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

From: Louis DeLosSantos <[email-masked]>
Date: Fri, 26 May 2023 10:11:43 -0400

On Thu, May 25, 2023 at 11:01:34PM -0700, Yonghong Song wrote:
> 
> 
> 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 <[email-masked]>
> > ---
> >   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 */
> >   };
> > 

> 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.

Agreed. 

My initial feelings were there is no real use case to query against the Kernel's
`all` table. 

The response from John will dictate if this remains the case, as the suggestion
of using a new flag bit will nullify this issue, I think.

If it stays tho, I will def add details in the commit message around this on next
rev.