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Date: Mon, 4 Mar 2019 23:48:52 +0100 From: Dietmar Eggemann <dietmar.eggemann@....com> To: "Joel Fernandes (Google)" <joel@...lfernandes.org>, linux-kernel@...r.kernel.org Cc: Andrew Morton <akpm@...ux-foundation.org>, ast@...nel.org, atishp04@...il.com, dancol@...gle.com, Dan Williams <dan.j.williams@...el.com>, gregkh@...uxfoundation.org, Guenter Roeck <groeck@...omium.org>, Jonathan Corbet <corbet@....net>, karim.yaghmour@...rsys.com, Kees Cook <keescook@...omium.org>, kernel-team@...roid.com, linux-doc@...r.kernel.org, linux-kselftest@...r.kernel.org, linux-trace-devel@...r.kernel.org, Manoj Rao <linux@...ojrajarao.com>, Masahiro Yamada <yamada.masahiro@...ionext.com>, mhiramat@...nel.org, qais.yousef@....com, rdunlap@...radead.org, rostedt@...dmis.org, Shuah Khan <shuah@...nel.org>, yhs@...com Subject: Re: [PATCH v4 1/2] Provide in-kernel headers for making it easy to extend the kernel On 3/1/19 5:08 PM, Joel Fernandes (Google) 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. > > 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 > > Additional notes: > (1) external modules must be built on the same arch as the host that > built vmlinux. This can be done either in a qemu emulated chroot on the > target, or natively. This is due to host arch dependency of kernel > scripts. > > (2) > 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. > > (3) 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@...lfernandes.org> > --- > > Changes since v3: > - Blank tar was being generated because of a one line I > forgot to push. It is updated now. > - Added module.lds since arm64 needs it to build modules. Tested on x86 with eBPF scripts and exporting an alternative BCC_KERNEL_SOURCE. Tested-by: Dietmar Eggemann <dietmar.eggemann@....com>
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