netdev - [PATCH 1/2] bpf: add table ID to bpf_fib

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Message-Id: <20230505-bpf-add-tbid-fib-lookup-v1-1-fd99f7162e76@gmail.com>
Date: Thu, 25 May 2023 10:27:59 -0400
From: Louis DeLosSantos <louis.delos.devel@...il.com>
To: bpf@...r.kernel.org, netdev@...r.kernel.org
Cc: Martin KaFai Lau <martin.lau@...ux.dev>, 
 Stanislav Fomichev <sdf@...gle.com>, razor@...ckwall.org, 
 Louis DeLosSantos <louis.delos.devel@...il.com>
Subject: [PATCH 1/2] 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` flag and the
`tbid` field in `struct bpf_fib_lookup` is greater then 0, the `tbid`
field 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@...il.com>
---
 include/uapi/linux/bpf.h       | 17 ++++++++++++++---
 net/core/filter.c              | 12 ++++++++++++
 tools/include/uapi/linux/bpf.h | 17 ++++++++++++++---
 3 files changed, 40 insertions(+), 6 deletions(-)

diff --git a/include/uapi/linux/bpf.h b/include/uapi/linux/bpf.h
index 1bb11a6ee6676..2096fbb328a9b 100644
--- a/include/uapi/linux/bpf.h
+++ b/include/uapi/linux/bpf.h
@@ -3167,6 +3167,8 @@ union bpf_attr {
  *		**BPF_FIB_LOOKUP_DIRECT**
  *			Do a direct table lookup vs full lookup using FIB
  *			rules.
+ *			If *params*->tbid is non-zero, this value designates
+ *			a routing table ID to perform the lookup against.
  *		**BPF_FIB_LOOKUP_OUTPUT**
  *			Perform lookup from an egress perspective (default is
  *			ingress).
@@ -6881,9 +6883,18 @@ 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` flag, a
+		 * specific routing table to use for the fib lookup.
+		 */
+		__u32	tbid;
+	};
+
 	__u8	smac[6];     /* ETH_ALEN */
 	__u8	dmac[6];     /* ETH_ALEN */
 };
diff --git a/net/core/filter.c b/net/core/filter.c
index 451b0ec7f2421..6f710aa0a54b3 100644
--- a/net/core/filter.c
+++ b/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 (params->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 (params->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;
diff --git a/tools/include/uapi/linux/bpf.h b/tools/include/uapi/linux/bpf.h
index 1bb11a6ee6676..2096fbb328a9b 100644
--- a/tools/include/uapi/linux/bpf.h
+++ b/tools/include/uapi/linux/bpf.h
@@ -3167,6 +3167,8 @@ union bpf_attr {
  *		**BPF_FIB_LOOKUP_DIRECT**
  *			Do a direct table lookup vs full lookup using FIB
  *			rules.
+ *			If *params*->tbid is non-zero, this value designates
+ *			a routing table ID to perform the lookup against.
  *		**BPF_FIB_LOOKUP_OUTPUT**
  *			Perform lookup from an egress perspective (default is
  *			ingress).
@@ -6881,9 +6883,18 @@ 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` flag, a
+		 * specific routing table to use for the fib lookup.
+		 */
+		__u32	tbid;
+	};
+
 	__u8	smac[6];     /* ETH_ALEN */
 	__u8	dmac[6];     /* ETH_ALEN */
 };

-- 
2.40.1