Steffen Trumtrar wrote:
> For the skb, the timestamp is read from the header before the XDP
> program runs, so XDP_PASS keeps it even if the program changes the
> packet. XDP therefore works with rx timestamping enabled.
>
> Add xmo_rx_timestamp so XDP programs can read the timestamp with
> bpf_xdp_metadata_rx_timestamp(). It returns -ENODATA if timestamping
> is not enabled or the device did not timestamp the packet. Install the
> metadata ops also if only VIRTIO_NET_F_RX_TSTAMP is negotiated;
> rx_hash returns -ENODATA then.
>
> Like virtnet_xdp_rx_hash(), the callback finds the header relative to
> xdp->data, so it has to be called before bpf_xdp_adjust_head().
>
> Signed-off-by: Steffen Trumtrar <[email protected]>
> Assisted-by: Claude:claude-opus-5-5
> ---
> drivers/net/virtio_net.c | 26 ++++++++++++++++++++++++++
> 1 file changed, 26 insertions(+)
>
> diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
> index 23af10c613b48..e79b65653fad7 100644
> --- a/drivers/net/virtio_net.c
> +++ b/drivers/net/virtio_net.c
> @@ -6848,8 +6848,30 @@ static int virtnet_xdp_rx_hash(const struct xdp_md
> *_ctx, u32 *hash,
> return 0;
> }
>
> +static int virtnet_xdp_rx_timestamp(const struct xdp_md *_ctx, u64
> *timestamp)
> +{
> + const struct xdp_buff *xdp = (void *)_ctx;
> + const struct virtio_net_common_hdr *hdr;
> + struct virtnet_info *vi;
> + u64 tstamp;
> +
> + vi = netdev_priv(xdp->rxq->dev);
> + if (!READ_ONCE(vi->hwts_rx_en))
> + return -ENODATA;
> +
> + hdr = (const struct virtio_net_common_hdr *)(xdp->data - vi->hdr_len);
Reading `hdr->tstamp` in place from `xdp->data - vi->hdr_len` violates the
XDP
metadata area contract (`Documentation/networking/xdp-rx-metadata.rst`:
drivers
must copy device-reserved metadata out of `[data_hard_start, data)` before
presenting the frame to XDP) and breaks standard XDP metadata consumers:
- `hdr->tstamp` sits at `[xdp->data - 8, xdp->data)`. Standard XDP
programs—such
as `tools/testing/selftests/bpf/progs/xdp_hw_metadata.c`—call
`bpf_xdp_adjust_meta(ctx, -(int)sizeof(struct xdp_meta))` first,
initialize
fields in `meta` (e.g. `meta->hint_valid = 0` at `[xdp->data - 4,
xdp->data)`),
and only then call `bpf_xdp_metadata_rx_timestamp(ctx,
&meta->rx_timestamp)`.
Initializing `meta` overwrites `hdr->tstamp` (and `hdr->hash_v1_hdr`)
before
this kfunc runs.
- Calling `bpf_xdp_adjust_head()` before reading metadata also shifts
`xdp->data`,
reading arbitrary bytes at `xdp->data - vi->hdr_len`.
Wrap `struct xdp_buff` in a driver struct on the stack (`container_of`, and
in
`struct xdp_buff_xsk`'s `cb` verified with `XSK_CHECK_PRIV_TYPE()` for
AF_XDP,
matching `gve`/`igc`/`ice`/`mlx5`) and stash `tstamp` (already extracted in
`receive_buf()` and `virtnet_receive_xsk_buf()` before XDP runs) and the
hash
fields before `bpf_prog_run_xdp()`.
