On Thu, Mar 07, 2019 at 10:03:43AM -0500, Joel Fernandes wrote: > On Thu, Mar 07, 2019 at 09:58:24AM +0100, Geert Uytterhoeven wrote: > > Hi Joel, > > > > On Fri, Mar 1, 2019 at 5:10 PM Joel Fernandes (Google) > > <joel@xxxxxxxxxxxxxxxxx> wrote: > > > Introduce in-kernel headers and other artifacts which are made available > > > as an archive through proc (/proc/kheaders.tar.xz file). This archive makes > > > it possible to build kernel modules, run eBPF programs, and other > > > tracing programs that need to extend the kernel for tracing purposes > > > without any dependency on the file system having headers and build > > > artifacts. > > > > > > On Android and embedded systems, it is common to switch kernels but not > > > have kernel headers available on the file system. Raw kernel headers > > > also cannot be copied into the filesystem like they can be on other > > > distros, due to licensing and other issues. There's no linux-headers > > > package on Android. Further once a different kernel is booted, any > > > headers stored on the file system will no longer be useful. By storing > > > the headers as a compressed archive within the kernel, we can avoid these > > > issues that have been a hindrance for a long time. > > > > > > The feature is also buildable as a module just in case the user desires > > > it not being part of the kernel image. This makes it possible to load > > > and unload the headers on demand. A tracing program, or a kernel module > > > builder can load the module, do its operations, and then unload the > > > module to save kernel memory. The total memory needed is 3.8MB. > > > > > > The code to read the headers is based on /proc/config.gz code and uses > > > the same technique to embed the headers. > > > > > > To build a module, the below steps have been tested on an x86 machine: > > > modprobe kheaders > > > rm -rf $HOME/headers > > > mkdir -p $HOME/headers > > > tar -xvf /proc/kheaders.tar.xz -C $HOME/headers >/dev/null > > > cd my-kernel-module > > > make -C $HOME/headers M=$(pwd) modules > > > rmmod kheaders > > > > As the usage pattern will be accessing the individual files, what about > > implementing a file system that provides read-only access to the internal > > kheaders archive? > > > > mount kheaders $HOME/headers -t kheaders > > I thought about it already. This is easier said than done though. The archive > is compressed from 40MB to 3.6MB. If we leave it uncompressed in RAM, then it > will take up the entire 40MB of RAM and in Android we don't even use > disk-based swap. > > So we will need some kind of intra file compressed memory representation that > a filesystem can use for the backing store. I thought of RAM-backed squashfs > but it requires squashfs-tools to be installed at build time (which my host > distro itself didn't have). > > It is just so much easier to use tar + xz at build time, and leave the > decompression task to the user. After decompression, the files will live on > the disk and the page-cache mechanism will free memory when/if the files fall > off the LRUs. > > WDYT? I think the compressed tarball is much simpler/easier overall. If someone really wants the filesystem, they just uncompress it into a tmpfs mount. It's much less moving kernel code to worry about. thanks, greg k-h
