linux-kernel - Re: [PATCH net v2] veth: xdp: use head instead of hard

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Date:   Thu, 2 Apr 2020 11:06:19 +0200
From:   Jesper Dangaard Brouer <brouer@...hat.com>
To:     Toshiaki Makita <toshiaki.makita1@...il.com>
Cc:     Mao Wenan <maowenan@...wei.com>, davem@...emloft.net,
        ast@...nel.org, daniel@...earbox.net, kuba@...nel.org,
        hawk@...nel.org, john.fastabend@...il.com, kafai@...com,
        songliubraving@...com, yhs@...com, andriin@...com,
        jwi@...ux.ibm.com, jianglidong3@...com, edumazet@...gle.com,
        netdev@...r.kernel.org, linux-kernel@...r.kernel.org,
        bpf@...r.kernel.org, kernel-janitors@...r.kernel.org,
        brouer@...hat.com
Subject: Re: [PATCH net v2] veth: xdp: use head instead of hard_start

On Thu, 2 Apr 2020 09:47:03 +0900
Toshiaki Makita <toshiaki.makita1@...il.com> 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@....ntt.co.jp>  
> 
> FYI: This mail address is deprecated.
> 
> > 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>  
> 
> FWIW,
> 
> Acked-by: Toshiaki Makita <toshiaki.makita1@...il.com>

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.

[1] http://vger.kernel.org/~davem/net-next.html
-- 
Best regards,
  Jesper Dangaard Brouer
  MSc.CS, Principal Kernel Engineer at Red Hat
  LinkedIn: http://www.linkedin.com/in/brouer