selftests/ring-buffer: Add test for out-of-bound pgoff mapping

Extend the ring-buffer mapping test coverage by checking an out-of-bound
pgoff which has proven to be problematic in the past.

Link: https://lore.kernel.org/r/20241218170318.2814991-1-vdonnefort@google.com
Cc: Shuah Khan <skhan@linuxfoundation.org>
Cc: linux-kselftest@vger.kernel.org
Signed-off-by: Vincent Donnefort <vdonnefort@google.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>

Author: vdonnefort

tools/testing/selftests/ring-buffer/map_test.c

@@ -233,12 +233,18 @@ TEST_F(map, data_mmap)
 	ASSERT_NE(data, MAP_FAILED);
 	munmap(data, data_len);
 
-	/* Overflow the available subbufs by 1 */
+	/* Offset within ring-buffer bounds, mapping size overflow */
 	meta_len += desc->meta->subbuf_size * 2;
 	data = mmap(NULL, data_len, PROT_READ, MAP_SHARED,
 		    desc->cpu_fd, meta_len);
 	ASSERT_EQ(data, MAP_FAILED);
 
+	/* Offset outside ring-buffer bounds */
+	data_len = desc->meta->subbuf_size * desc->meta->nr_subbufs;
+	data = mmap(NULL, data_len, PROT_READ, MAP_SHARED,
+		    desc->cpu_fd, data_len + (desc->meta->subbuf_size * 2));
+	ASSERT_EQ(data, MAP_FAILED);
+
 	/* Verify meta-page padding */
 	if (desc->meta->meta_page_size > getpagesize()) {
 		data_len = desc->meta->meta_page_size;