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, 18 insertions, 0 deletions

diff --git a/tests/kernel-module-install/Makefile b/tests/kernel-module-install/Makefile
new file mode 100644
index 00000000..9b8bf2bc
--- /dev/null
+++ b/tests/kernel-module-install/Makefile
@@ -0,0 +1,18 @@
+.PHONY: kernelrelease modules modules_install
+
+kernelrelease:
+	@printf '%s\n' test-kernel
+
+modules:
+	@module=$$(basename "$$(dirname "$(M)")"); \
+	printf 'test module %s\n' "$$module" > "$(M)/$$module.ko"
+
+modules_install:
+	@test "$(M)" != ""
+	@test -d "$(M)"
+	@if test "$${FAIL_MODULES_INSTALL:-0}" = 1; then exit 42; fi
+	@module=$$(basename "$$(dirname "$(M)")"); \
+	test -f "$(M)/$$module.ko"; \
+	destination="$(INSTALL_MOD_PATH)/lib/modules/test-kernel/extra"; \
+	mkdir -p "$$destination"; \
+	cp "$(M)/$$module.ko" "$$destination/$$module.ko"