efi_loader: function to get GUID for variable name

In multiple places we need the default GUID matching a variable name.
The patch provides a library function. For secure boot related variables
like 'PK', 'KEK', 'db' a lookup table is used. For all other variable
names EFI_GLOBAL_VARIABLE is returned.

Signed-off-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>
Reviewed-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>

include/efi_variable.h

@@ -256,6 +256,14 @@ efi_status_t efi_init_secure_state(void);
 enum efi_auth_var_type efi_auth_var_get_type(const u16 *name,
 					     const efi_guid_t *guid);
 
+/**
+ * efi_auth_var_get_guid() - get the predefined GUID for a variable name
+ *
+ * @name:	name of UEFI variable
+ * Return:	guid of UEFI variable
+ */
+const efi_guid_t *efi_auth_var_get_guid(const u16 *name);
+
 /**
  * efi_get_next_variable_name_mem() - Runtime common code across efi variable
  *                                    implementations for GetNextVariable()

lib/efi_loader/efi_var_common.c

@@ -385,6 +385,15 @@ enum efi_auth_var_type efi_auth_var_get_type(const u16 *name,
 	return EFI_AUTH_VAR_NONE;
 }
 
+const efi_guid_t *efi_auth_var_get_guid(const u16 *name)
+{
+	for (size_t i = 0; i < ARRAY_SIZE(name_type); ++i) {
+		if (!u16_strcmp(name, name_type[i].name))
+			return name_type[i].guid;
+	}
+	return &efi_global_variable_guid;
+}
+
 /**
  * efi_get_var() - read value of an EFI variable
  *