netdev - Re: [xdp-hints] Re: [PATCH bpf-next 06/11] xdp: Carry over xdp metadata into skb context

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Date:   Mon, 21 Nov 2022 10:47:44 -0800
From:   Jakub Kicinski <kuba@...nel.org>
To:     Stanislav Fomichev <sdf@...gle.com>
Cc:     Toke Høiland-Jørgensen <toke@...hat.com>,
        Jesper Dangaard Brouer <jbrouer@...hat.com>,
        bpf@...r.kernel.org, brouer@...hat.com, ast@...nel.org,
        daniel@...earbox.net, andrii@...nel.org, martin.lau@...ux.dev,
        song@...nel.org, yhs@...com, john.fastabend@...il.com,
        kpsingh@...nel.org, haoluo@...gle.com, jolsa@...nel.org,
        David Ahern <dsahern@...il.com>,
        Willem de Bruijn <willemb@...gle.com>,
        Anatoly Burakov <anatoly.burakov@...el.com>,
        Alexander Lobakin <alexandr.lobakin@...el.com>,
        Magnus Karlsson <magnus.karlsson@...il.com>,
        Maryam Tahhan <mtahhan@...hat.com>, xdp-hints@...-project.net,
        netdev@...r.kernel.org
Subject: Re: [xdp-hints] Re: [PATCH bpf-next 06/11] xdp: Carry over xdp
 metadata into skb context

On Mon, 21 Nov 2022 09:53:02 -0800 Stanislav Fomichev wrote:
> > Jakub was objecting to putting it in the UAPI header, but didn't we
> > already agree that this wasn't necessary?
> >
> > I.e., if we just define
> >
> > struct xdp_skb_metadata *bpf_xdp_metadata_export_to_skb()
> >
> > as a kfunc, the xdp_skb_metadata struct won't appear in any UAPI headers
> > and will only be accessible via BTF? And we can put the actual data
> > wherever we choose, since that bit is nicely hidden behind the kfunc,
> > while the returned pointer still allows programs to access it.
> >
> > We could even make that kfunc smart enough that it checks if the field
> > is already populated and just return the pointer to the existing data
> > instead of re-populating it int his case (with a flag to override,
> > maybe?).  
> 
> Even if we only expose it via btf, I think the fact that we still
> expose a somewhat fixed layout is the problem?
> I'm not sure the fact that we're not technically putting in the uapi
> header is the issue here, but maybe I'm wrong?
> Jakub?

Until the device metadata access from BPF is in bpf-next the only
opinion I have on this is something along the lines of "not right now".

I may be missing some concerns / perspectives, in which case - when
is the next "BPF office hours" meeting?