Re: [PATCH v4 1/2] Provide in-kernel headers for making it easy to extend the kernel

[Joel Fernandes <joel@joelfernandes.org>]

On Thu, Mar 07, 2019 at 04:23:03PM +0100, Greg KH wrote:
> > > On Fri, Mar 1, 2019 at 5:10 PM Joel Fernandes (Google)
> > > <joel@joelfernandes.org> 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.

Agreed, I also feel the same. thanks,

 - Joel