Strip Linux kernel sources according to .config

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

Compile everything and use atime to find out which files were not used. It might not be very accurate but it's probably worth a try.

Answer 2

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?

Answer 3

Kernel Minimization Script :

A project inspired by this question and providing an easy answer... It contains a Python script that generate a minimized sources code during build time. The new minimized source tree will only contain used sources. (project page)

Info :

The script is tested working with the kernel v4.14.x, however building the kernel one more time from those generated minimized sources require to copy make files and Kconfig files etc... at least we could easily isolate only used source for investigations and development

Usage :

cd /kernel/sources
make 
wget https://github.com/Hitachi-India-Pvt-Ltd-RD/minimization/raw/master/minimize.py
export PATH=$PATH:`pwd`
make C=2 CHECK=minimize.py CF="-mindir ../path-to-minimized-source-tree/"

Note & Reminder :

If we are building within and against the targeted machine, we also have the make localmodconfig command that shrink the current config file with only the currently used modules, if used before "Minimization" it will generate further more stripped sources