diff --git a/tests/modetest/modetest.c b/tests/modetest/modetest.c
index 9b1aa537..cc96015f 100644
--- a/tests/modetest/modetest.c
+++ b/tests/modetest/modetest.c
@@ -817,7 +817,7 @@ struct pipe_arg {
 	unsigned int num_cons;
 	uint32_t crtc_id;
 	char mode_str[64];
-	char format_str[5];
+	char format_str[8]; /* need to leave room for "_BE" and terminating \0 */
 	float vrefresh;
 	unsigned int fourcc;
 	drmModeModeInfo *mode;
@@ -841,7 +841,7 @@ struct plane_arg {
 	unsigned int old_fb_id;
 	struct bo *bo;
 	struct bo *old_bo;
-	char format_str[5]; /* need to leave room for terminating \0 */
+	char format_str[8]; /* need to leave room for "_BE" and terminating \0 */
 	unsigned int fourcc;
 };
 
@@ -2032,8 +2032,9 @@ static int parse_connector(struct pipe_arg *pipe, const char *arg)
 	}
 
 	if (*p == '@') {
-		strncpy(pipe->format_str, p + 1, 4);
-		pipe->format_str[4] = '\0';
+		len = sizeof(pipe->format_str) - 1;
+		strncpy(pipe->format_str, p + 1, len);
+		pipe->format_str[len] = '\0';
 	}
 
 	pipe->fourcc = util_format_fourcc(pipe->format_str);
@@ -2047,6 +2048,7 @@ static int parse_connector(struct pipe_arg *pipe, const char *arg)
 
 static int parse_plane(struct plane_arg *plane, const char *p)
 {
+	unsigned int len;
 	char *end;
 
 	plane->plane_id = strtoul(p, &end, 10);
@@ -2085,8 +2087,9 @@ static int parse_plane(struct plane_arg *plane, const char *p)
 	}
 
 	if (*end == '@') {
-		strncpy(plane->format_str, end + 1, 4);
-		plane->format_str[4] = '\0';
+		len = sizeof(plane->format_str) - 1;
+		strncpy(plane->format_str, end + 1, len);
+		plane->format_str[len] = '\0';
 	} else {
 		strcpy(plane->format_str, "XR24");
 	}