ipv4: fib: Don't ignore error route in local/main tables.

[ Upstream commit b72f0db ]

When CONFIG_IP_MULTIPLE_TABLES is enabled but no rule is added,
fib_lookup() performs route lookup directly on two tables.

Since the first lookup does not properly bail out, the result
of an error route in the merged local/main table could be
overwritten by another route in the default table:

  # unshare -n
  # ip link set lo up
  # ip route add 192.168.0.0/24 dev lo table 253
  # ip route add unreachable 192.168.0.0/24
  # ip route get 192.168.0.1
  192.168.0.1 dev lo table default uid 0
      cache <local>

Once a random rule is added, the error route is respected:

  # ip rule add table 0
  # ip rule del table 0
  # ip route get 192.168.0.1
  RTNETLINK answers: No route to host

Let's fix the inconsistent behaviour.

Fixes: f4530fa ("ipv4: Avoid overhead when no custom FIB rules are installed.")
Signed-off-by: Kuniyuki Iwashima <kuniyu@google.com>
Reviewed-by: Ido Schimmel <idosch@nvidia.com>
Link: https://patch.msgid.link/20260619212753.3367244-1-kuniyu@google.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>

Author: q2ven

include/net/ip_fib.h

@@ -370,7 +370,7 @@ static inline int fib_lookup(struct net *net, struct flowi4 *flp,
 			     struct fib_result *res, unsigned int flags)
 {
 	struct fib_table *tb;
-	int err = -ENETUNREACH;
+	int err = -EAGAIN;
 
 	flags |= FIB_LOOKUP_NOREF;
 	if (net->ipv4.fib_has_custom_rules)
@@ -384,17 +384,16 @@ static inline int fib_lookup(struct net *net, struct flowi4 *flp,
 	if (tb)
 		err = fib_table_lookup(tb, flp, res, flags);
 
-	if (!err)
+	if (err != -EAGAIN)
 		goto out;
 
 	tb = rcu_dereference_rtnl(net->ipv4.fib_default);
 	if (tb)
 		err = fib_table_lookup(tb, flp, res, flags);
 
-out:
 	if (err == -EAGAIN)
 		err = -ENETUNREACH;
-
+out:
 	rcu_read_unlock();
 
 	return err;