Yocto: Why is struct module undefined during building an external kernel module

Question

I am trying to add the example external kernel module hello-mod from meta-skeleton/recipes-kernel to my image using the Yocto build system. When I try to compile it with bitbake hello-mod, however, it fails at a second stage complaining that struct module has no member named name on the line .name = KBUILD_MODNAME,. It seems as if struct module is not being defined even though linux/module.h has been included. This is the generic module code that doesn't seem to compile:

#include <linux/module.h>
#include <linux/vermagic.h>
#include <linux/compiler.h>

MODULE_INFO(vermagic, VERMAGIC_STRING);
MODULE_INFO(name, KBUILD_MODNAME);

__visible struct module __this_module
__attribute__((section(".gnu.linkonce.this_module"))) = {
    .name = KBUILD_MODNAME,
    .init = init_module,
#ifdef CONFIG_MODULE_UNLOAD
    .exit = cleanup_module,
#endif
    .arch = MODULE_ARCH_INIT,
};

#ifdef RETPOLINE
MODULE_INFO(retpoline, "Y");
#endif

static const char __module_depends[]
__used
__attribute__((section(".modinfo"))) =
"depends=";

I am unable to see anything wrong in the compilation log, make is being invoked (correctly, to the best of knowledge) with the following line:

make -C /path/to/poky-rocko/build/tmp/work-shared/duovero/kernel-source \ 
M=/path/to/poky-rocko/build/tmp/work/duovero-poky-linux-gnueabi/hello-mod/0.1-r0

and I can confirm that module.h exists in the directory: /path/to/poky-rocko/build/tmp/work-shared/duovero/kernel-source/include/linux

So I am running out of and looking for ideas on how to debug this issue. For the record, these are the current versions of the layers, kernel etc:

poky is 9915e071bcadd7c4d5363a067c529889851d37a5 (rocko) from git.yoctoproject.org
linux is 9dc30ff9a115559cc55673d0b1d3c576402d073e from git.kernel.org

Any help or hints would really be appreciated!

"It seems as if struct module is not being defined..." - No, the error struct module has no member named name means that the structure is defined, but has no specified field. An error about using undefined structure type looks different. — Tsyvarev
– Tsyvarev, Commented May 14, 2018 at 21:32
I haven't modified the example, just included it into my layer. I agree that the error indicates that something is defining struct module, however, struct module as defined in <linux/module.h> definitely has a member called name. — mallwright
– mallwright, Commented May 15, 2018 at 10:08
Do you have CONFIG_MODULES=y in your kernel .config? That's probably a silly question because it shouldn't get to stage 2 if it is not defined, but the struct module full declaration does depend on the CONFIG_MODULES macro being defined. — Ian Abbott
– Ian Abbott, Commented May 15, 2018 at 13:42
Thanks for the feedback Ian, as you guessed though, CONFIG_MODULES=y is indeed defined. — mallwright
– mallwright, Commented May 16, 2018 at 7:56

mallwright · Accepted Answer · 2018-05-16 14:12:11Z

It seems to be an issue with the Yocto build system. The hello-mod module builds when I follow these steps.

Check out a version of the kernel 4.16.x, add hello-mod to MACHINE_ESSENTIAL_EXTRA _RRECOMMENDS and watch the build fail.
Check out a copy of another kernel 4.16.y by setting SRCREV to the corresponding commit id, continue the build and watch it succeed.

So far I have observed this when moving from 4.16.5 to 4.16.8 and vice versa. So at this point I am pretty sure that this is an issue with the build system.

I had a close look at the output of bitbake -e using grep (against "hello" and "MACHINE_ESSENTIAL_EXTRA_RRECOMMENDS") and diff but couldn't find any significant differences. I also checked the compilation and configuration logs, the only interesting thing I saw was in the configuration logs where on the second run this is printed:

DEBUG: Executing shell function do_configure
NOTE: make KERNEL_SRC=/path/to/poky-rocko/build/tmp/work-shared/duovero/kernel-source clean
rm -f *.o *~ core .depend .*.cmd *.ko *.mod.c
rm -f Module.markers Module.symvers modules.order
rm -rf .tmp_versions Modules.symvers
DEBUG: Shell function do_configure finished
DEBUG: Executing python function do_qa_configure
DEBUG: Python function do_qa_configure finished

while on the first run, we only have:

DEBUG: Executing shell function do_configure
DEBUG: Shell function do_configure finished
DEBUG: Executing python function do_qa_configure
DEBUG: Python function do_qa_configure finished

Although this may have just occurred since this was the second run of the configure task for hello-mod. This seems very much like a bug in the build system to me. To this end, I have reported the issue on Bugzilla:

https://bugzilla.yoctoproject.org/show_bug.cgi?id=12748

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