netdev - Re: [PATCH net-next 09/33] veth: adjust hard_start offset on redirect XDP frames

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Message-ID: <87368t3rnh.fsf@toke.dk>
Date:   Fri, 24 Apr 2020 16:05:54 +0200
From:   Toke Høiland-Jørgensen <toke@...hat.com>
To:     Jesper Dangaard Brouer <brouer@...hat.com>, sameehj@...zon.com
Cc:     Toshiaki Makita <toshiaki.makita1@...il.com>,
        Mao Wenan <maowenan@...wei.com>,
        Toshiaki Makita <toshiaki.makita1@...il.com>,
        Jesper Dangaard Brouer <brouer@...hat.com>,
        netdev@...r.kernel.org, bpf@...r.kernel.org, zorik@...zon.com,
        akiyano@...zon.com, gtzalik@...zon.com,
        Daniel Borkmann <borkmann@...earbox.net>,
        Alexei Starovoitov <alexei.starovoitov@...il.com>,
        John Fastabend <john.fastabend@...il.com>,
        Alexander Duyck <alexander.duyck@...il.com>,
        Jeff Kirsher <jeffrey.t.kirsher@...el.com>,
        David Ahern <dsahern@...il.com>,
        Willem de Bruijn <willemdebruijn.kernel@...il.com>,
        Ilias Apalodimas <ilias.apalodimas@...aro.org>,
        Lorenzo Bianconi <lorenzo@...nel.org>,
        Saeed Mahameed <saeedm@...lanox.com>,
        steffen.klassert@...unet.com
Subject: Re: [PATCH net-next 09/33] veth: adjust hard_start offset on redirect XDP frames

Jesper Dangaard Brouer <brouer@...hat.com> writes:

> When native XDP redirect into a veth device, the frame arrives in the
> xdp_frame structure. It is then processed in veth_xdp_rcv_one(),
> which can run a new XDP bpf_prog on the packet. Doing so requires
> converting xdp_frame to xdp_buff, but the tricky part is that
> xdp_frame memory area is located in the top (data_hard_start) memory
> area that xdp_buff will point into.
>
> The current code tried to protect the xdp_frame area, by assigning
> xdp_buff.data_hard_start past this memory. This results in 32 bytes
> less headroom to expand into via BPF-helper bpf_xdp_adjust_head().
>
> This protect step is actually not needed, because BPF-helper
> bpf_xdp_adjust_head() already reserve this area, and don't allow
> BPF-prog to expand into it. Thus, it is safe to point data_hard_start
> directly at xdp_frame memory area.
>
> Cc: Toshiaki Makita <toshiaki.makita1@...il.com>
> Fixes: 9fc8d518d9d5 ("veth: Handle xdp_frames in xdp napi ring")
> Reported-by: Mao Wenan <maowenan@...wei.com>
> Signed-off-by: Jesper Dangaard Brouer <brouer@...hat.com>
> Acked-by: Toshiaki Makita <toshiaki.makita1@...il.com>

Acked-by: Toke Høiland-Jørgensen <toke@...hat.com>