[PATCH net] ipv4/route: do not check saddr dev if iif is LOOPBACK_IFINDEX

[Hangbin Liu]

Jianlin reported a bug that for IPv4, ip route get from src_addr would fail
if src_addr is not an address on local system.

\# ip route get 1.1.1.1 from 2.2.2.2
RTNETLINK answers: Invalid argument

While IPv6 would get the default route

\# ip -6 route get 1111::1 from 2222::2
1111::1 from 2222::2 via fe80:52:0:4982::1fe dev eth0 proto ra src \
        2620::2cf:e2ff:fe40:37 metric 1024 hoplimit 64 pref medium

Fix it by disabling the __ip_dev_find() check if iif is LOOPBACK_IFINDEX.

Reported-by: Jianlin Shi <ji...@redhat.com>
Signed-off-by: Hangbin Liu <liuhang...@gmail.com>
---
 net/ipv4/route.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/net/ipv4/route.c b/net/ipv4/route.c
index 517300d587a7..1be78e8f9e3c 100644
--- a/net/ipv4/route.c
+++ b/net/ipv4/route.c
@@ -2512,7 +2512,8 @@ struct rtable *ip_route_output_key_hash_rcu(struct net 
*net, struct flowi4 *fl4,
                        goto make_route;
                }
 
-               if (!(fl4->flowi4_flags & FLOWI_FLAG_ANYSRC)) {
+               if (!(fl4->flowi4_flags & FLOWI_FLAG_ANYSRC) &&
+                   fl4->flowi4_iif != LOOPBACK_IFINDEX) {
                        /* It is equivalent to inet_addr_type(saddr) == 
RTN_LOCAL */
                        if (!__ip_dev_find(net, fl4->saddr, false))
                                goto out;
-- 
2.19.2