Messages in this thread Patch in this message |  | | From | Jason Xing <> | Date | Sat, 14 Jan 2023 20:05:44 +0800 | Subject | Re: [PATCH net] tcp: avoid the lookup process failing to get sk in ehash table |
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On Sat, Jan 14, 2023 at 5:45 PM Eric Dumazet <edumazet@google.com> wrote: > > On Thu, Jan 12, 2023 at 7:54 AM Jason Xing <kerneljasonxing@gmail.com> wrote: > > > > From: Jason Xing <kernelxing@tencent.com> > > > > While one cpu is working on looking up the right socket from ehash > > table, another cpu is done deleting the request socket and is about > > to add (or is adding) the big socket from the table. It means that > > we could miss both of them, even though it has little chance. > > > > Let me draw a call trace map of the server side. > > CPU 0 CPU 1 > > ----- ----- > > tcp_v4_rcv() syn_recv_sock() > > inet_ehash_insert() > > -> sk_nulls_del_node_init_rcu(osk) > > __inet_lookup_established() > > -> __sk_nulls_add_node_rcu(sk, list) > > > > Notice that the CPU 0 is receiving the data after the final ack > > during 3-way shakehands and CPU 1 is still handling the final ack. > > > > Why could this be a real problem? > > This case is happening only when the final ack and the first data > > receiving by different CPUs. Then the server receiving data with > > ACK flag tries to search one proper established socket from ehash > > table, but apparently it fails as my map shows above. After that, > > the server fetches a listener socket and then sends a RST because > > it finds a ACK flag in the skb (data), which obeys RST definition > > in RFC 793. > > > > Many thanks to Eric for great help from beginning to end. > > > > Fixes: 5e0724d027f0 ("tcp/dccp: fix hashdance race for passive sessions") > > Signed-off-by: Jason Xing <kernelxing@tencent.com> > > --- > > net/ipv4/inet_hashtables.c | 10 ++++++++++ > > 1 file changed, 10 insertions(+) > > > > diff --git a/net/ipv4/inet_hashtables.c b/net/ipv4/inet_hashtables.c > > index 24a38b56fab9..18f88cb4efcb 100644 > > --- a/net/ipv4/inet_hashtables.c > > +++ b/net/ipv4/inet_hashtables.c > > @@ -650,7 +650,16 @@ bool inet_ehash_insert(struct sock *sk, struct sock *osk, bool *found_dup_sk) > > spin_lock(lock); > > if (osk) { > > WARN_ON_ONCE(sk->sk_hash != osk->sk_hash); > > + if (sk_hashed(osk)) > > + /* Before deleting the node, we insert a new one to make > > + * sure that the look-up=sk process would not miss either > > + * of them and that at least one node would exist in ehash > > + * table all the time. Otherwise there's a tiny chance > > + * that lookup process could find nothing in ehash table. > > + */ > > + __sk_nulls_add_node_rcu(sk, list); > > In our private email exchange, I suggested to insert sk at the _tail_ > of the hash bucket. >
Yes, I noticed that. At that time I kept considering the race condition of the RCU itself, not the scene you mentioned as below.
> Inserting it at the _head_ would still leave a race condition, because > a concurrent reader might > have already started the bucket traversal, and would not see 'sk'.
Thanks for the detailed explanation. Now I see why. I'll replace it with __sk_nulls_add_node_tail_rcu() function and send the v2 patch.
By the way, I checked the removal of TIMEWAIT socket which is included in this patch. I write down the call-trace: inet_hash_connect() -> __inet_hash_connect() -> if (sk_unhashed(sk)) { inet_ehash_nolisten(sk, (struct sock *)tw, NULL); -> inet_ehash_insert(sk, osk, found_dup_sk); Therefore, this patch covers the timewait case.
Thanks, Jason
> > Thanks. > > > ret = sk_nulls_del_node_init_rcu(osk); > > + goto unlock; > > } else if (found_dup_sk) { > > *found_dup_sk = inet_ehash_lookup_by_sk(sk, list); > > if (*found_dup_sk) > > @@ -660,6 +669,7 @@ bool inet_ehash_insert(struct sock *sk, struct sock *osk, bool *found_dup_sk) > > if (ret) > > __sk_nulls_add_node_rcu(sk, list); > > > > +unlock: > > spin_unlock(lock); > > > > return ret; > > -- > > 2.37.3 > >
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