This is a helper program to clean unused files from a project source directory. It's main purpose is for reducing the size of a Linux Kernel source tree.

lk-reducer is based on "cleanmysourcetree" by Jann Horn. Original idea by Joshua J. Drake. This is a modified version of the second implementation of this tool. You can find the unmodified second implementation at https://git.thejh.net/?p=cleanmysourcetree.git

Motivation

The Linux Kernel has grown terribly bloated over the years. It contains hundreds of drivers and other code that are often not needed to build a single kernel. This creates a great deal of noise -- especially when conducting something like a source code audit. Wouldn't it be nice if we could get rid of everything we don't need?

How it Works

This program uses the Linux inotify subsystem to monitor for accesses and modifications within a directory hierarchy. By watching which files are and are not accessed during a successful build, we can determine which files are unnecessary and thus able to be removed.

NOTE: inotify doesn't work well with symbolic links, so an extra step is needed to copy those.

Dependencies

This code depends on the "uthash" library. You can install it via a package if your distribution has one (uthash-dev on Ubuntu) or from the original repository at https://github.com/troydhanson/uthash

Limitations

Modern Linux kernels limit the amount of inotify watches that a user can use. If your attempt to run lk-reducer fails with an error indicating no more watches are allowed, you will need to increase the fs.inotify.max_user_watches kernel parameter.

dev:0:~$ sudo sysctl fs.inotify.max_user_watches=16384
dev:0:~$

Usage

To use this program, start by invoking it with the path to the Linux kernel source you're planning to reduce. If no target directory is given, it defaults to the current directory.

dev:0:~$ lk-reducer /android/source/kernel/msm
dev:0:msm$

After installing monitoring, it will spawn a fresh shell within the provided directory. Build the kernel as you would normally. After you're done, clean up the build and exit the sub-shell.

dev:0:msm$ make marlin_defconfig
dev:0:msm$ make -j$(NPROCS)
dev:0:msm$ make mrproper

Exiting will transfer control back to the reducer, which will proceed to generate a file containing all of the files and their statuses in "lk-reducer.out". You can use this file to then do further manipulations to the source tree. The possible statuses are Accessed, Untouched, or Generated.

You could delete all the untouched files and directories:

dev:0:msm$ grep ^U lk-reducer.out | cut -c 3- | xargs rm -vf "$F"

You could copy all of the essential files and directories to a new location:

dev:0:msm$ grep ^A lk-reducer.out | cut -c 3- > lk-reducer-keep.out
dev:0:msm$ cpio -pdm ../msm-reduced/ < lk-reducer-keep.out
dev:0:msm$ # Or, use tar if cpio isn't installed.
dev:0:msm$ tar cf - -T lk-reducer-keep.out | tar xf - -C ../msm-reduced/

As we said previously, symbolic links aren't handled properly, so you might want to copy those separately.

dev:0:msm$ find . -type l -print | cpio -pdm ../msm-reduced/

Sample Use

This is what it looked like when we used it in practice:

dev:0:msm$ du -hs --exclude .git .
750M    .
dev:0:msm$ lk-reducer
dropping you into an interactive shell now. compile the project, then exit the shell.
dev:0:msm$ make marlin_defconfig
  HOSTCC  scripts/basic/fixdep
  HOSTCC  scripts/kconfig/conf.o
  SHIPPED scripts/kconfig/zconf.tab.c
  SHIPPED scripts/kconfig/zconf.lex.c
  SHIPPED scripts/kconfig/zconf.hash.c
  HOSTCC  scripts/kconfig/zconf.tab.o
  HOSTLD  scripts/kconfig/conf
drivers/soc/qcom/Kconfig:381:warning: choice value used outside its choice group
drivers/soc/qcom/Kconfig:386:warning: choice value used outside its choice group
#
# configuration written to .config
#
dev:0:msm$ make -j30 > ../msm-marlin-make.out 2>&1
dev:0:msm$ make mrproper
  CLEAN   .
  CLEAN   arch/arm64/kernel/vdso
  CLEAN   arch/arm64/kernel
  CLEAN   crypto/asymmetric_keys
  CLEAN   kernel/time
  CLEAN   kernel
  CLEAN   lib
  CLEAN   net/wireless
  CLEAN   security/selinux
  CLEAN   usr
  CLEAN   arch/arm64/boot/dts/htc
  CLEAN   arch/arm64/boot
  CLEAN   .tmp_versions
  CLEAN   scripts/basic
  CLEAN   scripts/dtc
  CLEAN   scripts/kconfig
  CLEAN   scripts/mod
  CLEAN   scripts/selinux/genheaders
  CLEAN   scripts/selinux/mdp
  CLEAN   scripts
  CLEAN   include/config include/generated arch/arm64/include/generated
  CLEAN   .config .version include/generated/uapi/linux/version.h Module.symvers
dev:0:msm$ exit
exit
processing remaining events...
inotify event collection phase is over, dumping results to "lk-reducer.out"...
cleanup complete
dev:0:msm$ grep -h ^A lk-reducer.out | sort -u | cut -c 3- | grep -v '\./\.git\/' > lk-reducer-keep.out
dev:0:msm$ mkdir ../msm-marlin-reduced
dev:0:msm$ tar cf - -T lk-reducer-keep.out | tar xf - -C ../msm-marlin-reduced/
dev:0:msm$ du -hs ../msm-marlin-reduced/
132M    ../msm-marlin-reduced/

750M down to 132M!! That's a lot less noise!

Authors

This project is a collaboration between Joshua J. Drake (@jduck) and Jann Horn (@TheJH).