Subject: Re: [RFC bpf-next 1/2] net: bridge: add unstable br_fdb_find_port_from_ifindex helper

From: Toke Høiland-Jørgensen <[email-masked]>
Date: Mon, 24 Jan 2022 18:50:57 +0100

[ snip to focus on the API ]

> +int br_fdb_find_port_from_ifindex(struct xdp_md *xdp_ctx,
> +				  struct bpf_fdb_lookup *opt,
> +				  u32 opt__sz)
> +{
> +	struct xdp_buff *ctx = (struct xdp_buff *)xdp_ctx;
> +	struct net_bridge_port *port;
> +	struct net_device *dev;
> +	int ret = -ENODEV;
> +
> +	BUILD_BUG_ON(sizeof(struct bpf_fdb_lookup) != NF_BPF_FDB_OPTS_SZ);
> +	if (!opt || opt__sz != sizeof(struct bpf_fdb_lookup))
> +		return -ENODEV;

Why is the BUILD_BUG_ON needed? Or why is the NF_BPF_FDB_OPTS_SZ
constant even needed?

> +	rcu_read_lock();

This is not needed when the function is only being called from XDP...

> +
> +	dev = dev_get_by_index_rcu(dev_net(ctx->rxq->dev), opt->ifindex);
> +	if (!dev)
> +		goto out;
> +
> +	if (unlikely(!netif_is_bridge_port(dev)))
> +		goto out;
> +
> +	port = br_port_get_check_rcu(dev);
> +	if (unlikely(!port || !port->br))
> +		goto out;
> +
> +	dev = __br_fdb_find_port(port->br->dev, opt->addr, opt->vid, true);
> +	if (dev)
> +		ret = dev->ifindex;
> +out:
> +	rcu_read_unlock();
> +
> +	return ret;
> +}
> +
>  struct net_bridge_fdb_entry *br_fdb_find_rcu(struct net_bridge *br,
>  					     const unsigned char *addr,
>  					     __u16 vid)
> diff --git a/net/bridge/br_private.h b/net/bridge/br_private.h
> index 2661dda1a92b..64d4f1727da2 100644
> --- a/net/bridge/br_private.h
> +++ b/net/bridge/br_private.h
> @@ -18,6 +18,7 @@
>  #include <linux/if_vlan.h>
>  #include <linux/rhashtable.h>
>  #include <linux/refcount.h>
> +#include <linux/bpf.h>
>  
>  #define BR_HASH_BITS 8
>  #define BR_HASH_SIZE (1 << BR_HASH_BITS)
> @@ -2094,4 +2095,15 @@ void br_do_proxy_suppress_arp(struct sk_buff *skb, struct net_bridge *br,
>  void br_do_suppress_nd(struct sk_buff *skb, struct net_bridge *br,
>  		       u16 vid, struct net_bridge_port *p, struct nd_msg *msg);
>  struct nd_msg *br_is_nd_neigh_msg(struct sk_buff *skb, struct nd_msg *m);
> +
> +#define NF_BPF_FDB_OPTS_SZ	12
> +struct bpf_fdb_lookup {
> +	u8	addr[ETH_ALEN]; /* ETH_ALEN */
> +	u16	vid;
> +	u32	ifindex;
> +};

It seems like addr and ifindex should always be required, right? So why
not make them regular function args? That way the ptr to eth addr could
be a ptr directly to the packet header (saving a memcpy), and the common
case(?) could just pass a NULL opts struct?

> +int br_fdb_find_port_from_ifindex(struct xdp_md *xdp_ctx,
> +				  struct bpf_fdb_lookup *opt,
> +				  u32 opt__sz);

It should probably be documented that the return value is an ifindex as
well; I guess one of the drawbacks of kfunc's relative to regular
helpers is that there is no convention for how to document their usage -
maybe we should fix that before we get too many of them? :)

-Toke