[…]Enter the Linux Kernel reducer, or lk-reducer for short. This tool helps avoid some of the problems we have discussed thus far. It works by monitoring file system access while building the Linux kernel. (By no means is its utility limited to the Linux Kernel, but it is simply where we found a need.) This is possible because providing the Linux Kernel source code is a legal requirement under the Linux GNU Public License (LGPL). By monitoring the build process, we are able to determine which files from within the source tree have been used to build the final kernel image. Some files will inevitably not get used and thus we can determine that they are not needed. Therefore, they can be eliminated from review during a source code audit. This saves time during both manual and automated source code reviews. The current incarnation of the tool is a slight modification of an implementation by Jann Horn. The first incarnation consisted of using strace(1) and a bunch of shell scripts to monitor calls to the open(2) system call. Jann developed his version around the Linux inotify subsystem. His implementation is much more clean and performant. My only modifications were to let the user decide how to process the monitoring results based on a data file. The tool generates a file called “lk-reducer.out” that shows whether each file was Accessed, Untouched, or Generated. Let us see it in action![…]

http://www.droidsec.org/blogs/2017/05/22/a-simple-tool-for-linux-kernel-audits.html