bpf: Add table ID to bpf_fib_lookup BPF helper

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

include/uapi/linux/bpf.h

@@ -3177,6 +3177,10 @@ union bpf_attr {
  *		**BPF_FIB_LOOKUP_DIRECT**
  *			Do a direct table lookup vs full lookup using FIB
  *			rules.
+ *		**BPF_FIB_LOOKUP_TBID**
+ *			Used with BPF_FIB_LOOKUP_DIRECT.
+ *			Use the routing table ID present in *params*->tbid
+ *			for the fib lookup.
  *		**BPF_FIB_LOOKUP_OUTPUT**
  *			Perform lookup from an egress perspective (default is
  *			ingress).
@@ -6831,6 +6835,7 @@ enum {
 	BPF_FIB_LOOKUP_DIRECT  = (1U << 0),
 	BPF_FIB_LOOKUP_OUTPUT  = (1U << 1),
 	BPF_FIB_LOOKUP_SKIP_NEIGH = (1U << 2),
+	BPF_FIB_LOOKUP_TBID    = (1U << 3),
 };
 
 enum {
@@ -6891,9 +6896,19 @@ struct bpf_fib_lookup {
 		__u32		ipv6_dst[4];  /* in6_addr; network order */
 	};
 
-	/* output */
-	__be16	h_vlan_proto;
-	__be16	h_vlan_TCI;
+	union {
+		struct {
+			/* output */
+			__be16	h_vlan_proto;
+			__be16	h_vlan_TCI;
+		};
+		/* input: when accompanied with the
+		 * 'BPF_FIB_LOOKUP_DIRECT | BPF_FIB_LOOKUP_TBID` flags, a
+		 * specific routing table to use for the fib lookup.
+		 */
+		__u32	tbid;
+	};
+
 	__u8	smac[6];     /* ETH_ALEN */
 	__u8	dmac[6];     /* ETH_ALEN */
 };

net/core/filter.c

@@ -5803,6 +5803,12 @@ static int bpf_ipv4_fib_lookup(struct net *net, struct bpf_fib_lookup *params,
 		u32 tbid = l3mdev_fib_table_rcu(dev) ? : RT_TABLE_MAIN;
 		struct fib_table *tb;
 
+		if (flags & BPF_FIB_LOOKUP_TBID) {
+			tbid = params->tbid;
+			/* zero out for vlan output */
+			params->tbid = 0;
+		}
+
 		tb = fib_get_table(net, tbid);
 		if (unlikely(!tb))
 			return BPF_FIB_LKUP_RET_NOT_FWDED;
@@ -5936,6 +5942,12 @@ static int bpf_ipv6_fib_lookup(struct net *net, struct bpf_fib_lookup *params,
 		u32 tbid = l3mdev_fib_table_rcu(dev) ? : RT_TABLE_MAIN;
 		struct fib6_table *tb;
 
+		if (flags & BPF_FIB_LOOKUP_TBID) {
+			tbid = params->tbid;
+			/* zero out for vlan output */
+			params->tbid = 0;
+		}
+
 		tb = ipv6_stub->fib6_get_table(net, tbid);
 		if (unlikely(!tb))
 			return BPF_FIB_LKUP_RET_NOT_FWDED;
@@ -6008,7 +6020,7 @@ static int bpf_ipv6_fib_lookup(struct net *net, struct bpf_fib_lookup *params,
 #endif
 
 #define BPF_FIB_LOOKUP_MASK (BPF_FIB_LOOKUP_DIRECT | BPF_FIB_LOOKUP_OUTPUT | \
-			     BPF_FIB_LOOKUP_SKIP_NEIGH)
+			     BPF_FIB_LOOKUP_SKIP_NEIGH | BPF_FIB_LOOKUP_TBID)
 
 BPF_CALL_4(bpf_xdp_fib_lookup, struct xdp_buff *, ctx,
 	   struct bpf_fib_lookup *, params, int, plen, u32, flags)

tools/include/uapi/linux/bpf.h

@@ -3177,6 +3177,10 @@ union bpf_attr {
  *		**BPF_FIB_LOOKUP_DIRECT**
  *			Do a direct table lookup vs full lookup using FIB
  *			rules.
+ *		**BPF_FIB_LOOKUP_TBID**
+ *			Used with BPF_FIB_LOOKUP_DIRECT.
+ *			Use the routing table ID present in *params*->tbid
+ *			for the fib lookup.
  *		**BPF_FIB_LOOKUP_OUTPUT**
  *			Perform lookup from an egress perspective (default is
  *			ingress).
@@ -6831,6 +6835,7 @@ enum {
 	BPF_FIB_LOOKUP_DIRECT  = (1U << 0),
 	BPF_FIB_LOOKUP_OUTPUT  = (1U << 1),
 	BPF_FIB_LOOKUP_SKIP_NEIGH = (1U << 2),
+	BPF_FIB_LOOKUP_TBID    = (1U << 3),
 };
 
 enum {
@@ -6891,9 +6896,19 @@ struct bpf_fib_lookup {
 		__u32		ipv6_dst[4];  /* in6_addr; network order */
 	};
 
-	/* output */
-	__be16	h_vlan_proto;
-	__be16	h_vlan_TCI;
+	union {
+		struct {
+			/* output */
+			__be16	h_vlan_proto;
+			__be16	h_vlan_TCI;
+		};
+		/* input: when accompanied with the
+		 * 'BPF_FIB_LOOKUP_DIRECT | BPF_FIB_LOOKUP_TBID` flags, a
+		 * specific routing table to use for the fib lookup.
+		 */
+		__u32	tbid;
+	};
+
 	__u8	smac[6];     /* ETH_ALEN */
 	__u8	dmac[6];     /* ETH_ALEN */
 };