Is there a Buildroot setting for making kernel source available to compile against? - embedded-linux

Question

I've been given a .zip file containing source for a proprietary kernel module. Once unzip'd, there is an install script that needs to be run. The install script untar's the actual source and builds the kernel module. It requires kernel headers to compile against.
Here is my Buildroot .mk file:
FOOCO_VERSION = 1.0
FOOCO_SOURCE = cust_kernel_drvr.zip
FOOCO_SITE = /mnt/third-party/fooco
FOOCO_SITE_METHOD = local
define FOOCO_CONFIGURE_CMDS
unzip $(#D)/$(FOOCO_SOURCE) -d $(#D)
endef
define FOOCO_BUILD_CMDS
chmod +x $(#D)/TOOLS/Linux_x64/DRIVER/install
cd $(#D)/TOOLS/Linux_x64/DRIVER; $(SHELL) ./install
rm -rf $(#D)
endef
$(eval $(generic-package))
This results in the following log output and error:
(Note: I enabled debugging that shows the start and end of each step.)
DEBUG: start | rsync | fooco
>>> fooco 1.0 Syncing from source dir /mnt/third-party/fooco
rsync -au --chmod=u=rwX,go=rX --exclude .svn --exclude .git --exclude .hg --exclude .bzr --exclude CVS /mnt/third-party/fooco/ /root/buildroot-2022.02.1/output/build/fooco-1.0
DEBUG: end | rsync | fooco
DEBUG: start | configure | fooco
>>> fooco 1.0 Configuring
unzip /root/buildroot-2022.02.1/output/build/fooco-1.0/cust_kernel_drvr.zip -d /root/buildroot-2022.02.1/output/build/fooco-1.0
Archive: /root/buildroot-2022.02.1/output/build/foofo-1.0/cust_kernel_drvr.zip
[snip]
creating: /root/buildroot-2022.02.1/output/build/fooco-1.0/TOOLS/Linux_x64/
creating: /root/buildroot-2022.02.1/output/build/fooco-1.0/TOOLS/Linux_x64/DRIVER/
inflating: /root/buildroot-2022.02.1/output/build/fooco-1.0/TOOLS/Linux_x64/DRIVER/install
inflating: /root/buildroot-2022.02.1/output/build/fooco-1.0/TOOLS/Linux_x64/DRIVER/cust_kernel_drvr-1.2.0.15-0.noarch.rpm
inflating: /root/buildroot-2022.02.1/output/build/fooco-1.0/TOOLS/Linux_x64/DRIVER/cust_kernel_drvr.tar.gz
inflating: /root/buildroot-2022.02.1/output/build/fooco-1.0/TOOLS/Linux_x64/DRIVER/license_gpl.txt
[snip]
DEBUG: end | configure | fooco
DEBUG: start | build | fooco
>>> fooco 1.0 Building
chmod +x /root/buildroot-2022.02.1/output/build/fooco-1.0/TOOLS/Linux_x64/DRIVER/install
cd /root/buildroot-2022.02.1/output/build/fooco-1.0/TOOLS/Linux_x64/DRIVER; /bin/bash ./install
Extracting archive..OK!
Compiling the driver...Error: make[1]: Entering directory '/root/buildroot-2022.02.1/output/build/fooco-1.0/TOOLS/Linux_x64/DRIVER/fooco_cust/src/linux/driver'
common.mk:82: *** Kernel header files not in any of the expected locations.
common.mk:83: *** Install the appropriate kernel development package, e.g.
common.mk:84: *** kernel-devel, for building kernel modules and try again. Stop.
make[1]: Leaving directory '/root/buildroot-2022.02.1/output/build/fooco-1.0/TOOLS/Linux_x64/DRIVER/fooco_cust/src/linux/driver'
Error: unable to find driver file (fooco_cust.ko) in /root/buildroot-2022.02.1/output/build/fooco-1.0/TOOLS/Linux_x64/DRIVER/fooco_cust/src/linux/driver
rm -rf /root/buildroot-2022.02.1/output/build/fooco-1.0
DEBUG: end | build | fooco
touch: cannot touch '/root/buildroot-2022.02.1/output/build/fooco-1.0/.stamp_built': No such file or directory
make: *** [/root/buildroot-2022.02.1/output/build/fooco-1.0/.stamp_built] Error 1
package/pkg-generic.mk:289: recipe for target
'/root/buildroot-2022.02.1/output/build/fooco-1.0/.stamp_built' failed
I found that the make files that came with the kernel module are looking in several places for the kernel headers:
/lib/modules/${BUILD_KERNEL}/source \
/lib/modules/${BUILD_KERNEL}/build \
/usr/src/linux-${BUILD_KERNEL} \
/usr/src/linux-$(${BUILD_KERNEL} | sed 's/-.*//') \
/usr/src/kernel-headers-${BUILD_KERNEL} \
/usr/src/kernel-source-${BUILD_KERNEL} \
/usr/src/linux-$(${BUILD_KERNEL} | sed 's/\([0-9]*\.[0-9]*\)\..*/\1/') \
/usr/src/linux \
/usr/src/kernels/${BUILD_KERNEL} \
/usr/src/kernels
Why is the kernel source not visible to this build? I thought that, since Buildroot is building the kernel as part of the overall process, the header files would be available for subsequent kernel module compiles. Am I missing a setting? I feel that I'm not understanding the Buildroot process in a basic way, even after referring to the manual many times.
I'm using Buildroot 2022.02.1 and kernel 5.15.33.

Answer 1

Your download/extract logic is very convoluted. You should really use something like this:
FOO_SITE = /mnt/third-party/fooco
FOO_SOURCE = cust_kernel_drvr.zip
FOO_SITE_METHOD = file
define FOO_EXTRACT_CMDS
unzip $(FOO_DLDIR)/$(FOOCO_SOURCE) -d $(#D)
endef
Regarding the build issue: it is impossible to help without studying the specific build system of this kernel module. Very likely you will need to pass some environment variables to tell the build system where your kernel source code is located, and possibly other things. But without looking at the specific details, it's impossible to help you.
You can have a look at how standard out of tree kernel modules are handled by looking at the package/pkg-kernel-module.mk code. However, that will not be directly useful to a package like yours that uses a custom installation script.

Answer 2

The magic was the LINUX_DIR variable which, according to Buildroot user manual:
contains the path to where the Linux kernel has been extracted and built.
I was able to patch the install script to send this variable to the make file that was looking for the kernel.