Once again, we are seeing the extended GNU make syntax in action. This makefile is read twice on a typical build. When the makefile is invoked from the command line, it notices that the KERNELRELEASE variable has not been set. It locates the kernel source directory by taking advantage of the fact that the symbolic link build in the installed modules directory points back at the kernel build tree. If you are not actually running the kernel that you are building for, you can supply a KERNELDIR= option on the command line, set the KERNELDIR environment variable, or rewrite the line that sets KERNELDIR in the makefile. Once the kernel source tree has been found, the makefile invokes the default: target, which runs a second make command (parameterized in the makefile as $(MAKE)) to invoke the kernel build system as described previously. On the second reading, the makefile sets obj-m, and the kernel makefiles take care of actually building the module.