netdev - Re: [PATCH v14 bpf-next 00/18] mvneta: introduce XDP multi-buffer support

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Message-ID: <CAADnVQL15NAqbswXedF0r2om8SOiMQE80OSjbyCA56s-B4y8zA@mail.gmail.com>
Date:   Fri, 17 Sep 2021 11:43:07 -0700
From:   Alexei Starovoitov <alexei.starovoitov@...il.com>
To:     Jakub Kicinski <kuba@...nel.org>
Cc:     Lorenzo Bianconi <lorenzo@...nel.org>, bpf <bpf@...r.kernel.org>,
        Network Development <netdev@...r.kernel.org>,
        Lorenzo Bianconi <lorenzo.bianconi@...hat.com>,
        "David S. Miller" <davem@...emloft.net>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>, shayagr@...zon.com,
        John Fastabend <john.fastabend@...il.com>,
        David Ahern <dsahern@...nel.org>,
        Jesper Dangaard Brouer <brouer@...hat.com>,
        Eelco Chaudron <echaudro@...hat.com>,
        Jason Wang <jasowang@...hat.com>,
        Alexander Duyck <alexander.duyck@...il.com>,
        Saeed Mahameed <saeed@...nel.org>,
        "Fijalkowski, Maciej" <maciej.fijalkowski@...el.com>,
        "Karlsson, Magnus" <magnus.karlsson@...el.com>,
        tirthendu.sarkar@...el.com,
        Toke Høiland-Jørgensen <toke@...hat.com>
Subject: Re: [PATCH v14 bpf-next 00/18] mvneta: introduce XDP multi-buffer support

On Fri, Sep 17, 2021 at 11:33 AM Jakub Kicinski <kuba@...nel.org> wrote:
>
> > >
> > > Alternatively how about we produce a variation on skb_header_pointer()
> > > (use on-stack buffer or direct access if the entire region is in one
> > > frag).
>
> If bpf_xdp_load_bytes() / bpf_xdp_store_bytes() works for most we
> can start with that. In all honesty I don't know what the exact
> use cases for looking at data are, either. I'm primarily worried
> about exposing the kernel internals too early.

I don't mind the xdp equivalent of skb_load_bytes,
but skb_header_pointer() idea is superior.
When we did xdp with data/data_end there was no refine_retval_range
concept in the verifier (iirc or we just missed that opportunity).
We'd need something more advanced: a pointer with valid range
refined by input argument 'len' or NULL.
The verifier doesn't have such thing yet, but it fits as a combination of
value_or_null plus refine_retval_range.
The bpf_xdp_header_pointer() and bpf_skb_header_pointer()
would probably simplify bpf programs as well.
There would be no need to deal with data/data_end.