Subject: Re: [PATCH bpf-next] bpf,sockmap: fix sk->sk_forward_alloc warn_on in sk_stream_kill_queues

From: Jakub Sitnicki <[email-masked]>
Date: Mon, 30 May 2022 18:37:16 +0200

On Sat, May 28, 2022 at 09:54 AM +08, wangyufen wrote:
> 在 2022/5/28 5:37, Cong Wang 写道:
>> On Tue, May 24, 2022 at 03:53:11PM +0800, Wang Yufen wrote:
>>> During TCP sockmap redirect pressure test, the following warning is triggered:
>>> WARNING: CPU: 3 PID: 2145 at net/core/stream.c:205 sk_stream_kill_queues+0xbc/0xd0
>>> CPU: 3 PID: 2145 Comm: iperf Kdump: loaded Tainted: G        W         5.10.0+ #9
>>> Call Trace:
>>>   inet_csk_destroy_sock+0x55/0x110
>>>   inet_csk_listen_stop+0xbb/0x380
>>>   tcp_close+0x41b/0x480
>>>   inet_release+0x42/0x80
>>>   __sock_release+0x3d/0xa0
>>>   sock_close+0x11/0x20
>>>   __fput+0x9d/0x240
>>>   task_work_run+0x62/0x90
>>>   exit_to_user_mode_prepare+0x110/0x120
>>>   syscall_exit_to_user_mode+0x27/0x190
>>>   entry_SYSCALL_64_after_hwframe+0x44/0xa9
>>>
>>> The reason we observed is that:
>>> When the listener is closing, a connection may have completed the three-way
>>> handshake but not accepted, and the client has sent some packets. The child
>>> sks in accept queue release by inet_child_forget()->inet_csk_destroy_sock(),
>>> but psocks of child sks have not released.
>>>
>> Hm, in this scenario, how does the child socket end up in the sockmap?
>> Clearly user-space does not have a chance to get an fd yet.
>>
>> And, how does your patch work? Since the child sock does not even inheirt
>> the sock proto after clone (see the comments above tcp_bpf_clone()) at
>> all?
>>
>> Thanks.
>> .
> My test cases are as follows:
>
> __section("sockops")
> int bpf_sockmap(struct bpf_sock_ops *skops)
> {
>     switch (skops->op) {
>         case BPF_SOCK_OPS_PASSIVE_ESTABLISHED_CB:
>         case BPF_SOCK_OPS_ACTIVE_ESTABLISHED_CB:
>             ...
>             bpf_sock_hash_update(skops, &sock_ops_map, &key, BPF_NOEXIST);
>             break;
>         ...
> }

Right, when processing the final ACK in tcp_rcv_state_process(), we
invoke the BPF_SOCK_OPS_PASSIVE_ESTABLISHED_CB BPF callback.

This gives a chance to install sockmap sk_prot callbacks.

An accept() without ever calling accept() ;-)

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