This Python script will generate minimized source code with the stripping techniques below.
The minimized source tree will consist of only used source code at compile time:

• configuration conditionals such as #ifdef #if #endif will be eliminated.
• #define macros remain as are.
• #include sentences remain as are.

Currently this minimization script is applicable to Linux kernel and Busybox project.

The minimized sources will be in more human-readable form, and it contains less lines of code.
Such source code transformation helps us make code inspection easier and more efficient in general.
Also, it's going to be beneficial for test or verification by having eliminated unused code.

The original idea comes from a post in stackoverflow.
Strip Linux kernel sources according to .config

The minimization script requires the following commands working in the host machine.

• diffstat
• diff
• echo
• file
• gcc (and everything else that is needed to build Linux Kernel or BusyBox)
• python (2.x and 3.x compatibility is supported)

1. Navigate to your kernel tree directory.
   example:

$ cd linux-4.4.1

2. copy minimize.py to your kernel tree directory.

3. prepare configuration file
   Prepare your tuned .config and put it to the kernel tree directory.
   Or generate .config by make command.
   example:

$ make allnoconfig

4. add the script directory path
   example:

$ export PATH=$PATH:`pwd`

5. run make with the following options.

$ make C=1 CHECK=minimize.py CF="-mindir ../minimized-tree/"

use C=1 to perform minimization only for (re)compilation target files.
use C=2 to perform minimization for all the source files regardless of whether they are compilation target or not.
specify any output directory at -mindir option in CF flag
C, CHECK flags are mandatory. -mindir option in CF flag is optional, the default minimized tree location is ../minimized-tree if CF is omitted.

Or, minimization is also applicable for partial build target.
example:

$ make drivers C=1 CHECK=minimize.py CF="-mindir ../minimized-tree/"

If successful, compilation and minimization will be executed at the same time,
and the minimized kernel tree will be generated under ../minimized-tree/.
Note that only the target C source files will be minimized. The other file contents(included header etc) remain as are for a reason.

The script also works with other projects provided that their Makefile support source CHECK(sparse by default) option via C=1 flag.
For example, this is directly applicable to BusyBox with exactly the same workflow.
example:

$ wget http://busybox.net/downloads/busybox-1.24.1.tar.bz2
$ tar jxf busybox-1.24.1.tar.bz2
$ cd busybox-1.24.1
$ make defconfig
$ export PATH=$PATH:<path to minimize.py>
$ make C=1 CHECK=minimize.py CF="-mindir ../minimized-busybox/"

On completed, the minimized BusyBox sources will be generated under directory ../minimized-busybox/.
In the example above, there will be 505 C files(with NO #ifdef blocks) in the minimized tree,
where there were originally 629 C files(with many complicated #ifdef conditional blocks).

There is a minimization summary info available. In the minimized tree directory, a text file diffstat.log and minimize.patch will be generated.
If you run minimize.py with the filepath of diffstat.log, it will display summary info like this.

$ ./minimize.py ../minimized-busybox/diffstat.log 
296 out of 505 compiled C files have been minimized.
Unused 20460 lines(11% of the original C code) have been removed.

For BusyBox, you can build the minimized source tree with the same make commands.
You can copy the minimized C files to the busybox project directory by overwriting the original ones, then make them.
Among the minimized built products, busybox_unstripped.out and busybox_unstripped.out are exactly the same as the original ones(md5sum matches).
The executables busybox and busybox_unstripped differ in some points(build time stamp etc),
but the disassembled code (output of objdump -d busybox) exactly matches with the original ones also.

For the Linux Kernel, we confirmed objdump -d vmlinux.o exatcly matches between the minimized and the original built product in allnoconfig condition.
For compilation of the minimized Kernel Tree, allnoconfig, defconfig(x86), and cross-environment omap2plus_defconfig(arm-linux-gnueabi) are confirmed successful.
For compilation of the minimized BusyBox, allnoconfig and defconfig(x86) are confirmed successful.
Other kernel configs or architectures are yet to be determined.

1. Consider extending this technique to other build system such as CMake.
2. Combine analysis execution at one time (like --with-spatch option).
3. Try other kernel/busybox config or othre architectures.

• Works on either Python 2.x or 3.x without any modification.
• For Linux Kernel, we've confirmed 4.0.7, 4.3.3, and 4.4.1 are applicable.
• For BusyBox, we've confirmed 1.24.1 is applicable.

• relevant discussions in the SIL2LinuxMP mailing list
  The mailing list is currently only available in SIL2LinuxMP community.

If you have problems, questions, ideas or suggestions, please contact below, or post to the SIL2LinuxMP mailing list.

• Desai Krishnaji [email protected]
• Kotaro Hashimoto [email protected]