Test listns() pagination.
List namespaces in batches.

Signed-off-by: Christian Brauner <brauner@kernel.org>
---
 tools/testing/selftests/namespaces/listns_test.c | 53 ++++++++++++++++++++++++
 1 file changed, 53 insertions(+)

diff --git a/tools/testing/selftests/namespaces/listns_test.c b/tools/testing/selftests/namespaces/listns_test.c
index 64249502ac49..7dff63a00263 100644
--- a/tools/testing/selftests/namespaces/listns_test.c
+++ b/tools/testing/selftests/namespaces/listns_test.c
@@ -115,4 +115,57 @@ TEST(listns_filter_by_type)
 	}
 }
 
+/*
+ * Test listns() pagination.
+ * List namespaces in batches.
+ */
+TEST(listns_pagination)
+{
+	struct ns_id_req req = {
+		.size = sizeof(req),
+		.spare = 0,
+		.ns_id = 0,
+		.ns_type = 0,
+		.spare2 = 0,
+		.user_ns_id = 0,
+	};
+	__u64 batch1[2], batch2[2];
+	ssize_t ret1, ret2;
+
+	/* Get first batch */
+	ret1 = sys_listns(&req, batch1, ARRAY_SIZE(batch1), 0);
+	if (ret1 < 0) {
+		if (errno == ENOSYS)
+			SKIP(return, "listns() not supported");
+		TH_LOG("listns failed: %s (errno=%d)", strerror(errno), errno);
+		ASSERT_TRUE(false);
+	}
+	ASSERT_GE(ret1, 0);
+
+	if (ret1 == 0)
+		SKIP(return, "No namespaces found");
+
+	TH_LOG("First batch: %zd namespaces", ret1);
+
+	/* Get second batch using last ID from first batch */
+	if (ret1 == ARRAY_SIZE(batch1)) {
+		req.ns_id = batch1[ret1 - 1];
+		ret2 = sys_listns(&req, batch2, ARRAY_SIZE(batch2), 0);
+		ASSERT_GE(ret2, 0);
+
+		TH_LOG("Second batch: %zd namespaces (after ns_id=%llu)",
+		       ret2, (unsigned long long)req.ns_id);
+
+		/* If we got more results, verify IDs are monotonically increasing */
+		if (ret2 > 0) {
+			ASSERT_GT(batch2[0], batch1[ret1 - 1]);
+			TH_LOG("Pagination working: %llu > %llu",
+			       (unsigned long long)batch2[0],
+			       (unsigned long long)batch1[ret1 - 1]);
+		}
+	} else {
+		TH_LOG("All namespaces fit in first batch");
+	}
+}
+
 TEST_HARNESS_MAIN

-- 
2.47.3