net: Add l3mdev index to flow struct and avoid oif reset for port devices

The fundamental premise of VRF and l3mdev core code is binding a socket
to a device (l3mdev or netdev with an L3 domain) to indicate L3 scope.
Legacy code resets flowi_oif to the l3mdev losing any original port
device binding. Ben (among others) has demonstrated use cases where the
original port device binding is important and needs to be retained.
This patch handles that by adding a new entry to the common flow struct
that can indicate the l3mdev index for later rule and table matching
avoiding the need to reset flowi_oif.

In addition to allowing more use cases that require port device binds,
this patch brings a few datapath simplications:

1. l3mdev_fib_rule_match is only called when walking fib rules and
   always after l3mdev_update_flow. That allows an optimization to bail
   early for non-VRF type uses cases when flowi_l3mdev is not set. Also,
   only that index needs to be checked for the FIB table id.

2. l3mdev_update_flow can be called with flowi_oif set to a l3mdev
   (e.g., VRF) device. By resetting flowi_oif only for this case the
   FLOWI_FLAG_SKIP_NH_OIF flag is not longer needed and can be removed,
   removing several checks in the datapath. The flowi_iif path can be
   simplified to only be called if the it is not loopback (loopback can
   not be assigned to an L3 domain) and the l3mdev index is not already
   set.

3. Avoid another device lookup in the output path when the fib lookup
   returns a reject failure.

Note: 2 functional tests for local traffic with reject fib rules are
updated to reflect the new direct failure at FIB lookup time for ping
rather than the failure on packet path. The current code fails like this:

    HINT: Fails since address on vrf device is out of device scope
    COMMAND: ip netns exec ns-A ping -c1 -w1 -I eth1 172.16.3.1
    ping: Warning: source address might be selected on device other than: eth1
    PING 172.16.3.1 (172.16.3.1) from 172.16.3.1 eth1: 56(84) bytes of data.

    --- 172.16.3.1 ping statistics ---
    1 packets transmitted, 0 received, 100% packet loss, time 0ms

where the test now directly fails:

    HINT: Fails since address on vrf device is out of device scope
    COMMAND: ip netns exec ns-A ping -c1 -w1 -I eth1 172.16.3.1
    ping: connect: No route to host

Signed-off-by: David Ahern <dsahern@kernel.org>
Tested-by: Ben Greear <greearb@candelatech.com>
Link: https://lore.kernel.org/r/20220314204551.16369-1-dsahern@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

1 files changed, 5 insertions, 1 deletions

diff --git a/include/net/flow.h b/include/net/flow.h
index 58beb16a49b8..987bd511d652 100644
--- a/include/net/flow.h
+++ b/include/net/flow.h
@@ -29,6 +29,7 @@ struct flowi_tunnel {
 struct flowi_common {
 	int	flowic_oif;
 	int	flowic_iif;
+	int     flowic_l3mdev;
 	__u32	flowic_mark;
 	__u8	flowic_tos;
 	__u8	flowic_scope;
@@ -36,7 +37,6 @@ struct flowi_common {
 	__u8	flowic_flags;
 #define FLOWI_FLAG_ANYSRC		0x01
 #define FLOWI_FLAG_KNOWN_NH		0x02
-#define FLOWI_FLAG_SKIP_NH_OIF		0x04
 	__u32	flowic_secid;
 	kuid_t  flowic_uid;
 	struct flowi_tunnel flowic_tun_key;
@@ -70,6 +70,7 @@ struct flowi4 {
 	struct flowi_common	__fl_common;
 #define flowi4_oif		__fl_common.flowic_oif
 #define flowi4_iif		__fl_common.flowic_iif
+#define flowi4_l3mdev		__fl_common.flowic_l3mdev
 #define flowi4_mark		__fl_common.flowic_mark
 #define flowi4_tos		__fl_common.flowic_tos
 #define flowi4_scope		__fl_common.flowic_scope
@@ -102,6 +103,7 @@ static inline void flowi4_init_output(struct flowi4 *fl4, int oif,
 {
 	fl4->flowi4_oif = oif;
 	fl4->flowi4_iif = LOOPBACK_IFINDEX;
+	fl4->flowi4_l3mdev = 0;
 	fl4->flowi4_mark = mark;
 	fl4->flowi4_tos = tos;
 	fl4->flowi4_scope = scope;
@@ -132,6 +134,7 @@ struct flowi6 {
 	struct flowi_common	__fl_common;
 #define flowi6_oif		__fl_common.flowic_oif
 #define flowi6_iif		__fl_common.flowic_iif
+#define flowi6_l3mdev		__fl_common.flowic_l3mdev
 #define flowi6_mark		__fl_common.flowic_mark
 #define flowi6_scope		__fl_common.flowic_scope
 #define flowi6_proto		__fl_common.flowic_proto
@@ -177,6 +180,7 @@ struct flowi {
 	} u;
 #define flowi_oif	u.__fl_common.flowic_oif
 #define flowi_iif	u.__fl_common.flowic_iif
+#define flowi_l3mdev	u.__fl_common.flowic_l3mdev
 #define flowi_mark	u.__fl_common.flowic_mark
 #define flowi_tos	u.__fl_common.flowic_tos
 #define flowi_scope	u.__fl_common.flowic_scope