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

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Date:   Fri, 18 Nov 2022 15:05:30 +0100
From:   Jesper Dangaard Brouer <jbrouer@...hat.com>
To:     Stanislav Fomichev <sdf@...gle.com>, bpf@...r.kernel.org
Cc:     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>,
        Jakub Kicinski <kuba@...nel.org>,
        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: [PATCH bpf-next 06/11] xdp: Carry over xdp metadata into skb
 context

On 15/11/2022 04.02, Stanislav Fomichev wrote:
> Implement new bpf_xdp_metadata_export_to_skb kfunc which
> prepares compatible xdp metadata for kernel consumption.
> This kfunc should be called prior to bpf_redirect
> or when XDP_PASS'ing the frame into the kernel (note, the drivers
> have to be updated to enable consuming XDP_PASS'ed metadata).
> 
> veth driver is amended to consume this metadata when converting to skb.
> 
> Internally, XDP_FLAGS_HAS_SKB_METADATA flag is used to indicate
> whether the frame has skb metadata. The metadata is currently
> stored prior to xdp->data_meta. bpf_xdp_adjust_meta refuses
> to work after a call to bpf_xdp_metadata_export_to_skb (can lift
> this requirement later on if needed, we'd have to memmove
> xdp_skb_metadata).
> 

I think it is wrong to refuses using metadata area (bpf_xdp_adjust_meta)
when the function bpf_xdp_metadata_export_to_skb() have been called.
In my design they were suppose to co-exist, and BPF-prog was expected to
access this directly themselves.

With this current design, I think it is better to place the struct
xdp_skb_metadata (maybe call it xdp_skb_hints) after xdp_frame (in the
top of the frame).  This way we don't conflict with metadata and
headroom use-cases.  Plus, verifier will keep BPF-prog from accessing
this area directly (which seems to be one of the new design goals).

By placing it after xdp_frame, I think it would be possible to let veth 
unroll functions seamlessly access this info for XDP_REDIRECT'ed 
xdp_frame's.

WDYT?

--Jesper