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SubjectRe: net: hang in ip_finish_output
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On Mon, 2016-01-18 at 18:20 -0800, Eric Dumazet wrote:

> Same reason really.
>
> Right after sk2=socket(), setsockopt(sk2,...,SO_REUSEPORT, on) and
> bind(sk2, ...), but _before_ the connect(sk2) is done, sk2 is added into
> the soreuseport array, with a score which is smaller than the score of
> first socket sk1 found in hash table (I am speaking of the regular UDP
> hash table), if sk1 had the connect() done, giving a +8 to its score.
>
> So the bug has nothing to do with rcu or rcu_bh, it is just an infinite
> loop caused by different scores.
>
>
> hash bucket [X] -> sk1 -> sk2 -> NULL
>
> sk1 score = 14 (because it did a connect())
> sk2 score = 6
>
> I guess we should relax the test done after atomic_inc_not_zero_hint()
> to only test the base keys :
> (net, ipv6_only_sock, inet->inet_rcv_saddr & inet->inet_num)

One way to fix the issue it to not call reuseport_select_sock() if loop
was restarted, and fallback to the old mechanism : If the optimized
version might have a problem, just fallback to the safe thing.

IPv4 only patch :

diff --git a/net/ipv4/udp.c b/net/ipv4/udp.c
index dc45b538e237..8f1b2a9c90f8 100644
--- a/net/ipv4/udp.c
+++ b/net/ipv4/udp.c
@@ -500,6 +500,7 @@ static struct sock *udp4_lib_lookup2(struct net *net,
struct hlist_nulls_node *node;
int score, badness, matches = 0, reuseport = 0;
u32 hash = 0;
+ bool select_ok = true;

begin:
result = NULL;
@@ -512,14 +513,18 @@ begin:
badness = score;
reuseport = sk->sk_reuseport;
if (reuseport) {
- struct sock *sk2;
hash = udp_ehashfn(net, daddr, hnum,
saddr, sport);
- sk2 = reuseport_select_sock(sk, hash, skb,
+ if (select_ok) {
+ struct sock *sk2;
+
+ sk2 = reuseport_select_sock(sk, hash, skb,
sizeof(struct udphdr));
- if (sk2) {
- result = sk2;
- goto found;
+ if (sk2) {
+ result = sk2;
+ select_ok = false;
+ goto found;
+ }
}
matches = 1;
}
@@ -564,6 +569,7 @@ struct sock *__udp4_lib_lookup(struct net *net, __be32 saddr,
struct udp_hslot *hslot2, *hslot = &udptable->hash[slot];
int score, badness, matches = 0, reuseport = 0;
u32 hash = 0;
+ bool select_ok = true;

rcu_read_lock();
if (hslot->count > 10) {
@@ -601,14 +607,18 @@ begin:
badness = score;
reuseport = sk->sk_reuseport;
if (reuseport) {
- struct sock *sk2;
hash = udp_ehashfn(net, daddr, hnum,
saddr, sport);
- sk2 = reuseport_select_sock(sk, hash, skb,
+ if (select_ok) {
+ struct sock *sk2;
+
+ sk2 = reuseport_select_sock(sk, hash, skb,
sizeof(struct udphdr));
- if (sk2) {
- result = sk2;
- goto found;
+ if (sk2) {
+ select_ok = false;
+ result = sk2;
+ goto found;
+ }
}
matches = 1;
}




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