netdev - Re: [PATCH bpf-next 2/2] selftests/bpf: Enhance XDP Rx Metadata Handling

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <aGQNWXe6FBks8D3U@mini-arch>
Date: Tue, 1 Jul 2025 09:31:21 -0700
From: Stanislav Fomichev <stfomichev@...il.com>
To: Song Yoong Siang <yoong.siang.song@...el.com>
Cc: "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>,
	Daniel Borkmann <daniel@...earbox.net>,
	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>, 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 2/2] selftests/bpf: Enhance XDP Rx Metadata
 Handling

On 07/01, Song Yoong Siang wrote:
> Introduce the XDP_METADATA_SIZE macro to ensure that user applications can
> consistently retrieve the correct location of struct xdp_meta.
> 
> Prior to this commit, the XDP program adjusted the data_meta backward by
> the size of struct xdp_meta, while the user application retrieved the data
> by calculating backward from the data pointer. This approach only worked if
> xdp_buff->data_meta was equal to xdp_buff->data before calling
> bpf_xdp_adjust_meta.
> 
> With the introduction of XDP_METADATA_SIZE, both the XDP program and user
> application now calculate and identify the location of struct xdp_meta from
> the data pointer. This ensures the implementation remains functional even
> when there is device-reserved metadata, making the tests more portable
> across different NICs.
> 
> Signed-off-by: Song Yoong Siang <yoong.siang.song@...el.com>
> ---
>  tools/testing/selftests/bpf/prog_tests/xdp_metadata.c |  2 +-
>  tools/testing/selftests/bpf/progs/xdp_hw_metadata.c   | 10 +++++++++-
>  tools/testing/selftests/bpf/progs/xdp_metadata.c      |  8 +++++++-
>  tools/testing/selftests/bpf/xdp_hw_metadata.c         |  2 +-
>  tools/testing/selftests/bpf/xdp_metadata.h            |  7 +++++++
>  5 files changed, 25 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..72242ac1cdcd 100644
> --- a/tools/testing/selftests/bpf/progs/xdp_hw_metadata.c
> +++ b/tools/testing/selftests/bpf/progs/xdp_hw_metadata.c
> @@ -27,6 +27,7 @@ extern int bpf_xdp_metadata_rx_vlan_tag(const struct xdp_md *ctx,
>  SEC("xdp.frags")
>  int rx(struct xdp_md *ctx)
>  {
> +	int metalen_used, metalen_to_adjust;
>  	void *data, *data_meta, *data_end;
>  	struct ipv6hdr *ip6h = NULL;
>  	struct udphdr *udp = NULL;
> @@ -72,7 +73,14 @@ int rx(struct xdp_md *ctx)
>  		return XDP_PASS;
>  	}
>  
> -	err = bpf_xdp_adjust_meta(ctx, -(int)sizeof(struct xdp_meta));

[..]

> +	metalen_used = ctx->data - ctx->data_meta;

Is the intent here to query how much metadata has been consumed/reserved
by the driver? Looking at IGC it has the following code/comment:

	bi->xdp->data += IGC_TS_HDR_LEN;

	/* HW timestamp has been copied into local variable. Metadata
	 * length when XDP program is called should be 0.
	 */
	bi->xdp->data_meta += IGC_TS_HDR_LEN;

Are you sure that metadata size is correctly exposed to the bpf program?

My assumptions was that we should just unconditionally do bpf_xdp_adjust_meta
with -XDP_METADATA_SIZE and that should be good enough.