Re: [PATCH net v2] tcp: avoid creating multiple req socks with the same tuples

From: Eric Dumazet
Date: Fri Jun 14 2019 - 08:32:33 EST

On Fri, Jun 14, 2019 at 2:35 AM maowenan <maowenan@xxxxxxxxxx> 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.


SYN packet
lookup finds nothing.
<long delay, like hardware interrupts calling some buggy driver or something>

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 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)