[NEIGH]: Fix IP-over-ATM and ARP interaction.

The classical IP over ATM code maintains its own IPv4 <-> <ATM stuff>
ARP table, using the standard neighbour-table code. The
neigh_table_init function adds this neighbour table to a linked list
of all neighbor tables which is used by the functions neigh_delete()
neigh_add() and neightbl_set(), all called by the netlink code.

Once the ATM neighbour table is added to the list, there are two
tables with family == AF_INET there, and ARP entries sent via netlink
go into the first table with matching family. This is indeterminate
and often wrong.

To see the bug, on a kernel with CLIP enabled, create a standard IPv4
ARP entry by pinging an unused address on a local subnet. Then attempt
to complete that entry by doing

ip neigh replace <ip address> lladdr <some mac address> nud reachable

Looking at the ARP tables by using 

ip neigh show

will reveal two ARP entries for the same address. One of these can be
found in /proc/net/arp, and the other in /proc/net/atm/arp.

This patch adds a new function, neigh_table_init_no_netlink() which
does everything the neigh_table_init() does, except add the table to
the netlink all-arp-tables chain. In addition neigh_table_init() has a
check that all tables on the chain have a distinct address family.
The init call in clip.c is changed to call
neigh_table_init_no_netlink().

Since ATM ARP tables are rather more complicated than can currently be
handled by the available rtattrs in the netlink protocol, no
functionality is lost by this patch, and non-ATM ARP manipulation via
netlink is rescued. A more complete solution would involve a rtattr
for ATM ARP entries and some way for the netlink code to give
neigh_add and friends more information than just address family with
which to find the correct ARP table.

[ I've changed the assertion checking in neigh_table_init() to not
  use BUG_ON() while holding neigh_tbl_lock.  Instead we remember that
  we found an existing tbl with the same family, and after dropping
  the lock we'll give a diagnostic kernel log message and a stack dump.
  -DaveM ]
Signed-off-by: Simon Kelley <simon@thekelleys.org.uk>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/net/neighbour.h

@@ -211,6 +211,7 @@ struct neigh_table
 #define NEIGH_UPDATE_F_ADMIN			0x80000000
 
 extern void			neigh_table_init(struct neigh_table *tbl);
+extern void			neigh_table_init_no_netlink(struct neigh_table *tbl);
 extern int			neigh_table_clear(struct neigh_table *tbl);
 extern struct neighbour *	neigh_lookup(struct neigh_table *tbl,
 					     const void *pkey,

net/atm/clip.c

@@ -963,7 +963,7 @@ static struct file_operations arp_seq_fops = {
 static int __init atm_clip_init(void)
 {
 	struct proc_dir_entry *p;
-	neigh_table_init(&clip_tbl);
+	neigh_table_init_no_netlink(&clip_tbl);
 
 	clip_tbl_hook = &clip_tbl;
 	register_atm_ioctl(&clip_ioctl_ops);

net/core/neighbour.c

@@ -1326,8 +1326,7 @@ void neigh_parms_destroy(struct neigh_parms *parms)
 	kfree(parms);
 }
 
-
-void neigh_table_init(struct neigh_table *tbl)
+void neigh_table_init_no_netlink(struct neigh_table *tbl)
 {
 	unsigned long now = jiffies;
 	unsigned long phsize;
@@ -1383,10 +1382,27 @@ void neigh_table_init(struct neigh_table *tbl)
 
 	tbl->last_flush = now;
 	tbl->last_rand	= now + tbl->parms.reachable_time * 20;
+}
+
+void neigh_table_init(struct neigh_table *tbl)
+{
+	struct neigh_table *tmp;
+
+	neigh_table_init_no_netlink(tbl);
 	write_lock(&neigh_tbl_lock);
+	for (tmp = neigh_tables; tmp; tmp = tmp->next) {
+		if (tmp->family == tbl->family)
+			break;
+	}
 	tbl->next	= neigh_tables;
 	neigh_tables	= tbl;
 	write_unlock(&neigh_tbl_lock);
+
+	if (unlikely(tmp)) {
+		printk(KERN_ERR "NEIGH: Registering multiple tables for "
+		       "family %d\n", tbl->family);
+		dump_stack();
+	}
 }
 
 int neigh_table_clear(struct neigh_table *tbl)
@@ -2657,6 +2673,7 @@ EXPORT_SYMBOL(neigh_rand_reach_time);
 EXPORT_SYMBOL(neigh_resolve_output);
 EXPORT_SYMBOL(neigh_table_clear);
 EXPORT_SYMBOL(neigh_table_init);
+EXPORT_SYMBOL(neigh_table_init_no_netlink);
 EXPORT_SYMBOL(neigh_update);
 EXPORT_SYMBOL(neigh_update_hhs);
 EXPORT_SYMBOL(pneigh_enqueue);