netdev - Re: [RFC bpf-next 1/2] net: bridge: add unstable br_fdb_find_port_from

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Date:   Wed, 26 Jan 2022 12:11:18 -0800
From:   Andrii Nakryiko <andrii.nakryiko@...il.com>
To:     Kumar Kartikeya Dwivedi <memxor@...il.com>
Cc:     Lorenzo Bianconi <lorenzo@...nel.org>,
        Toke Høiland-Jørgensen <toke@...hat.com>,
        bpf <bpf@...r.kernel.org>, Networking <netdev@...r.kernel.org>,
        Lorenzo Bianconi <lorenzo.bianconi@...hat.com>,
        "David S. Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        David Ahern <dsahern@...nel.org>,
        Yoshiki Komachi <komachi.yoshiki@...il.com>,
        Jesper Dangaard Brouer <brouer@...hat.com>
Subject: Re: [RFC bpf-next 1/2] net: bridge: add unstable br_fdb_find_port_from_ifindex
 helper

On Wed, Jan 26, 2022 at 4:05 AM Kumar Kartikeya Dwivedi
<memxor@...il.com> wrote:
>
> 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).

Without reading the rest of the thread, bpf_core_type_size(struct
bpf_fdb_lookup) would be a CO-RE-relocatable way to get the actual
size of the type in the kernel.

>
> 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