Re: [BUG] VPN broken in net-next

[David Miller <davem@davemloft.net>]

From: Julian Anastasov <ja@ssi.bg>
Date: Fri, 4 Mar 2011 10:39:55 +0200 (EET)

> On Thu, 3 Mar 2011, David Miller wrote:
> 
>> I suspect that even if we need to handle prefixes, we can still use
>> the hash for optimistic lookup, and fallback to a local table FIB
>> inspection if that fails.
> 
> 	Yes, as ip_route_output_slow uses __ip_dev_find for
> fl4_src there should be some kind of fallback to local table,
> so that traffic from 127.0.0.2 to 127.0.0.3 or other local
> subnets on loopback can work. Another option is to use
> inet_addr_onlink but I suspect people can add many addresses
> on loopback: inet_addr_onlink(loopback_indev, addr, 0)

I just got back to this, sorry for taking so long :-)

Here is the patch I've come up with and will commit to
net-2.6, thanks!

--------------------
ipv4: Fallback to FIB local table in __ip_dev_find().

In commit 9435eb1cf0b76b323019cebf8d16762a50a12a19
("ipv4: Implement __ip_dev_find using new interface address hash.")
we reimplemented __ip_dev_find() so that it doesn't have to
do a full FIB table lookup.

Instead, it consults a hash table of addresses configured to
interfaces.

This works identically to the old code in all except one case,
and that is for loopback subnets.

The old code would match the loopback device for any IP address
that falls within a subnet configured to the loopback device.

Handle this corner case by doing the FIB lookup.

We could implement this via inet_addr_onlink() but:

1) Someone could configure many addresses to loopback and
   inet_addr_onlink() is a simple list traversal.

2) We know the old code works.

Reported-by: Julian Anastasov <ja@ssi.bg>
Signed-off-by: David S. Miller <davem@davemloft.net>
---
 net/ipv4/devinet.c |   16 ++++++++++++++++
 1 files changed, 16 insertions(+), 0 deletions(-)

diff --git a/net/ipv4/devinet.c b/net/ipv4/devinet.c
index d5a4553..5345b0b 100644
--- a/net/ipv4/devinet.c
+++ b/net/ipv4/devinet.c
@@ -64,6 +64,8 @@
 #include <net/rtnetlink.h>
 #include <net/net_namespace.h>
 
+#include "fib_lookup.h"
+
 static struct ipv4_devconf ipv4_devconf = {
 	.data = {
 		[IPV4_DEVCONF_ACCEPT_REDIRECTS - 1] = 1,
@@ -151,6 +153,20 @@ struct net_device *__ip_dev_find(struct net *net, __be32 addr, bool devref)
 			break;
 		}
 	}
+	if (!result) {
+		struct flowi4 fl4 = { .daddr = addr };
+		struct fib_result res = { 0 };
+		struct fib_table *local;
+
+		/* Fallback to FIB local table so that communication
+		 * over loopback subnets work.
+		 */
+		local = fib_get_table(net, RT_TABLE_LOCAL);
+		if (local &&
+		    !fib_table_lookup(local, &fl4, &res, FIB_LOOKUP_NOREF) &&
+		    res.type == RTN_LOCAL)
+			result = FIB_RES_DEV(res);
+	}
 	if (result && devref)
 		dev_hold(result);
 	rcu_read_unlock();
-- 
1.7.4.1