Question: Move BPF_SK_LOOKUP ahead of connected UDP sk lookup?

[Philo Lu <lulie@linux.alibaba.com>]

Hi all, I wonder if it is feasible to move BPF_SK_LOOKUP ahead of 
connected UDP sk lookup?

That is something like:
(i.e., move connected udp socket lookup behind bpf sk lookup prog)
```
diff --git a/net/ipv4/udp.c b/net/ipv4/udp.c
index ddb86baaea6c8..9a1408775bcb1 100644
--- a/net/ipv4/udp.c
+++ b/net/ipv4/udp.c
@@ -493,13 +493,6 @@ struct sock *__udp4_lib_lookup(const struct net 
*net, __be32 saddr,
         slot2 = hash2 & udptable->mask;
         hslot2 = &udptable->hash2[slot2];

-       /* Lookup connected or non-wildcard socket */
-       result = udp4_lib_lookup2(net, saddr, sport,
-                                 daddr, hnum, dif, sdif,
-                                 hslot2, skb);
-       if (!IS_ERR_OR_NULL(result) && result->sk_state == TCP_ESTABLISHED)
-               goto done;
-
         /* Lookup redirect from BPF */
         if (static_branch_unlikely(&bpf_sk_lookup_enabled) &&
             udptable == net->ipv4.udp_table) {
@@ -512,6 +505,13 @@ struct sock *__udp4_lib_lookup(const struct net 
*net, __be32 saddr,
                 }
         }

+       /* Lookup connected or non-wildcard socket */
+       result = udp4_lib_lookup2(net, saddr, sport,
+                                 daddr, hnum, dif, sdif,
+                                 hslot2, skb);
+       if (!IS_ERR_OR_NULL(result) && result->sk_state == TCP_ESTABLISHED)
+               goto done;
+
         /* Got non-wildcard socket or error on first lookup */
         if (result)
                 goto done;
```

This will be useful, e.g., if there are many concurrent udp sockets of a 
same ip:port, where udp4_lib_lookup2() may induce high softirq overhead, 
because it computes score for all sockets of the ip:port. With bpf 
sk_lookup prog, we can implement 4-tuple hash for udp socket lookup to 
solve the problem (if bpf prog runs before udp4_lib_lookup2).

Currently, in udp, bpf sk lookup runs after connected socket lookup. 
IIUC, this is because the early version of SK_LOOKUP[0] modified 
local_ip/local_port to redirect socket. This may interact wrongly with 
udp lookup because udp uses score to select socket, and setting 
local_ip/local_port cannot guarantee the result socket selected. 
However, now we get socket directly from map in bpf sk_lookup prog, so 
the above problem no longer exists.

So is there any other problem on it？Or I'll try to work on it and commit 
patches later.

[0]https://lore.kernel.org/bpf/20190618130050.8344-1-jakub@cloudflare.com/

Thank you for your time.
-- 
Philo