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Add ability to specify routing table ID to the `bpf_fib_lookup` BPF helper. A new field `tbid` is added to `struct bpf_fib_lookup` used as parameters to the `bpf_fib_lookup` BPF helper. When the helper is called with the `BPF_FIB_LOOKUP_DIRECT` and `BPF_FIB_LOOKUP_TBID` flags the `tbid` field in `struct bpf_fib_lookup` will be used as the table ID for the fib lookup. If the `tbid` does not exist the fib lookup will fail with `BPF_FIB_LKUP_RET_NOT_FWDED`. The `tbid` field becomes a union over the vlan related output fields in `struct bpf_fib_lookup` and will be zeroed immediately after usage. This functionality is useful in containerized environments. For instance, if a CNI wants to dictate the next-hop for traffic leaving a container it can create a container-specific routing table and perform a fib lookup against this table in a "host-net-namespace-side" TC program. This functionality also allows `ip rule` like functionality at the TC layer, allowing an eBPF program to pick a routing table based on some aspect of the sk_buff. As a concrete use case, this feature will be used in Cilium's SRv6 L3VPN datapath. When egress traffic leaves a Pod an eBPF program attached by Cilium will determine which VRF the egress traffic should target, and then perform a FIB lookup in a specific table representing this VRF's FIB. Signed-off-by: Louis DeLosSantos <louis.delos.devel@gmail.com> Signed-off-by: Daniel Borkmann <daniel@iogearbox.net> Link: https://lore.kernel.org/bpf/20230505-bpf-add-tbid-fib-lookup-v2-1-0a31c22c748c@gmail.com