Re: [PATCH 1/2] bpf: add table ID to bpf_fib_lookup BPF helper

[Yonghong Song <yhs@meta.com>]

On 5/25/23 7:27 AM, Louis DeLosSantos wrote:
> 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.

I think table id 0 is legal in the kernel, right?
It is probably okay to consider table id 0 not supported to
simplify the user interface. But it would be great to
add some explanations in the commit message.

> 
> 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>
> ---
>   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 */
>   };
>