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

[Kumar Kartikeya Dwivedi <memxor@gmail.com>]

On Wed, Jan 26, 2022 at 05:12:15PM IST, Lorenzo Bianconi wrote:
> > [ 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?
>
> I added it to be symmetric with respect to ct counterpart

But the constant needs to be an enum, not a define, otherwise it will not be
emitted to BTF, I added it so that one could easily check the struct 'version'
(because sizeof is not relocated in BPF programs).

Yes, bpf_core_field_exists and would also work, but the size is fixed anyway and
we need to check it, so it felt better to give it a name and also make it
visible to BPF programs at the same time.

>
>  [...]

--
Kartikeya