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

[Alexei Starovoitov]

On Thu, Apr 2, 2020 at 2:06 AM Jesper Dangaard Brouer <brouer@xxxxxxxxxx> wrote:
>
> On Thu, 2 Apr 2020 09:47:03 +0900
> Toshiaki Makita <toshiaki.makita1@xxxxxxxxx> wrote:
>
> > 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@xxxxxxxxxxxxx>
> >
> > FYI: This mail address is deprecated.
> >
> > > Fixes: 9fc8d518d9d5 ("veth: Handle xdp_frames in xdp napi ring")
> > > Reported-by: Mao Wenan <maowenan@xxxxxxxxxx>
> > > Signed-off-by: Jesper Dangaard Brouer <brouer@xxxxxxxxxx>
> >
> > FWIW,
> >
> > Acked-by: Toshiaki Makita <toshiaki.makita1@xxxxxxxxx>
>
> Thanks.
>
> I have updated your email and added your ack in my patchset.  I will
> submit this officially once net-next opens up again[1], as part my
> larger patchset for introducing XDP frame_sz.

It looks like bug fix to me.
The way I read it that behavior of bpf_xdp_adjust_head() is a bit
buggy with veth netdev,
so why wait ?