linux-kernel - Re: [PATCH v4 1/2] Provide in-kernel headers for making it easy to extend the kernel

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Date:   Mon, 11 Mar 2019 09:03:04 +0100
From:   Geert Uytterhoeven <geert@...ux-m68k.org>
To:     Karim Yaghmour <karim.yaghmour@...rsys.com>
Cc:     Greg KH <gregkh@...uxfoundation.org>,
        Joel Fernandes <joel@...lfernandes.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Alexei Starovoitov <ast@...nel.org>,
        atish patra <atishp04@...il.com>,
        Daniel Colascione <dancol@...gle.com>,
        Dan Williams <dan.j.williams@...el.com>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Guenter Roeck <groeck@...omium.org>,
        Jonathan Corbet <corbet@....net>,
        Kees Cook <keescook@...omium.org>,
        Android Kernel Team <kernel-team@...roid.com>,
        "open list:DOCUMENTATION" <linux-doc@...r.kernel.org>,
        "open list:KERNEL SELFTEST FRAMEWORK" 
        <linux-kselftest@...r.kernel.org>,
        linux-trace-devel@...r.kernel.org,
        Manoj Rao <linux@...ojrajarao.com>,
        Masahiro Yamada <yamada.masahiro@...ionext.com>,
        Masami Hiramatsu <mhiramat@...nel.org>,
        Qais Yousef <qais.yousef@....com>,
        Randy Dunlap <rdunlap@...radead.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Shuah Khan <shuah@...nel.org>, Yonghong Song <yhs@...com>
Subject: Re: [PATCH v4 1/2] Provide in-kernel headers for making it easy to
 extend the kernel

Hi Karim,

On Sat, Mar 9, 2019 at 10:44 PM Karim Yaghmour
<karim.yaghmour@...rsys.com> wrote:
> On 3/9/19 2:26 PM, Geert Uytterhoeven wrote:
> > So how does this work, with kernel images and kernel modules supplied
> > by separate parties, not "bound" by the same kernel headers/API, as they
> > can be replaced separately?
>
> The thing with Android is that there isn't a "one size fits all". Google
> provides guidance, policies and at least one example implementation
> through the Pixel lines.
>
> Each vendor however is allowed a great degree of latitude with regards
> to what they decide to do. Personally, if I was advising a team working
> on an Android device where Joel's patch was available as part of their
> kernel I would just recommend that they build it in -- i.e. not as a
> module. Hence, there would be no module chasing game.

OK, that makes sense, if kernel and modules are separate.
So kernel size increase due to including the headers does matter.

> With regards to Google's guidelines for manufacturers, though, Google
> states that CONFIG_MODVERSIONS needs to be enabled, see here:
> https://source.android.com/devices/architecture/kernel/modular-kernels
> FWIW, that doesn't mean that modules are actually used. Devices don't
> necessarily have to be using modules.

Personally, I had mixed experiences with CONFIG_MODVERSIONS.
But that was a looong time ago, before Android.
Things may have improved.

> > Isn't the need for kernel headers for user-space tools something different,
> > as this is limited to the uapi versions, which are less (almost not) subject
> > to change, compared to the kernel headers needed for compiling kernel
> > modules?
>
> Sorry, I should've been clearer. I'm including eBPF/BCC into the
> "user-space tools" here. That was in fact my prime motivation in
> encouraging Joel at the last LPC to look at this. I've been integrating
> the teaching of eBPF into my AOSP debugging and performance analysis
> class (see CC courseware here:
> http://www.opersys.com/training/android-debug-and-performance), and it
> was pretty messy trying to show people how to benefit from such tools
> under Android. Joel's present set of patches would obviate this problem.

OK.

Now about the actual solution: what is your opinion on embedding e.g.
a squashfs image in the kernel instead, which would be a more generic
solution, not adding more ABI to /proc?

Thanks!

Gr{oetje,eeting}s,

                        Geert

-- 
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@...ux-m68k.org

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
                                -- Linus Torvalds