On Thu, Feb 28, 2019 at 09:43:06AM -0500, Joel Fernandes wrote: > On Thu, Feb 28, 2019 at 11:17:51AM +0900, Masahiro Yamada wrote: > > Hi Joel, > > > > > > On Thu, Feb 28, 2019 at 4:40 AM 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. > > > > > > > > Please let me ask a question about the actual use-case. > > > > > > To build embedded systems including Android, > > I use an x86 build machine. > > > > In other words, I cross-compile vmlinux and in-tree modules. > > So, > > > > target-arch: arm64 > > host-arch: x86 > > > > > > > The other way we can make this work is using x86 usermode emulation inside a > chroot on the Android device which will make the earlier commands work. I verified the steps to build a module on my Pixel 3 (arm64) with Linux kernel for arm64 compiled on my x86 host: After building the headers, the steps were something like: 1.Build an x86 debian image with cross-gcc: sudo qemu-debootstrap --arch amd64 --include=make,gcc,gcc-aarch64-linux-gnu,perl,libelf1,python --variant=minbase $DIST $RUN_DIR http://ftp.us.debian.org/debian 2. Push qemu-x86_64-static (which I downloaded from the web) onto the device. 3. Tell binfmt_misc about qemu: echo ':qemu-x86_64:M::\x7fELF\x02\x01\x01\x00\x00\x00\x00\x00\x00\x00\x00\x00\x03\x00\x3e\x00: \xff\xff\xff\xff\xff\xff\xff\x00\xff\xff\xff\xff\xff\xff\xff\xff\xfe\xff\xff\xff:/qemu-x86_64-static:OC' > /proc/sys/fs/binfmt_misc/register 4. adb shell and then chroot into the image 5. follow all the steps in the commit message but set ARCH and CROSS_COMPILE appropriately. After Make, kernel module is cooked and ready :) thanks, - Joel
