Re: [PATCH net v2] veth: xdp: use head instead of hard_start

Toshiaki Makita
2020-04-02T00:47:14Z

On 2020/04/02 1:15, Jesper Dangaard Brouer wrote:
...
> [PATCH RFC net-next] veth: adjust hard_start offset on redirect XDP frames
>
> 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 <makita.toshiaki@lab.ntt.co.jp>

FYI: This mail address is deprecated.

> Fixes: 9fc8d518d9d5 ("veth: Handle xdp_frames in xdp napi ring")
> Reported-by: Mao Wenan <maowenan@huawei.com>
> Signed-off-by: Jesper Dangaard Brouer <brouer@redhat.com>

FWIW,

Acked-by: Toshiaki Makita <toshiaki.makita1@gmail.com>