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

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Date:   Wed, 29 Sep 2021 11:41:53 +0100
From:   Lorenz Bauer <lmb@...udflare.com>
To:     Jakub Kicinski <kuba@...nel.org>
Cc:     Lorenzo Bianconi <lorenzo@...nel.org>, bpf <bpf@...r.kernel.org>,
        Networking <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>,
        Maciej Fijalkowski <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 Thu, 16 Sept 2021 at 18:47, Jakub Kicinski <kuba@...nel.org> wrote:
>
> Won't applications end up building something like skb_header_pointer()
> based on bpf_xdp_adjust_data(), anyway? In which case why don't we
> provide them what they need?
>
> say:
>
> void *xdp_mb_pointer(struct xdp_buff *xdp_md, u32 flags,
>                      u32 offset, u32 len, void *stack_buf)
>
> flags and offset can be squashed into one u64 as needed. Helper returns
> pointer to packet data, either real one or stack_buf. Verifier has to
> be taught that the return value is NULL or a pointer which is safe with
> offsets up to @len.
>
> If the reason for access is write we'd also need:
>
> void *xdp_mb_pointer_flush(struct xdp_buff *xdp_md, u32 flags,
>                            u32 offset, u32 len, void *stack_buf)

Yes! This would be so much better than bpf_skb_load/store_bytes(),
especially if we can use it for both XDP and skb contexts as stated
elsewhere in this thread.

-- 
Lorenz Bauer  |  Systems Engineer
6th Floor, County Hall/The Riverside Building, SE1 7PB, UK

www.cloudflare.com