udp: ipv4: do not waste time in __udp4_lib_mcast_demux_lookup

Its too easy to add thousand of UDP sockets on a particular bucket,
and slow down an innocent multicast receiver.

Early demux is supposed to be an optimization, we should avoid spending
too much time in it.

It is interesting to note __udp4_lib_demux_lookup() only tries to
match first socket in the chain.

10 is the threshold we already have in __udp4_lib_lookup() to switch
to secondary hash.

Fixes: c8cbefced7faa ("udp: ipv4: Add udp early demux")
Signed-off-by: Eric Dumazet <edumazet@google.com>
Reported-by: David Held <drheld@google.com>
Cc: Shawn Bohrer <sbohrer@rgmadvisors.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

diff --git a/net/ipv4/udp.c b/net/ipv4/udp.c
index 185ed3e598027a475b6ed62c4a0e9f9838793f4d..d92f94b7e4025dd4779e75e6a75f2de560713778 100644 (file)
--- a/net/ipv4/udp.c
+++ b/net/ipv4/udp.c
@@ -1861,6 +1861,10 @@ static struct sock *__udp4_lib_mcast_demux_lookup(struct net *net,
        unsigned int count, slot = udp_hashfn(net, hnum, udp_table.mask);
        struct udp_hslot *hslot = &udp_table.hash[slot];
 
+       /* Do not bother scanning a too big list */
+       if (hslot->count > 10)
+               return NULL;
+
        rcu_read_lock();
 begin:
        count = 0;