of: of_property_read_string return -ENODATA when !length

When the length of the string is zero of_property_read_string should
return -ENODATA according to the description of the function.

However, of_property_read_string doesn't check prop->length. If
prop->length is zero, return -ENODATA.

Without this patch the following command in u-boot:

fdt set /chosen/node property-name

results in of_property_read_string returning -EILSEQ when attempting to
read property-name. With this patch, it returns -ENODATA as expected.
Signed-off-by: Stefano Stabellini <stefano.stabellini@xilinx.com>
Signed-off-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20220416003028.1315268-1-sstabellini@kernel.org

drivers/of/property.c

@@ -431,6 +431,9 @@ EXPORT_SYMBOL_GPL(of_property_read_variable_u64_array);
  * property does not have a value, and -EILSEQ if the string is not
  * null-terminated within the length of the property data.
  *
+ * Note that the empty string "" has length of 1, thus -ENODATA cannot
+ * be interpreted as an empty string.
+ *
  * The out_string pointer is modified only if a valid string can be decoded.
  */
 int of_property_read_string(const struct device_node *np, const char *propname,
@@ -439,7 +442,7 @@ int of_property_read_string(const struct device_node *np, const char *propname,
 	const struct property *prop = of_find_property(np, propname, NULL);
 	if (!prop)
 		return -EINVAL;
-	if (!prop->value)
+	if (!prop->length)
 		return -ENODATA;
 	if (strnlen(prop->value, prop->length) >= prop->length)
 		return -EILSEQ;