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

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Message-ID: <6d147702-d9be-458a-b79c-9b4269817d43@iogearbox.net>
Date: Thu, 3 Jul 2025 17:57:32 +0200
From: Daniel Borkmann <daniel@...earbox.net>
To: Song Yoong Siang <yoong.siang.song@...el.com>,
 "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, linux-doc@...r.kernel.org,
 linux-kernel@...r.kernel.org, bpf@...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 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?