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

[Liang Chen]

On Wed, Feb 7, 2024 at 10:27 PM Paolo Abeni <pabeni@xxxxxxxxxx> wrote:
>
> On Wed, 2024-02-07 at 10:54 +0800, Liang Chen wrote:
> > On Tue, Feb 6, 2024 at 6:44 PM Paolo Abeni <pabeni@xxxxxxxxxx> wrote:
> > >
> > > 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@xxxxxxxxxx> 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@xxxxxxxxx/
> > > > . 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?
> > >
> >
> > Normally, an XDP program accesses only the packet data. However,
> > there's also an XDP RX Metadata area, referenced by the data_meta
> > pointer. This pointer can be adjusted with bpf_xdp_adjust_meta to
> > point somewhere ahead of the data buffer, thereby granting the XDP
> > program access to the virtio header located immediately before the
>
> AFAICS bpf_xdp_adjust_meta() does not allow moving the meta_data before
> xdp->data_hard_start:
>
> https://elixir.bootlin.com/linux/latest/source/net/core/filter.c#L4210
>
> and virtio net set such field after the virtio_net_hdr:
>
> https://elixir.bootlin.com/linux/latest/source/drivers/net/virtio_net.c#L1218
> https://elixir.bootlin.com/linux/latest/source/drivers/net/virtio_net.c#L1420
>
> I don't see how the virtio hdr could be touched? Possibly even more
> important: if such thing is possible, I think is should be somewhat
> denied (for the same reason an H/W nic should prevent XDP from
> modifying its own buffer descriptor).

Thank you for highlighting this concern. The header layout differs
slightly between small and mergeable mode. Taking 'mergeable mode' as
an example, after calling xdp_prepare_buff the layout of xdp_buff
would be as depicted in the diagram below,

                      buf
                       |
                       v
        +--------------+--------------+-------------+
        | xdp headroom | virtio header| packet      |
        | (256 bytes)  | (20 bytes)   | content     |
        +--------------+--------------+-------------+
        ^                             ^
        |                             |
 data_hard_start                    data
                                  data_meta

If 'bpf_xdp_adjust_meta' repositions the 'data_meta' pointer a little
towards 'data_hard_start', it would point to the inline header, thus
potentially allowing the XDP program to access the inline header.

We will take a closer look on how to prevent the inline header from
being altered, possibly by borrowing some ideas from other
xdp_metadata_ops implementation.


Thanks,
Liang

>
> Cheers,
>
> Paolo
>