Building out-of-tree Linux kernel modules is pretty easy. The page https://www.kernel.org/doc/html/latest/kbuild/modules.html shows some basic info. There are some great ways to tinker with the CFLAGS (and other build flags).

The kernel module build uses rules/macros in the linux/scripts/Makefile.lib.

# deprecated old-style kernel build flags
EXTRA_CFLAGS+=-DEXTRA_CFLAGS

The kernel best practice to set build flags is through the ccflags variable.

# these flags added to all C compiles
ccflags-y += -Wall -Werror

File-specific flags can also be set.

# Set DEBUG flag when compiling hello.c
CFLAGS_hello.o += -DDEBUG

A useful feature is also removing flags. I’m currently working with a set of vendor code that has a warning triggered by a newer version of GCC. The compile is failing because the vendor makefiles use -Werror. I can’t modify the code due to license restrictions but I can modify the makefile.

# turn off the -Werror flag
ccflags-remove-y += -Werror

Similarly, can remove flags for a specific file.

# turn off -Werror just for hello.c
CFLAGS_REMOVE_hello.o += -Werror