linux-kernel - Re: [PATCH v3 1/2] Provide in-kernel headers for making it easy to extend the kernel

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Message-Id: <20190228173444.54403795211d80ba540b61cb@kernel.org>
Date:   Thu, 28 Feb 2019 17:34:44 +0900
From:   Masami Hiramatsu <mhiramat@...nel.org>
To:     "Joel Fernandes (Google)" <joel@...lfernandes.org>
Cc:     linux-kernel@...r.kernel.org,
        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, paulmck@...ux.vnet.ibm.com,
        "Peter Zijlstra (Intel)" <peterz@...radead.org>,
        qais.yousef@....com, rdunlap@...radead.org, rostedt@...dmis.org,
        Shuah Khan <shuah@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>, yhs@...com
Subject: Re: [PATCH v3 1/2] Provide in-kernel headers for making it easy to
 extend the kernel

Hi Joel,

On Wed, 27 Feb 2019 14:37:47 -0500
"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.

Hmm, isn't it easier to add kernel-headers package on Android?

> 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.

But it also requires to install build environment (tools etc.)
on the target system...

> 
> 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

It seems a bit complex, but no difference from compared with carrying
kheaders.tar.gz. I think we would better have a psudo filesystem
which can mount this compressed header file directly :) Then it becomes
simpler, like

modprobe headerfs
mkdir $HOME/headers
mount -t headerfs $HOME/headers

And this doesn't consume any disk-space.

Thank you,




-- 
Masami Hiramatsu <mhiramat@...nel.org>