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@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.
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