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

[Dietmar Eggemann <dietmar.eggemann@arm.com>]

On 3/1/19 5:08 PM, Joel Fernandes (Google) 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
> 
> Additional notes:
> (1) external modules must be built on the same arch as the host that
> built vmlinux. This can be done either in a qemu emulated chroot on the
> target, or natively. This is due to host arch dependency of kernel
> scripts.
> 
> (2)
> A limitation of module building with this is, since Module.symvers is
> not available in the archive due to a cyclic dependency with building of
> the archive into the kernel or module binaries, the modules built using
> the archive will not contain symbol versioning (modversion). This is
> usually not an issue since the idea of this patch is to build a kernel
> module on the fly and load it into the same kernel. An appropriate
> warning is already printed by the kernel to alert the user of modules
> not having modversions when built using the archive. For building with
> modversions, the user can use traditional header packages. For our
> tracing usecases, we build modules on the fly with this so it is not a
> concern.
> 
> (3) I have left IKHD_ST and IKHD_ED markers as is to facilitate
> future patches that would extract the headers from a kernel or module
> image.
> 
> Signed-off-by: Joel Fernandes (Google) <joel@joelfernandes.org>
> ---
> 
>      Changes since v3:
>      - Blank tar was being generated because of a one line I
>        forgot to push. It is updated now.
>      - Added module.lds since arm64 needs it to build modules.

Tested on x86 with eBPF scripts and exporting an alternative 
BCC_KERNEL_SOURCE.

Tested-by: Dietmar Eggemann <dietmar.eggemann@arm.com>