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 > 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. Masahiro > Additional notes: > (1) > 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. > > (2) 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> > --- -- Best Regards Masahiro Yamada
