bpf: Add BPF_FIB_LOOKUP_VLAN_INPUT flag to bpf_fib_lookup() helper

BPF_FIB_LOOKUP_VLAN resolves a VLAN egress. The reverse is also
useful: an XDP program receiving a VLAN-tagged frame on a physical
device wants the lookup to behave as if the packet had arrived on the
corresponding VLAN subinterface, so iif-based policy routing and VRF
table selection use the right ingress.

Add BPF_FIB_LOOKUP_VLAN_INPUT. When set, params->h_vlan_proto and
params->h_vlan_TCI are read as an input VLAN tag and the matching VLAN
device of params->ifindex is resolved with __vlan_find_dev_deep_rcu().
The device must be up and in the same network namespace as
params->ifindex (a VLAN device can be moved to another netns while
registered on its parent; receive would deliver into that other
namespace, which a lookup here cannot represent). If params->ifindex
is itself a VLAN device, its inner (QinQ) subinterface is matched.
For a bond or team, a tag on a port matches no device and returns
NOT_FWDED; pass the master's ifindex.
The lookup then runs with the resolved device as the ingress;
params->ifindex itself is not modified on the input side. When the
resolved device is enslaved to a VRF, both the full lookup (via the
l3mdev rule) and BPF_FIB_LOOKUP_DIRECT (via l3mdev_fib_table_rcu())
select the VRF's table from the resolved ingress. That follows from
feeding the resolved device to the flow as the ingress
(fl4.flowi4_iif = dev->ifindex), which is what makes l3mdev resolve
the VRF master from the subinterface rather than from
params->ifindex.

The two failure classes get different treatment on purpose. A
h_vlan_proto other than 802.1Q/802.1ad is API misuse and returns
-EINVAL, since it would otherwise reach the WARN in vlan_proto_idx()
with a program-controlled value. An unmatched VID, a device that is
down, or one in another namespace is a data outcome and returns
BPF_FIB_LKUP_RET_NOT_FWDED, matching the DIRECT path when
fib_get_table() finds no table and mirroring real ingress, where the
receive path drops such frames. A VID of 0 (a priority tag) is looked
up literally and normally fails the same way; receive instead
processes such frames untagged, so callers should not set the flag for
priority tags. Proceeding on the physical device for any of these
would be fail-open for the policy-routing cases above.

The h_vlan fields share a union with tbid, so the flag cannot be
combined with BPF_FIB_LOOKUP_TBID. It describes ingress, so it also
cannot be combined with BPF_FIB_LOOKUP_OUTPUT. Both combinations
return -EINVAL; restricting now keeps a later relaxation backward
compatible. Combining with BPF_FIB_LOOKUP_VLAN is allowed: the tag is
consumed on the ingress side and the egress tag is written on
success.

Under !CONFIG_VLAN_8021Q the __vlan_find_dev_deep_rcu() stub returns
NULL, so every lookup with a valid proto returns NOT_FWDED, which is
correct since no VLAN device can exist.

Suggested-by: Toke Høiland-Jørgensen <toke@redhat.com>
Signed-off-by: Avinash Duduskar <avinash.duduskar@gmail.com>
Reviewed-by: Toke Høiland-Jørgensen <toke@redhat.com>
Reviewed-by: Emil Tsalapatis <emil@etsalapatis.com>
Link: https://lore.kernel.org/bpf/20260713162305.1237211-3-avinash.duduskar@gmail.com
Signed-off-by: Kumar Kartikeya Dwivedi <memxor@gmail.com>

include/uapi/linux/bpf.h

@@ -3557,6 +3557,22 @@ union bpf_attr {
  *			This flag is only valid for XDP programs; tc programs
  *			receive -EINVAL since they can redirect to the VLAN
  *			device directly.
+ *		**BPF_FIB_LOOKUP_VLAN_INPUT**
+ *			Treat *params*->h_vlan_proto and *params*->h_vlan_TCI
+ *			as an input VLAN tag and run the lookup as if ingress
+ *			had happened on the VLAN subinterface carrying that tag
+ *			on *params*->ifindex. The VID is the low 12 bits of
+ *			*params*->h_vlan_TCI; *params*->h_vlan_proto must be
+ *			ETH_P_8021Q or ETH_P_8021AD in network byte order, else
+ *			**-EINVAL**. If *params*->ifindex is itself a VLAN
+ *			device, its inner (QinQ) subinterface is matched; for a
+ *			bond or team, pass the master's ifindex. An unmatched
+ *			tag, a down device, or one in another namespace returns
+ *			**BPF_FIB_LKUP_RET_NOT_FWDED**, mirroring real ingress.
+ *			A VID of 0 is looked up literally, so do not set this
+ *			flag for priority-tagged frames. Cannot be combined with
+ *			**BPF_FIB_LOOKUP_TBID** or **BPF_FIB_LOOKUP_OUTPUT**
+ *			(returns **-EINVAL**).
  *
  *		*ctx* is either **struct xdp_md** for XDP programs or
  *		**struct sk_buff** tc cls_act programs.
@@ -7365,6 +7381,7 @@ enum {
 	BPF_FIB_LOOKUP_SRC     = (1U << 4),
 	BPF_FIB_LOOKUP_MARK    = (1U << 5),
 	BPF_FIB_LOOKUP_VLAN    = (1U << 6),
+	BPF_FIB_LOOKUP_VLAN_INPUT = (1U << 7),
 };
 
 enum {
@@ -7435,7 +7452,9 @@ struct bpf_fib_lookup {
 			/*
 			 * output with BPF_FIB_LOOKUP_VLAN: set from the
 			 * resolved egress VLAN device (see the flag); zeroed
-			 * on other successful lookups.
+			 * on other successful lookups. input with
+			 * BPF_FIB_LOOKUP_VLAN_INPUT: the VLAN tag to scope
+			 * the lookup by.
 			 */
 			__be16	h_vlan_proto;
 			__be16	h_vlan_TCI;