Re: [PATCH bpf-next,v3 2/2] selftests/bpf: Enhance XDP Rx metadata handling

[Jesper Dangaard Brouer <hawk@xxxxxxxxxx>]

On 02/07/2025 18.57, Song Yoong Siang wrote:
> Introduce the XDP_METADATA_SIZE macro as a conservative measure to
> accommodate any metadata areas reserved by Ethernet devices.

This seems like a sloppy workaround :-(

To me, the problem arise because AF_XDP is lacking the ability to
communicate the size of the data_meta area.  If we had this capability,
then we could allow the IGC driver to take some of the space, have the
BPF-prog expand it futher (bpf_xdp_adjust_meta) and then userspace
AF_XDP would simply be able to see the size of the data_meta area, and
apply the struct xdp_meta at right offset.


> Signed-off-by: Song Yoong Siang <yoong.siang.song@xxxxxxxxx>
> ---
>   tools/testing/selftests/bpf/prog_tests/xdp_metadata.c | 2 +-
>   tools/testing/selftests/bpf/progs/xdp_hw_metadata.c   | 2 +-
>   tools/testing/selftests/bpf/progs/xdp_metadata.c      | 2 +-
>   tools/testing/selftests/bpf/xdp_hw_metadata.c         | 2 +-
>   tools/testing/selftests/bpf/xdp_metadata.h            | 7 +++++++
>   5 files changed, 11 insertions(+), 4 deletions(-)
>
> diff --git a/tools/testing/selftests/bpf/prog_tests/xdp_metadata.c b/tools/testing/selftests/bpf/prog_tests/xdp_metadata.c
> index 19f92affc2da..8d6c2633698b 100644
> --- a/tools/testing/selftests/bpf/prog_tests/xdp_metadata.c
> +++ b/tools/testing/selftests/bpf/prog_tests/xdp_metadata.c
> @@ -302,7 +302,7 @@ static int verify_xsk_metadata(struct xsk *xsk, bool sent_from_af_xdp)
>   
>   	/* custom metadata */
>   
> -	meta = data - sizeof(struct xdp_meta);
> +	meta = data - XDP_METADATA_SIZE;
>   
>   	if (!ASSERT_NEQ(meta->rx_timestamp, 0, "rx_timestamp"))
>   		return -1;
> diff --git a/tools/testing/selftests/bpf/progs/xdp_hw_metadata.c b/tools/testing/selftests/bpf/progs/xdp_hw_metadata.c
> index 330ece2eabdb..3766f58d3486 100644
> --- a/tools/testing/selftests/bpf/progs/xdp_hw_metadata.c
> +++ b/tools/testing/selftests/bpf/progs/xdp_hw_metadata.c
> @@ -72,7 +72,7 @@ int rx(struct xdp_md *ctx)
>   		return XDP_PASS;
>   	}
>   
> -	err = bpf_xdp_adjust_meta(ctx, -(int)sizeof(struct xdp_meta));
> +	err = bpf_xdp_adjust_meta(ctx, -(int)XDP_METADATA_SIZE);
>   	if (err) {
>   		__sync_add_and_fetch(&pkts_fail, 1);
>   		return XDP_PASS;
> diff --git a/tools/testing/selftests/bpf/progs/xdp_metadata.c b/tools/testing/selftests/bpf/progs/xdp_metadata.c
> index 09bb8a038d52..5cada85fe0f4 100644
> --- a/tools/testing/selftests/bpf/progs/xdp_metadata.c
> +++ b/tools/testing/selftests/bpf/progs/xdp_metadata.c
> @@ -73,7 +73,7 @@ int rx(struct xdp_md *ctx)
>   
>   	/* Reserve enough for all custom metadata. */
>   
> -	ret = bpf_xdp_adjust_meta(ctx, -(int)sizeof(struct xdp_meta));
> +	ret = bpf_xdp_adjust_meta(ctx, -(int)XDP_METADATA_SIZE);
>   	if (ret != 0)
>   		return XDP_DROP;
>   
> diff --git a/tools/testing/selftests/bpf/xdp_hw_metadata.c b/tools/testing/selftests/bpf/xdp_hw_metadata.c
> index 3d8de0d4c96a..a529d55d4ff4 100644
> --- a/tools/testing/selftests/bpf/xdp_hw_metadata.c
> +++ b/tools/testing/selftests/bpf/xdp_hw_metadata.c
> @@ -223,7 +223,7 @@ static void verify_xdp_metadata(void *data, clockid_t clock_id)
>   {
>   	struct xdp_meta *meta;
>   
> -	meta = data - sizeof(*meta);
> +	meta = data - XDP_METADATA_SIZE;
>   
>   	if (meta->hint_valid & XDP_META_FIELD_RSS)
>   		printf("rx_hash: 0x%X with RSS type:0x%X\n",
> diff --git a/tools/testing/selftests/bpf/xdp_metadata.h b/tools/testing/selftests/bpf/xdp_metadata.h
> index 87318ad1117a..2dfd3bf5e7bb 100644
> --- a/tools/testing/selftests/bpf/xdp_metadata.h
> +++ b/tools/testing/selftests/bpf/xdp_metadata.h
> @@ -50,3 +50,10 @@ struct xdp_meta {
>   	};
>   	enum xdp_meta_field hint_valid;
>   };
> +
> +/* XDP_METADATA_SIZE must be at least the size of struct xdp_meta. An additional
> + * 32 bytes of padding is included as a conservative measure to accommodate any
> + * metadata areas reserved by Ethernet devices. If the device-reserved metadata
> + * exceeds 32 bytes, this value will need adjustment.
> + */
> +#define XDP_METADATA_SIZE	(sizeof(struct xdp_meta) + 32)