Re: [PATCH net-next v5] virtio_net: Support RX hash XDP hint

[Paolo Abeni <pabeni@redhat.com>]

On Sat, 2024-02-03 at 10:56 +0800, Liang Chen wrote:
> On Sat, Feb 3, 2024 at 12:20 AM Jesper Dangaard Brouer <hawk@kernel.org> wrote:
> > On 02/02/2024 13.11, Liang Chen wrote:
[...]
> > > @@ -1033,6 +1039,16 @@ static void put_xdp_frags(struct xdp_buff *xdp)
> > >       }
> > >   }
> > > 
> > > +static void virtnet_xdp_save_rx_hash(struct virtnet_xdp_buff *virtnet_xdp,
> > > +                                  struct net_device *dev,
> > > +                                  struct virtio_net_hdr_v1_hash *hdr_hash)
> > > +{
> > > +     if (dev->features & NETIF_F_RXHASH) {
> > > +             virtnet_xdp->hash_value = hdr_hash->hash_value;
> > > +             virtnet_xdp->hash_report = hdr_hash->hash_report;
> > > +     }
> > > +}
> > > +
> > 
> > Would it be possible to store a pointer to hdr_hash in virtnet_xdp_buff,
> > with the purpose of delaying extracting this, until and only if XDP
> > bpf_prog calls the kfunc?
> > 
> 
> That seems to be the way v1 works,
> https://lore.kernel.org/all/20240122102256.261374-1-liangchen.linux@gmail.com/
> . But it was pointed out that the inline header may be overwritten by
> the xdp prog, so the hash is copied out to maintain its integrity.

Why? isn't XDP supposed to get write access only to the pkt
contents/buffer?

if the XDP program can really change the virtio_net_hdr, that looks
potentially dangerous/bug prone regardless of this patch.

Thanks,

Paolo