Add fu_bytes_get_data_safe() to check for zero sized data

Fixes https://github.com/fwupd/fwupd/issues/3122

libfwupdplugin/fu-common.c

@@ -3306,6 +3306,38 @@ fu_battery_state_to_string (FuBatteryState battery_state)
 	return NULL;
 }
 
+/**
+ * fu_bytes_get_data_safe:
+ * @bytes: a #GBytes
+ * @bufsz: (out) (optional): location to return size of byte data
+ * @error: A #GError or %NULL
+ *
+ * Get the byte data in the #GBytes. This data should not be modified.
+ * This function will always return the same pointer for a given #GBytes.
+ *
+ * If the size of @bytes is zero, then %NULL is returned and the @error is set,
+ * which differs in behaviour to that of g_bytes_get_data().
+ *
+ * This may be useful when calling g_mapped_file_new() on a zero-length file.
+ *
+ * Returns: a pointer to the byte data, or %NULL.
+ *
+ * Since: 1.6.0
+ **/
+const guint8 *
+fu_bytes_get_data_safe (GBytes *bytes, gsize *bufsz, GError **error)
+{
+	const guint8 *buf = g_bytes_get_data (bytes, bufsz);
+	if (buf == NULL) {
+		g_set_error (error,
+			     G_IO_ERROR,
+			     G_IO_ERROR_INVALID_DATA,
+			     "invalid data");
+		return NULL;
+	}
+	return buf;
+}
+
 /**
  * fu_xmlb_builder_insert_kv:
  * @bn: #JsonBuilder

libfwupdplugin/fu-common.h

@@ -201,6 +201,9 @@ void		 fu_common_dump_bytes		(const gchar	*log_domain,
 GBytes		*fu_common_bytes_align		(GBytes		*bytes,
 						 gsize		 blksz,
 						 gchar		 padval);
+const guint8	*fu_bytes_get_data_safe		(GBytes		*bytes,
+						 gsize		*bufsz,
+						 GError		**error);
 gboolean	 fu_common_bytes_is_empty	(GBytes		*bytes);
 gboolean	 fu_common_bytes_compare	(GBytes		*bytes1,
 						 GBytes		*bytes2,

libfwupdplugin/fu-self-test.c

@@ -1068,6 +1068,45 @@ fu_common_store_cab_error_missing_file_func (void)
 	g_assert_null (silo);
 }
 
+static void
+fu_common_bytes_get_data_func (void)
+{
+	const gchar *fn = "/tmp/fwupdzero";
+	const guint8 *buf;
+	gboolean ret;
+	g_autoptr(GBytes) bytes1 = NULL;
+	g_autoptr(GBytes) bytes2 = NULL;
+	g_autoptr(GBytes) bytes3 = NULL;
+	g_autoptr(GError) error = NULL;
+	g_autoptr(GMappedFile) mmap = NULL;
+
+	/* create file with zero size */
+	ret = g_file_set_contents (fn, NULL, 0, &error);
+	g_assert_no_error (error);
+	g_assert_true (ret);
+
+	/* check we got zero sized data */
+	bytes1 = fu_common_get_contents_bytes (fn, &error);
+	g_assert_no_error (error);
+	g_assert_nonnull (bytes1);
+	g_assert_cmpint (g_bytes_get_size (bytes1), ==, 0);
+	g_assert_nonnull (g_bytes_get_data (bytes1, NULL));
+
+	/* do the same with an mmap mapping, which returns NULL on empty file */
+	mmap = g_mapped_file_new (fn, FALSE, &error);
+	g_assert_no_error (error);
+	g_assert_nonnull (mmap);
+	bytes2 = g_mapped_file_get_bytes (mmap);
+	g_assert_nonnull (bytes2);
+	g_assert_cmpint (g_bytes_get_size (bytes2), ==, 0);
+	g_assert_null (g_bytes_get_data (bytes2, NULL));
+
+	/* use the safe function */
+	buf = fu_bytes_get_data_safe (bytes2, NULL, &error);
+	g_assert_error (error, G_IO_ERROR, G_IO_ERROR_INVALID_DATA);
+	g_assert_null (buf);
+}
+
 static void
 fu_common_store_cab_error_size_func (void)
 {
@@ -2664,6 +2703,7 @@ main (int argc, char **argv)
 	g_test_add_func ("/fwupd/common{cab-error-wrong-checksum}", fu_common_store_cab_error_wrong_checksum_func);
 	g_test_add_func ("/fwupd/common{cab-error-missing-file}", fu_common_store_cab_error_missing_file_func);
 	g_test_add_func ("/fwupd/common{cab-error-size}", fu_common_store_cab_error_size_func);
+	g_test_add_func ("/fwupd/common{bytes-get-data}", fu_common_bytes_get_data_func);
 	g_test_add_func ("/fwupd/common{spawn)", fu_common_spawn_func);
 	g_test_add_func ("/fwupd/common{spawn-timeout)", fu_common_spawn_timeout_func);
 	g_test_add_func ("/fwupd/common{firmware-builder}", fu_common_firmware_builder_func);

libfwupdplugin/fwupdplugin.map

@@ -716,6 +716,7 @@ LIBFWUPDPLUGIN_1.6.0 {
     fu_battery_state_to_string;
     fu_byte_array_align_up;
     fu_byte_array_set_size_full;
+    fu_bytes_get_data_safe;
     fu_common_align_up;
     fu_context_add_compile_version;
     fu_context_add_firmware_gtype;

plugins/uefi-dbx/fu-efi-image.c

@@ -110,7 +110,9 @@ fu_efi_image_new (GBytes *data, GError **error)
 	g_autoptr(GPtrArray) checksum_regions = NULL;
 
 	/* verify this is a DOS file */
-	buf = g_bytes_get_data (data, &bufsz);
+	buf = fu_bytes_get_data_safe (data, &bufsz, error);
+	if (buf == NULL)
+		return NULL;
 	if (!fu_common_read_uint16_safe (buf, bufsz,
 					 _DOS_OFFSET_SIGNATURE,
 					 &dos_sig, G_LITTLE_ENDIAN, error))