On Tue, Feb 27, 2024 at 4:42 AM John Fastabend [off-list ref] wrote:

Jason Wang wrote:

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On Fri, Feb 23, 2024 at 9:42 AM Xuan Zhuo [off-list ref] wrote:

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On Fri, 09 Feb 2024 13:57:25 +0100, Paolo Abeni [off-list ref] wrote:

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On Fri, 2024-02-09 at 18:39 +0800, Liang Chen wrote:

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On Wed, Feb 7, 2024 at 10:27 PM Paolo Abeni [off-list ref] wrote:

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On Wed, 2024-02-07 at 10:54 +0800, Liang Chen wrote:

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On Tue, Feb 6, 2024 at 6:44 PM Paolo Abeni [off-list ref] wrote:

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On Sat, 2024-02-03 at 10:56 +0800, Liang Chen wrote:

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On Sat, Feb 3, 2024 at 12:20 AM Jesper Dangaard Brouer [off-list ref] wrote:

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On 02/02/2024 13.11, Liang Chen wrote:

[...]

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@@ -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/ (local)
. 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.

Fairly late to the thread sorry. Given above layout does it make sense to
just delay extraction to the kfunc as suggested above? Sure the XDP program
could smash the entry in virtio header, but this is already the case for
anything else there. A program writing over the virtio header is likely
buggy anyways. Worse that might happen is bad rss values and mappings?

Thank you for raising the concern. I am not quite sure if the XDP
program is considered buggy, as it is agnostic to the layout of the
inline header.
Let's say an XDP program calls bpf_xdp_adjust_meta to adjust data_meta
to point to the inline header and overwrites it without even knowing
of its existence. Later, when the XDP program invokes the kfunc to
retrieve the hash, incorrect data would be returned. In this case, the
XDP program seems to be doing everything legally but ends up with the
wrong hash data.

Thanks,
Liang

I like seeing more use cases for the hints though.

Thanks!
John