > Aside from that, the patch as-is also fails BPF CI in a lot of places, 
> > please
> > make sure to check selftests:
> >
> > https://github.com/kernel-patches/bpf/runs/5537367301?check_suite_focus=true 
> >
> >
> >   [...]
> >   #145/73 sockmap_listen/sockmap IPv6 test_udp_redir:OK
> >   #145/74 sockmap_listen/sockmap IPv6 test_udp_unix_redir:OK
> >   #145/75 sockmap_listen/sockmap Unix test_unix_redir:OK
> >   #145/76 sockmap_listen/sockmap Unix test_unix_redir:OK
> >   ./test_progs:test_ops_cleanup:1424: map_delete: expected 
> > EINVAL/ENOENT: Operation not supported
> >   test_ops_cleanup:FAIL:1424
> >   ./test_progs:test_ops_cleanup:1424: map_delete: expected 
> > EINVAL/ENOENT: Operation not supported
> >   test_ops_cleanup:FAIL:1424
> >   #145/77 sockmap_listen/sockhash IPv4 TCP test_insert_invalid:FAIL
> >   ./test_progs:test_ops_cleanup:1424: map_delete: expected 
> > EINVAL/ENOENT: Operation not supported
> >   test_ops_cleanup:FAIL:1424
> >   ./test_progs:test_ops_cleanup:1424: map_delete: expected 
> > EINVAL/ENOENT: Operation not supported
> >   test_ops_cleanup:FAIL:1424
> >   #145/78 sockmap_listen/sockhash IPv4 TCP test_insert_opened:FAIL
> >   ./test_progs:test_ops_cleanup:1424: map_delete: expected 
> > EINVAL/ENOENT: Operation not supported
> >   test_ops_cleanup:FAIL:1424
> >   ./test_progs:test_ops_cleanup:1424: map_delete: expected 
> > EINVAL/ENOENT: Operation not supported
> >   test_ops_cleanup:FAIL:1424
> >   #145/79 sockmap_listen/sockhash IPv4 TCP test_insert_bound:FAIL
> >   ./test_progs:test_ops_cleanup:1424: map_delete: expected 
> > EINVAL/ENOENT: Operation not supported
> >   test_ops_cleanup:FAIL:1424
> >   ./test_progs:test_ops_cleanup:1424: map_delete: expected 
> > EINVAL/ENOENT: Operation not supported
> >   test_ops_cleanup:FAIL:1424
> >   [...]
> >
> > Thanks,
> > Daniel
> > .
> 
> I'm not sure about this patch. The main purpose is to point out the 
> possible problems
> 
> when the socket is deleted from the map.I'm sorry for the trouble.
> 
> Thanks.

If you want to delete a socket you should flush it first. To do this
stop redirecting traffic to it and then read all the data out. At
the moment its a bit tricky to know when the recieving socket is
empty though. Adding a flag on delete to only delete when the
ingress qlen == 0 might be a possibility if you need delete to
work and are trying to work out how to safely delete sockets.