cmake: fix kernel module installation

Install each module from its real build directory, propagate staged install roots, and fail when kbuild modules_install fails. Add a rootless CTest and CI regression for successful and failed installs.

Best practices followed:

Use the standard CMake DESTDIR and kbuild INSTALL_MOD_PATH staging interfaces:
https://cmake.org/cmake/help/latest/envvar/DESTDIR.html
https://docs.kernel.org/kbuild/modules.html#install-mod-path

Check child command status with execute_process RESULT_VARIABLE:
https://cmake.org/cmake/help/latest/command/execute_process.html

Register repeatable tests with CTest add_test:
https://cmake.org/cmake/help/latest/command/add_test.html

1 files changed, 35 insertions, 0 deletions

diff --git a/cmake/install-kernel-module.cmake b/cmake/install-kernel-module.cmake
new file mode 100644
index 00000000..1350e7c9
--- /dev/null
+++ b/cmake/install-kernel-module.cmake
@@ -0,0 +1,35 @@
+if (NOT DEFINED KERNEL_BUILD_DIR OR KERNEL_BUILD_DIR STREQUAL "")
+	message(FATAL_ERROR "Kernel build directory is not set")
+endif ()
+
+if (NOT DEFINED KERNEL_MODULE_BUILD_DIR OR
+    KERNEL_MODULE_BUILD_DIR STREQUAL "")
+	message(FATAL_ERROR "Kernel module build directory is not set")
+endif ()
+
+set(_install_command
+	make -C "${KERNEL_BUILD_DIR}"
+	"M=${KERNEL_MODULE_BUILD_DIR}"
+	modules_install
+)
+
+# DESTDIR is the conventional CMake staging root.  The kernel build system
+# uses INSTALL_MOD_PATH for the same purpose, so propagate it explicitly.
+if (NOT "$ENV{DESTDIR}" STREQUAL "")
+	list(APPEND _install_command "INSTALL_MOD_PATH=$ENV{DESTDIR}")
+endif ()
+
+execute_process(
+	COMMAND ${_install_command}
+	RESULT_VARIABLE _install_result
+)
+
+if (NOT "${_install_result}" STREQUAL "0")
+	message(FATAL_ERROR
+		"Failed to install kernel module from "
+		"${KERNEL_MODULE_BUILD_DIR} (exit status: ${_install_result})"
+	)
+endif ()
+
+unset(_install_command)
+unset(_install_result)