udp: ipv4: do not waste time in __udp4_lib_mcast_demux_lookup

[ Upstream commit 63c6f81cdde58c41da62a8d8a209592e42a0203e ]

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: 421b3885bf6d5 ("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>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

diff --git a/net/ipv4/udp.c b/net/ipv4/udp.c
index 4468e1adc094a1f6f12eb20cf7c79a9b9187826d..54a5fe92de5a5cf50a6e6d25eba774fd93765f22 100644 (file)
--- a/net/ipv4/udp.c
+++ b/net/ipv4/udp.c
@@ -1834,6 +1834,10 @@ 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;