No longer works on recent kernels because kallsyms_lookup_name is unexported.

[Hello1024]

# make
make -C /lib/modules/5.8.0-43-generic/build M=/home/foo/devmem-full-access modules
make[1]: Entering directory '/usr/src/linux-headers-5.8.0-43-generic'
  MODPOST /home/foo/devmem-full-access/Module.symvers
ERROR: modpost: "kallsyms_lookup_name" [/home/foo/devmem-full-access/devmem-full-access.ko] undefined!
make[2]: *** [scripts/Makefile.modpost:111: /home/foo/devmem-full-access/Module.symvers] Error 1
make[2]: *** Deleting file '/home/foo/devmem-full-access/Module.symvers'
make[1]: *** [Makefile:1693: modules] Error 2
make[1]: Leaving directory '/usr/src/linux-headers-5.8.0-43-generic'
make: *** [Makefile:12: all] Error 2

Looks like this code will need to be migrated to the livepatch API.