Linux Kernel out of tree Driver Compilation failed with BUILD_BUG_ON_ZERO - linux

Question

On a Raspberry Pi 3, I am compiling a custom Linux Kernel and want to include an open source software PWM kernel driver.
I cloned the following repository for the Linux source:
https://github.com/raspberrypi/linux.git
I cloned the following repository for the Software PWM:
https://github.com/dagon666/rpi_SoftPwm
After successfully building the kernel, I compile the software PWM driver per the README like this:
pi#raspberrypi:~/Desktop/rpi_SoftPwm $ make M=/home/pi/Desktop/rpi_SoftPwm -C /home/pi/linux/ modules
The compilation fails with the error below:
make: Entering directory '/home/pi/linux'
CC [M] /home/pi/Desktop/rpi_SoftPwm/pwm.o
In file included from ./include/linux/thread_info.h:11:0,
from ./include/asm-generic/preempt.h:4,
from ./arch/arm/include/generated/asm/preempt.h:1,
from ./include/linux/preempt.h:59,
from ./include/linux/spinlock.h:50,
from ./include/linux/seqlock.h:35,
from ./include/linux/time.h:5,
from ./include/linux/stat.h:18,
from ./include/linux/module.h:10,
from /home/pi/Desktop/rpi_SoftPwm/pwm.c:2:
./include/linux/bug.h:37:45: error: negative width in bit-field ‘<anonymous>’
#define BUILD_BUG_ON_ZERO(e) (sizeof(struct { int:-!!(e); }))
^
./include/linux/kernel.h:854:3: note: in expansion of macro ‘BUILD_BUG_ON_ZERO’
BUILD_BUG_ON_ZERO((perms) & 2) + \
^
./include/linux/sysfs.h:102:12: note: in expansion of macro ‘VERIFY_OCTAL_PERMISSIONS’
.mode = VERIFY_OCTAL_PERMISSIONS(_mode) }, \
^
/home/pi/Desktop/rpi_SoftPwm/pwm.c:155:2: note: in expansion of macro ‘__ATTR’
__ATTR(export, 0222, NULL, export_store),
^
./include/linux/bug.h:37:45: error: negative width in bit-field ‘<anonymous>’
#define BUILD_BUG_ON_ZERO(e) (sizeof(struct { int:-!!(e); }))
^
./include/linux/kernel.h:854:3: note: in expansion of macro ‘BUILD_BUG_ON_ZERO’
BUILD_BUG_ON_ZERO((perms) & 2) + \
^
./include/linux/sysfs.h:102:12: note: in expansion of macro ‘VERIFY_OCTAL_PERMISSIONS’
.mode = VERIFY_OCTAL_PERMISSIONS(_mode) }, \
^
/home/pi/Desktop/rpi_SoftPwm/pwm.c:156:2: note: in expansion of macro ‘__ATTR’
__ATTR(unexport, 0222, NULL, unexport_store),
^
scripts/Makefile.build:299: recipe for target '/home/pi/Desktop/rpi_SoftPwm/pwm.o' failed
make[1]: *** [/home/pi/Desktop/rpi_SoftPwm/pwm.o] Error 1
Makefile:1490: recipe for target '_module_/home/pi/Desktop/rpi_SoftPwm' failed
make: *** [_module_/home/pi/Desktop/rpi_SoftPwm] Error 2
make: Leaving directory '/home/pi/linux'
I've tried to research this on Google and have not seen a solution. The failure has to do with the __ATTR macro. This used to compile with the older Linux Kernel 3.19, however now Raspbian is up to Linux 4+.
How can I get this to compile? Thanks.

Answer 1

Linux kernel developers tend to create attributes (files under /sys) non-writable for non-root users. This is what macro VERIFY_OCTAL_PERMISSIONS checks: permissions shouldn't have flag S_IWOTH (second bit) set.
Replace permissions 0222 to 0220, so compilation will succeed.