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

[Jakub Sitnicki <jakub@cloudflare.com>]

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@cloudflare.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)
>  {

[...]