Several commodity operating systems have used kernel extensions to extend or replace their functionalities. Generally, since the kernel extensions are executed in the same address space with the kernel, a mere fault in the extensions may lead the whole system to be corrupted. So naturally, studies on the kernel extension are mainly proposed with the goal of isolating extension faults from the system. However, previous schemes require the static analysis of the extension module and the modification of kernel source code. The goal of this paper is to remove such overhead stages. This paper proposes Sentry; a lightweight kernel subsystem that provides dependable execution environment for the kernel extensions. We show the efficiency of Sentry through practical implementation on Linux.