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

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

quoted

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

quoted

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

[...]

quoted

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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
packet data.

Thanks,
Liang

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

Thanks,

Paolo