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

From: Masami Hiramatsu <mhiramat@kernel.org>
To: "Joel Fernandes (Google)" <joel@joelfernandes.org>
Cc: linux-kernel@vger.kernel.org,
	Andrew Morton <akpm@linux-foundation.org>,
	ast@kernel.org, atishp04@gmail.com, dancol@google.com,
	Dan Williams <dan.j.williams@intel.com>,
	gregkh@linuxfoundation.org, Guenter Roeck <groeck@chromium.org>,
	Jonathan Corbet <corbet@lwn.net>,
	karim.yaghmour@opersys.com, Kees Cook <keescook@chromium.org>,
	kernel-team@android.com, linux-doc@vger.kernel.org,
	linux-kselftest@vger.kernel.org,
	linux-trace-devel@vger.kernel.org,
	Manoj Rao <linux@manojrajarao.com>,
	Masahiro Yamada <yamada.masahiro@socionext.com>,
	mhiramat@kernel.org, paulmck@linux.vnet.ibm.com,
	"Peter Zijlstra (Intel)" <peterz@infradead.org>,
	qais.yousef@arm.com, rdunlap@infradead.org, rostedt@goodmis.org,
	Shuah Khan <shuah@kernel.org>,
	Thomas Gleixner <tglx@linutronix.de>,
	yhs@fb.com
Subject: Re: [PATCH v3 1/2] Provide in-kernel headers for making it easy to extend the kernel
Date: Thu, 28 Feb 2019 17:34:44 +0900	[thread overview]
Message-ID: <20190228173444.54403795211d80ba540b61cb@kernel.org> (raw)
In-Reply-To: <20190227193748.132301-1-joel@joelfernandes.org>

Hi Joel,

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

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@kernel.org>