netdev - RE: [PATCH bpf-next,v3 1/2] doc: enhance explanation of XDP Rx metadata layout and METADATA

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Message-ID: <IA3PR11MB9254E3549D984FFB1305C01DD842A@IA3PR11MB9254.namprd11.prod.outlook.com>
Date: Fri, 4 Jul 2025 00:33:21 +0000
From: "Song, Yoong Siang" <yoong.siang.song@...el.com>
To: Daniel Borkmann <daniel@...earbox.net>, "David S . Miller"
	<davem@...emloft.net>, Eric Dumazet <edumazet@...gle.com>, Jakub Kicinski
	<kuba@...nel.org>, Paolo Abeni <pabeni@...hat.com>, Simon Horman
	<horms@...nel.org>, Jonathan Corbet <corbet@....net>, Alexei Starovoitov
	<ast@...nel.org>, Jesper Dangaard Brouer <hawk@...nel.org>, John Fastabend
	<john.fastabend@...il.com>, Stanislav Fomichev <sdf@...ichev.me>, "Andrii
 Nakryiko" <andrii@...nel.org>, Martin KaFai Lau <martin.lau@...ux.dev>,
	Eduard Zingerman <eddyz87@...il.com>, Song Liu <song@...nel.org>, "Yonghong
 Song" <yonghong.song@...ux.dev>, KP Singh <kpsingh@...nel.org>, Hao Luo
	<haoluo@...gle.com>, Jiri Olsa <jolsa@...nel.org>, Mykola Lysenko
	<mykolal@...com>, Shuah Khan <shuah@...nel.org>
CC: "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
	"linux-doc@...r.kernel.org" <linux-doc@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"bpf@...r.kernel.org" <bpf@...r.kernel.org>,
	"linux-kselftest@...r.kernel.org" <linux-kselftest@...r.kernel.org>
Subject: RE: [PATCH bpf-next,v3 1/2] doc: enhance explanation of XDP Rx
 metadata layout and METADATA_SIZE

On Thursday, July 3, 2025 11:58 PM, Daniel Borkmann <daniel@...earbox.net> wrote:
>On 7/2/25 6:57 PM, Song Yoong Siang wrote:
>[...]
>> +It is important to note that some devices may utilize the ``data_meta`` area for
>> +their own purposes. For example, the IGC device utilizes ``IGC_TS_HDR_LEN``
>> +bytes of the ``data_meta`` area for receiving hardware timestamps. Therefore,
>> +the XDP program should ensure that it does not overwrite any existing metadata.
>> +The metadata layout of such device is depicted below::
>> +
>> +  +----------+-----------------+--------------------------+------+
>> +  | headroom | custom metadata | device-reserved metadata | data |
>> +  +----------+-----------------+--------------------------+------+
>> +             ^                                            ^
>> +             |                                            |
>> +   xdp_buff->data_meta                              xdp_buff->data
>
>Imho, this section is misleading to developers. Suppose you're a XDP program writer
>and you want to implement a generic native BPF program (independent of the underlying
>NIC). Does this mean, the expectation is to dig into driver code to gather whether
>or not a driver is prepopulating and how much of it? What are the implications if the
>data is overwritten? For example, in Cilium today we use the buffer described here
>as device-reserved metadata and override it. How will users know what breaks?

Thanks for your input.

A generic XDP program can always check the size of device-reserved metadata by
"ctx->data - ctx->data_meta" and avoid overwrite it, as shown in code below in my
v1 submission [1]. This requires driver to expose the metadata length used [2].
However, I dint have good justification for making the metadata length user-visible.
So, I submitted this v3 to keep it simple. Any thoughts?

+	metalen_used = ctx->data - ctx->data_meta;
+	metalen_to_adjust = XDP_METADATA_SIZE - metalen_used;
+	if (metalen_to_adjust < (int)sizeof(struct xdp_meta))
+		return XDP_DROP;
+
+	ret = bpf_xdp_adjust_meta(ctx, -metalen_to_adjust);

[1] https://lore.kernel.org/netdev/20250701042940.3272325-3-yoong.siang.song@intel.com/
[2] https://lore.kernel.org/netdev/20250701080955.3273137-1-yoong.siang.song@intel.com/