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From:   Masahiro Yamada <yamada.masahiro@socionext.com>
Date:   Sat, 2 Mar 2019 11:13:07 +0900
Message-ID: <CAK7LNAShVt=zOPP1TUuBbR_H8kmc+zuLAyRXucjMMKrALkW_6A@mail.gmail.com>
Subject: Re: [PATCH v3 1/2] Provide in-kernel headers for making it easy to
 extend the kernel
To:     Joel Fernandes <joel@joelfernandes.org>
Cc:     Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        Alexei Starovoitov <ast@kernel.org>,
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On Sat, Mar 2, 2019 at 3:03 AM Joel Fernandes <joel@joelfernandes.org> wrote:
>
> On Fri, Mar 01, 2019 at 03:25:05PM +0900, Masahiro Yamada wrote:
> [...]
> > > > 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.
> > > >
> > >
> > > You are right, the above commands in the commit message work only if the
> > > host/target are same arch due to scripts.
> > >
> > > However we can build with arm64 device connected to a host, like this (which
> > > I tested):
> > >
> > > adb shell modprobe kheaders; adb pull /proc/kheaders.tar.xz
> > > 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) ARCH=arm64 CROSS_COMPILE=aarch64- modules
> > > adb push test.ko /data/; adb shell rmmod kheaders
> > >
> > > The other way we can make this work is using x86 usermode emulation inside a
> > > chroot on the Android device which will make the earlier commands work. One
> > > thing to note is that Android also runs on x86 hardware so the commands in
> > > the commit message will work even for x86 Android targets already.
> > >
> > > Also note that this the "module building" part is really only one of the
> > > usecases. eBPF is another which needs the headers - and the headers are vast
> > > majority of the archive. Headers take 3.1MB out of 3.6MB of the archive on
> > > arm64 builds.
> > >
> > > How do you want to proceed here, should I mention these points in the commit
> > > message?
> >
> >
> >
> > I do not request a re-spin just for a matter of commit log,
> > but this version produces an empty tarball.
> > So, you will have a chance to update the patch anyway.
> >
> > In the next version, it would be nice to note that
> > "external modules must be built on the same host arch
> > as built vmlinux".
>
> Ok, I respun it with 1 more minor nit for arm64 building. Please take a look.


I have not checked code-diff in v3 yet.

Anyway, I will add comments to v4
if I notice something.


> > Let me ask one more question.
> >
> > I guess this patch is motivated by
> > how difficult to convey kernel headers
> > from vendors to users.
> >
> > In that situation, how will the user find
> > the right compiler to use for building external modules?
> >
> >
> >
> >
> > Greg KH said:
> >
> > We don't ever support the system of loading a module built with anything
> > other than the _exact_ same compiler than the kernel was.
> >
> >
> > For the full context, see this:
> > https://lore.kernel.org/patchwork/patch/836247/#1031547
>
> IMO this issue is not related to this patch but is just an issue with
> building external modules in general.


I do not think it is an issue of the build system, at least.

As far as I understood Greg's comment, it is troublesome
without the assumption that vmlinux and modules are built
by the same compiler.

It is related to this patch since this patch assumes use-cases
where external modules are built in a completely different environment,
where a different compiler is probably installed.


> It is up to the user to use the right
> compiler, etc. I will let Greg comment more on that.


> thanks,
>
>  - Joel
>


-- 
Best Regards
Masahiro Yamada