Question

Is there any efficient way (maybe by abusing the gcc preprocessor?) to get a set of stripped kernel sources where all code not needed according to .config is left out?

Answer 1

Well got some steps into a solution.

First, one can obtain the used compiler commands by

make KBUILD_VERBOSE=1 | tee build.log
grep '^  gcc' build.log

For now, I select only one gcc command line for further steps. For example the build of kernel/kmod.c, it looks like:

gcc <LIST OF MANY OPTIONS> -c -o kernel/kmod.o kernel/kmod.c

I now remove the option -c, -o ... and add -E, thus disabling compilation and writing preprocessor output to the screen. Further I add -fdirectives-only to prevent macro expansion and -undef to remove the GNU defined macro definitions. -nostdinc to remove the standard c headers is already added by the kernel makefile.

Now includes are still included and thus expanded on the preprocessor output. Thus I pipe the input file through grep removing them: grep -v '#include' kernel/kmod.c. Now only one include is left: autoconf.h is included by the Makefile’s command line. This is great as it actually defines the macros used by #ifdef CONFIG_... to select the active kernel code.

The only thing left is to filter out the preprocessor comments and the remaining #defines from autoconf.h by means of grep -v '^#'.

The whole pipe looks like:

grep -v '#include' kernel/kmod.c | gcc -E -fdirectives-only -undef <ORIGINAL KERNEL BUILD GCC OPTIONS WITHOUT -c AND -o ...> - |grep -v '^#'

and the result is a filtered version of kernel/kmod.c containing the code that is actually build into kmod.o.

Questions remain: How to do that for the whole source tree? Are there files that are actually build but never used and stripped at linking?