netdev - Re: [PATCH bpf-next v3 11/16] selftests/bpf: Dump skb metadata on verification failure

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Message-ID: <871pmottmm.fsf@cloudflare.com>
Date: Mon, 27 Oct 2025 13:29:53 +0100
From: Jakub Sitnicki <jakub@...udflare.com>
To: bpf@...r.kernel.org
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>,  Martin KaFai Lau
 <martin.lau@...ux.dev>,  Daniel Borkmann <daniel@...earbox.net>,  John
 Fastabend <john.fastabend@...il.com>,  Stanislav Fomichev
 <sdf@...ichev.me>,  Alexei Starovoitov <ast@...nel.org>,  Andrii Nakryiko
 <andrii@...nel.org>,  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>,  Arthur Fabre <arthur@...hurfabre.com>,  Jesper
 Dangaard Brouer <hawk@...nel.org>,  netdev@...r.kernel.org,
  kernel-team@...udflare.com
Subject: Re: [PATCH bpf-next v3 11/16] selftests/bpf: Dump skb metadata on
 verification failure

On Sun, Oct 26, 2025 at 03:18 PM +01, Jakub Sitnicki wrote:
> Add diagnostic output when metadata verification fails to help with
> troubleshooting test failures. Introduce a check_metadata() helper that
> prints both expected and received metadata to the BPF program's stderr
> stream on mismatch. The userspace test reads and dumps this stream on
> failure.
>
> Signed-off-by: Jakub Sitnicki <jakub@...udflare.com>
> ---

[...]

> diff --git a/tools/testing/selftests/bpf/progs/test_xdp_meta.c b/tools/testing/selftests/bpf/progs/test_xdp_meta.c
> index 11288b20f56c..74d7e2aab2ef 100644
> --- a/tools/testing/selftests/bpf/progs/test_xdp_meta.c
> +++ b/tools/testing/selftests/bpf/progs/test_xdp_meta.c
> @@ -27,6 +27,23 @@ static const __u8 meta_want[META_SIZE] = {
>  	0x31, 0x32, 0x33, 0x34, 0x35, 0x36, 0x37, 0x38,
>  };
>  
> +static bool check_metadata(const char *file, int line, __u8 *meta_have)
> +{
> +	if (!__builtin_memcmp(meta_have, meta_want, META_SIZE))
> +		return true;
> +
> +	bpf_stream_printk(BPF_STREAM_STDERR,
> +			  "FAIL:%s:%d: metadata mismatch\n"
> +			  "  have:\n    %pI6\n    %pI6\n"
> +			  "  want:\n    %pI6\n    %pI6\n",
> +			  file, line,
> +			  &meta_have[0x00], &meta_have[0x10],
> +			  &meta_want[0x00], &meta_have[0x10]);
                                             ^^^^^^^^^

FYI: AI review pointed to a copy-paste bug here.

> +	return false;
> +}
> +
> +#define check_metadata(meta_have) check_metadata(__FILE__, __LINE__, meta_have)
> +
>  SEC("tc")
>  int ing_cls(struct __sk_buff *ctx)
>  {

[...]