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
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
I am guessing the user will run these commands on the target system. In other words, external modules are native-compiled. So,
target-arch: arm64 host-arch: arm64
Is this correct?
If I understood the assumed use-case correctly, kheaders.tar.xw will contain host-programs compiled for x86, which will not work on the target system.
You are right, the above commands in the commit message work only if the host/target are same arch due to scripts.
However we can build with arm64 device connected to a host, like this (which I tested):
adb shell modprobe kheaders; adb pull /proc/kheaders.tar.xz 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) ARCH=arm64 CROSS_COMPILE=aarch64- modules adb push test.ko /data/; adb shell rmmod kheaders
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. One thing to note is that Android also runs on x86 hardware so the commands in the commit message will work even for x86 Android targets already.
Also note that this the "module building" part is really only one of the usecases. eBPF is another which needs the headers - and the headers are vast majority of the archive. Headers take 3.1MB out of 3.6MB of the archive on arm64 builds.
How do you want to proceed here, should I mention these points in the commit message?
thanks,
- Joel