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

3 files changed, 76 insertions, 1 deletions

diff --git a/tests/README.md b/tests/README.md
index 689a16d8..65afd254 100644
--- a/tests/README.md
+++ b/tests/README.md
@@ -2,6 +2,25 @@
 
 These tests are done for Ubuntu and Debian distros. Please use latest stable Debian or Ubuntu to run the tests.
 
+## Kernel module installation test
+
+The kernel module installation regression test is rootless and does not need
+kernel headers. It uses a fake kernel build tree to verify that `ipoe`,
+`vlan_mon`, and `ppposeq` are installed from their actual build directories,
+honor `DESTDIR`, and make installation fail when `modules_install` fails.
+
+Run it through CTest after configuring the project:
+
+```bash
+ctest --test-dir build --output-on-failure -R '^kernel-module-install$'
+```
+
+It can also be run directly from the repository root:
+
+```bash
+sh tests/kernel-module-install/test.sh cmake
+```
+
 ## Preparations
 
 Install pytest
@@ -100,4 +119,4 @@ If you want to re-run tests 'from scratch', you may want to remove coverage data
 # from root dir (parent for this dir)
 sudo gcovr -d # build report and delete
 sudo gcovr -d # check that data is deleted (any coverage = 0%)
-```
\ No newline at end of file
+```
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"
diff --git a/tests/kernel-module-install/test.sh b/tests/kernel-module-install/test.sh
new file mode 100644
index 00000000..1fbfd623
--- /dev/null
+++ b/tests/kernel-module-install/test.sh
@@ -0,0 +1,38 @@
+#!/bin/sh
+
+set -eu
+
+cmake_command=${1:-cmake}
+script_dir=$(CDPATH= cd -- "$(dirname -- "$0")" && pwd)
+source_dir=$(CDPATH= cd -- "${script_dir}/../.." && pwd)
+work_dir=$(mktemp -d)
+trap 'rm -rf "${work_dir}"' EXIT HUP INT TERM
+
+"${cmake_command}" \
+	-S "${source_dir}" \
+	-B "${work_dir}/build" \
+	-DBUILD_TESTING=OFF \
+	-DBUILD_DRIVER_ONLY=TRUE \
+	-DBUILD_IPOE_DRIVER=TRUE \
+	-DBUILD_VLAN_MON_DRIVER=TRUE \
+	-DBUILD_PPPOSEQ_DRIVER=TRUE \
+	-DIGNORE_GIT=TRUE \
+	-DKDIR="${script_dir}"
+
+"${cmake_command}" --build "${work_dir}/build" --parallel 2
+
+DESTDIR="${work_dir}/stage" \
+	"${cmake_command}" --install "${work_dir}/build"
+
+test -f "${work_dir}/stage/lib/modules/test-kernel/extra/ipoe.ko"
+test -f "${work_dir}/stage/lib/modules/test-kernel/extra/vlan_mon.ko"
+test -f "${work_dir}/stage/lib/modules/test-kernel/extra/ppposeq.ko"
+
+if FAIL_MODULES_INSTALL=1 DESTDIR="${work_dir}/failed-stage" \
+	"${cmake_command}" --install "${work_dir}/build" \
+	>"${work_dir}/failed-install.log" 2>&1; then
+	printf '%s\n' 'kernel module installation unexpectedly succeeded' >&2
+	exit 1
+fi
+
+grep -q 'Failed to install kernel module' "${work_dir}/failed-install.log"