sandbox: Add a function to load a relative file path

At present this implementation is specific to loading the test FDT. We
plan to load others, so create a generic function to handle this.

The path is now limited to 256 characters, to simplify the code.

When there is an empty argv[0] (which should not happen), the function now
just uses the path as is, with no prefix.

Signed-off-by: Simon Glass <sjg@chromium.org>

arch/sandbox/cpu/start.c

@@ -205,21 +205,19 @@ SANDBOX_CMDLINE_OPT_SHORT(default_fdt, 'D', 0,
 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;

arch/sandbox/cpu/state.c

@@ -396,6 +396,28 @@ bool autoboot_set_keyed(bool autoboot_keyed)
 	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;

arch/sandbox/include/asm/state.h

@@ -265,6 +265,20 @@ void state_reset_for_test(struct sandbox_state *state);
  */
 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
  */