On 2019/6/14 20:27, Eric Dumazet wrote:
> On Fri, Jun 14, 2019 at 2:35 AM maowenan <maowe...@huawei.com> wrote:
>>
>>
>>
>> On 2019/6/14 12:28, Eric Dumazet wrote:
>>>
>>>
>>> On 6/13/19 9:19 PM, maowenan wrote:
>>>>
>>>>
>>>> @Eric, for this issue I only want to check TCP_NEW_SYN_RECV sk, is it OK
>>>> like below?
>>>> + if (!osk && sk->sk_state == TCP_NEW_SYN_RECV)
>>>> + reqsk = __inet_lookup_established(sock_net(sk),
>>>> &tcp_hashinfo,
>>>> + sk->sk_daddr,
>>>> sk->sk_dport,
>>>> + sk->sk_rcv_saddr,
>>>> sk->sk_num,
>>>> +
>>>> sk->sk_bound_dev_if, sk->sk_bound_dev_if);
>>>> + if (unlikely(reqsk)) {
>>>>
>>>
>>> Not enough.
>>>
>>> If we have many cpus here, there is a chance another cpu has inserted a
>>> request socket, then
>>> replaced it by an ESTABLISH socket for the same 4-tuple.
>>
>> I try to get more clear about the scene you mentioned. And I have do some
>> testing about this, it can work well
>> when I use multiple cpus.
>>
>> The ESTABLISH socket would be from
>> tcp_check_req->tcp_v4_syn_recv_sock->tcp_create_openreq_child,
>> and for this path, inet_ehash_nolisten pass osk(NOT NULL), my patch won't
>> call __inet_lookup_established in inet_ehash_insert().
>>
>> When TCP_NEW_SYN_RECV socket try to inset to hash table, it will pass osk
>> with NULL, my patch will check whether reqsk existed
>> in hash table or not. If reqsk is existed, it just removes this reqsk and
>> dose not insert to hash table. Then the synack for this
>> reqsk can't be sent to client, and there is no chance to receive the ack
>> from client, so ESTABLISH socket can't be replaced in hash table.
>>
>> So I don't see the race when there are many cpus. Can you show me some clue?
>
> This is a bit silly.
> You focus on some crash you got on a given system, but do not see the real
> bug.
>
>
> CPU A
>
> SYN packet
> lookup finds nothing.
> Create a NEW_SYN_RECV
> <long delay, like hardware interrupts calling some buggy driver or something>
I agree that this is a special case.
I propose one point about the sequence of synack, if two synack with two
different
sequence since the time elapse 64ns, this issue disappear.
tcp_conn_request->tcp_v4_init_seq->secure_tcp_seq->seq_scale
static u32 seq_scale(u32 seq)
{
/*
* As close as possible to RFC 793, which
* suggests using a 250 kHz clock.
* Further reading shows this assumes 2 Mb/s networks.
* For 10 Mb/s Ethernet, a 1 MHz clock is appropriate.
* For 10 Gb/s Ethernet, a 1 GHz clock should be ok, but
* we also need to limit the resolution so that the u32 seq
* overlaps less than one time per MSL (2 minutes).
* Choosing a clock of 64 ns period is OK. (period of 274 s)
*/
return seq + (ktime_get_real_ns() >> 6);
}
So if the long delay larger than 64ns, the seq is difference.
>
> CPU B
> SYN packet
> -> inserts a NEW_SYN_RECV sends a SYNACK
> ACK packet
> -> replaces the NEW_SYN_RECV by ESTABLISH socket
>
> CPU A resumes.
> Basically a lookup (after taking the bucket spinlock) could either find :
> - Nothing (typical case where there was no race)
> - A NEW_SYN_RECV
> - A ESTABLISHED socket
> - A TIME_WAIT socket.
>
> You can not simply fix the "NEW_SYN_RECV" state case, and possibly add
> hard crashes (instead of current situation leading to RST packets)
>
> .
>
