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Date: Thu, 28 Feb 2019 18:27:51 -0500 From: Joel Fernandes <joel@...lfernandes.org> To: Masahiro Yamada <yamada.masahiro@...ionext.com> Cc: Linux Kernel Mailing List <linux-kernel@...r.kernel.org>, Andrew Morton <akpm@...ux-foundation.org>, Alexei Starovoitov <ast@...nel.org>, atish patra <atishp04@...il.com>, Daniel Colascione <dancol@...gle.com>, Dan Williams <dan.j.williams@...el.com>, Greg Kroah-Hartman <gregkh@...uxfoundation.org>, Guenter Roeck <groeck@...omium.org>, Jonathan Corbet <corbet@....net>, Karim Yaghmour <karim.yaghmour@...rsys.com>, Kees Cook <keescook@...omium.org>, kernel-team@...roid.com, "open list:DOCUMENTATION" <linux-doc@...r.kernel.org>, "open list:KERNEL SELFTEST FRAMEWORK" <linux-kselftest@...r.kernel.org>, linux-trace-devel@...r.kernel.org, Manoj Rao <linux@...ojrajarao.com>, Masami Hiramatsu <mhiramat@...nel.org>, Paul McKenney <paulmck@...ux.vnet.ibm.com>, "Peter Zijlstra (Intel)" <peterz@...radead.org>, qais.yousef@....com, Randy Dunlap <rdunlap@...radead.org>, Steven Rostedt <rostedt@...dmis.org>, Shuah Khan <shuah@...nel.org>, Thomas Gleixner <tglx@...utronix.de>, Yonghong Song <yhs@...com> Subject: Re: [PATCH v3 1/2] Provide in-kernel headers for making it easy to extend the kernel 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@...lfernandes.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. > > > > > > > > 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
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