On Mon, Mar 04, 2019 at 02:00:59PM +0000, Qais Yousef wrote: > On 03/01/19 11:08, 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@xxxxxxxxxxxxxxxxx> > > I could get the headers using this patch in both built-in and modules options. > > You can add my tested-and-reviewed-by: Qais Yousef <qais.yousef@xxxxxxx> > > I am not familiar with running kselftests so didn't get a chance to try the > next patch. Thanks a lot for testing! Will repost with the tags soon. - Joel
