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>
---
 .../bpf/prog_tests/xdp_context_test_run.c          | 24 ++++++++++++++++++---
 tools/testing/selftests/bpf/progs/test_xdp_meta.c  | 25 ++++++++++++++++++----
 2 files changed, 42 insertions(+), 7 deletions(-)

diff --git a/tools/testing/selftests/bpf/prog_tests/xdp_context_test_run.c b/tools/testing/selftests/bpf/prog_tests/xdp_context_test_run.c
index 93a1fbe6a4fd..db3027564261 100644
--- a/tools/testing/selftests/bpf/prog_tests/xdp_context_test_run.c
+++ b/tools/testing/selftests/bpf/prog_tests/xdp_context_test_run.c
@@ -171,6 +171,21 @@ static int write_test_packet(int tap_fd)
 	return 0;
 }
 
+static void dump_err_stream(const struct bpf_program *prog)
+{
+	char buf[512];
+	int ret;
+
+	ret = 0;
+	do {
+		ret = bpf_prog_stream_read(bpf_program__fd(prog),
+					   BPF_STREAM_STDERR, buf, sizeof(buf),
+					   NULL);
+		if (ret > 0)
+			fwrite(buf, sizeof(buf[0]), ret, stderr);
+	} while (ret > 0);
+}
+
 void test_xdp_context_veth(void)
 {
 	LIBBPF_OPTS(bpf_tc_hook, tc_hook, .attach_point = BPF_TC_INGRESS);
@@ -249,7 +264,8 @@ void test_xdp_context_veth(void)
 	if (!ASSERT_OK(ret, "send_test_packet"))
 		goto close;
 
-	ASSERT_TRUE(skel->bss->test_pass, "test_pass");
+	if (!ASSERT_TRUE(skel->bss->test_pass, "test_pass"))
+		dump_err_stream(tc_prog);
 
 close:
 	close_netns(nstoken);
@@ -314,7 +330,8 @@ static void test_tuntap(struct bpf_program *xdp_prog,
 	if (!ASSERT_OK(ret, "write_test_packet"))
 		goto close;
 
-	ASSERT_TRUE(*test_pass, "test_pass");
+	if (!ASSERT_TRUE(*test_pass, "test_pass"))
+		dump_err_stream(tc_prio_2_prog ? : tc_prio_1_prog);
 
 close:
 	if (tap_fd >= 0)
@@ -385,7 +402,8 @@ static void test_tuntap_mirred(struct bpf_program *xdp_prog,
 	if (!ASSERT_OK(ret, "write_test_packet"))
 		goto close;
 
-	ASSERT_TRUE(*test_pass, "test_pass");
+	if (!ASSERT_TRUE(*test_pass, "test_pass"))
+		dump_err_stream(tc_prog);
 
 close:
 	if (tap_fd >= 0)
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]);
+	return false;
+}
+
+#define check_metadata(meta_have) check_metadata(__FILE__, __LINE__, meta_have)
+
 SEC("tc")
 int ing_cls(struct __sk_buff *ctx)
 {
@@ -36,7 +53,7 @@ int ing_cls(struct __sk_buff *ctx)
 	if (meta_have + META_SIZE > data)
 		goto out;
 
-	if (__builtin_memcmp(meta_want, meta_have, META_SIZE))
+	if (!check_metadata(meta_have))
 		goto out;
 
 	test_pass = true;
@@ -54,7 +71,7 @@ int ing_cls_dynptr_read(struct __sk_buff *ctx)
 	bpf_dynptr_from_skb_meta(ctx, 0, &meta);
 	bpf_dynptr_read(meta_have, META_SIZE, &meta, 0, 0);
 
-	if (__builtin_memcmp(meta_want, meta_have, META_SIZE))
+	if (!check_metadata(meta_have))
 		goto out;
 
 	test_pass = true;
@@ -92,7 +109,7 @@ int ing_cls_dynptr_slice(struct __sk_buff *ctx)
 	if (!meta_have)
 		goto out;
 
-	if (__builtin_memcmp(meta_want, meta_have, META_SIZE))
+	if (!check_metadata(meta_have))
 		goto out;
 
 	test_pass = true;
@@ -153,7 +170,7 @@ int ing_cls_dynptr_offset_rd(struct __sk_buff *ctx)
 		goto out;
 	__builtin_memcpy(dst, src, chunk_len);
 
-	if (__builtin_memcmp(meta_want, meta_have, META_SIZE))
+	if (!check_metadata(meta_have))
 		goto out;
 
 	test_pass = true;

-- 
2.43.0