insmod: ERROR: could not insert module hello.ko

We tried to write a simple class driver framework which will create a directory in /sys/class, we were able to compile the code but got the following error while trying to install the driver as,

$ sudo insmod hello.ko 
insmod: ERROR: could not insert module hello.ko: Unknown symbol in module 

$ dmesg

[ 7514.340407] hello: Unknown symbol __class_register (err 0)
[ 7514.340436] hello: Unknown symbol class_unregister (err 0) 

Solution :

The function __class_create is exported only for GPL modules (exported with EXPORT_SYMBOL_GPL). So, you need to use a GPL license with MODULE_LICENSE macro to make use of that function.

Hence if you append your driver code with

MODULE_LICENSE("GPL"); 

and recompile driver and try insmod again, it will definately work and you will not get such errors.

Reference – https://stackoverflow.com/questions/29578931/unknown-symbol-class-create-err-0