diff --git a/arch/sandbox/cpu/start.c b/arch/sandbox/cpu/start.c
index 90a84e9..642be16 100644
--- a/arch/sandbox/cpu/start.c
+++ b/arch/sandbox/cpu/start.c
@@ -205,21 +205,19 @@
 static int sandbox_cmdline_cb_test_fdt(struct sandbox_state *state,
 				       const char *arg)
 {
-	const char *fmt = "/arch/sandbox/dts/test.dtb";
-	char *p;
+	char buf[256];
 	char *fname;
 	int len;
 
-	len = strlen(state->argv[0]) + strlen(fmt) + 1;
+	len = state_get_rel_filename("arch/sandbox/dts/test.dtb", buf,
+				     sizeof(buf));
+	if (len < 0)
+		return len;
+
 	fname = os_malloc(len);
 	if (!fname)
 		return -ENOMEM;
-	strcpy(fname, state->argv[0]);
-	p = strrchr(fname, '/');
-	if (!p)
-		p = fname + strlen(fname);
-	len -= p - fname;
-	snprintf(p, len, fmt);
+	strcpy(fname, buf);
 	state->fdt_fname = fname;
 
 	return 0;
diff --git a/arch/sandbox/cpu/state.c b/arch/sandbox/cpu/state.c
index e0d0112..787e021 100644
--- a/arch/sandbox/cpu/state.c
+++ b/arch/sandbox/cpu/state.c
@@ -396,6 +396,28 @@
 	return old_val;
 }
 
+int state_get_rel_filename(const char *rel_path, char *buf, int size)
+{
+	struct sandbox_state *state = state_get_current();
+	int rel_len, prog_len;
+	char *p;
+	int len;
+
+	rel_len = strlen(rel_path);
+	p = strrchr(state->argv[0], '/');
+	prog_len = p ? p - state->argv[0] : 0;
+
+	/* allow space for a / and a terminator */
+	len = prog_len + 1 + rel_len + 1;
+	if (len > size)
+		return -ENOSPC;
+	strncpy(buf, state->argv[0], prog_len);
+	buf[prog_len] = '/';
+	strcpy(buf + prog_len + 1, rel_path);
+
+	return len;
+}
+
 int state_init(void)
 {
 	state = &main_state;
diff --git a/arch/sandbox/include/asm/state.h b/arch/sandbox/include/asm/state.h
index 07c768a..12741ee 100644
--- a/arch/sandbox/include/asm/state.h
+++ b/arch/sandbox/include/asm/state.h
@@ -266,6 +266,20 @@
 void state_show(struct sandbox_state *state);
 
 /**
+ * state_get_rel_filename() - Get a filename relative to the executable
+ *
+ * This uses argv[0] to obtain a filename path
+ *
+ * @rel_path: Relative path to build, e.g. "arch/sandbox/dts/test.dtb". Must not
+ * have a trailing /
+ * @buf: Buffer to use to return the filename
+ * @size: Size of buffer
+ * @return length of filename (including terminator), -ENOSPC if @size is too
+ * small
+ */
+int state_get_rel_filename(const char *rel_path, char *buf, int size);
+
+/**
  * Initialize the test system state
  */
 int state_init(void);