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Date:   Fri, 1 Mar 2019 16:03:09 +0900
From:   Masami Hiramatsu <mhiramat@kernel.org>
To:     Joel Fernandes <joel@joelfernandes.org>
Cc:     linux-kernel@vger.kernel.org,
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        Shuah Khan <shuah@kernel.org>,
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Subject: Re: [PATCH v3 1/2] Provide in-kernel headers for making it easy to
 extend the kernel
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Hi Joel,

On Thu, 28 Feb 2019 22:26:11 -0500
Joel Fernandes <joel@joelfernandes.org> wrote:

> On Fri, Mar 01, 2019 at 11:28:26AM +0900, Masami Hiramatsu wrote:
> > Hi Joel,
> 
> Hi Masami,
> 
> > On Thu, 28 Feb 2019 10:00:54 -0500
> > Joel Fernandes <joel@joelfernandes.org> wrote:
> > 
> > > > Hmm, isn't it easier to add kernel-headers package on Android?
> > > 
> > > I have already been down that road. In the Android ecosystem, the Android
> > > teams only provide a "userspace system image" which goes on the system
> > > partition of the flash (and a couple other images are also provided but
> > > system is the main one). The system image cannot contain GPL source code. It
> > > is also not possible to put kernel headers for every kernel version on the
> > > system images that ship and is not practical. Android boots on 1000s of forked
> > > kernels. It does not make sense to provide headers on the system image for
> > > every kernel version and I already had many discussions on the subject with
> > > the teams, it is something that is just not done. Now for kernel modules,
> > > there's another image called the "vendor image" which is flashed onto the
> > > vendor parition, this is where kernel modules go.  This vendor image is not
> > > provided by Google for non-Pixel devices. So we have no control over what
> > > goes there BUT we do know that kernel modules that are enabled will go there,
> > > and we do have control over enforcing that certain kernel modules should be
> > > built and available as they are mandatory for Android to function properly.
> > > We would also possibly make it a built-in option as well. Anyway my point is
> > > keeping it in the kernel is really the easiest and the smartest choice IMO.
> > 
> > Sorry, I'm not convinced yet. This sounds like "because Android decided not
> > to put the header files on vendor partition, but kernel module is OK"
> > Why don't google ask vendors to put their kernel headers (or header tarball)
> > on vendor partition instead?
> 
> May be Google can do that, but I think you missed the point of the patches.
> Making it a module is not mandatory, people can build it into the kernel as
> well (CONFIG_IKHEADERS_PROC=y). In this case, the proc entry will be
> available on boot without any dependency on the filesystem. If you go through
> the other threads such as folks from ARM who replied, they just boot a kernel
> image for debug purpose and want headers on device available without any
> additional step of copying headers to the filesystem. And folks from Google
> also said that they wanted a built-in option.

Agreed. Making it built-in is reasonable, since it will not involves any
other files. :) 

> There are many usecases for this, I have often run into issues with Linux
> over the years not only with Android, but other distros, where I boot custom
> kernels with no linux-headers package. This is quite painful. It is
> convenient to have it as /proc file since the file is dependent on kernel
> being booted up and this will work across all Linux distros and systems. I
> feel that if you can keep an open mind about it, you will see that a lot of
> people will use this feature if it is accepted and there is a lot of positive
> feedback in earlier posts of this set.

I don't complain about having headers for custom boot kernel. I agree with you
that having kernel headers for debugging is always good. :)
So google recommends built-in, it is reasonable.

> > > > > 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.
> > > 
> > > I felt using a compressed tar is really the easiest way because of all the
> > > tools are already available.
> > 
> > As I asked above, if the pure tarball is useful, you can simply ask vendors
> > to put the header tarball on their vendor directory. I feel making it as
> > a module is not a right way.
> 
> I don't see what is the drawback of making it a module, it makes it well
> integrated into kernel build and ecosystem. I also didn't see any
> justification you're providing about why it cannot be a module. If you go
> through this and earlier threads, a lot of people are Ok with having a module
> option. And I asked several top kernel maintainers at LPC and many people
> suggested having it as a module.

I meant, if we have a tarball, we don't need any operation of loading/unloading
kmodules. But if we have this as built-in, yes, this would be much easier to
deploy it to device.

Anyway, having that option (make it as a module) is not bad. IMHO, that may
be more complicated than just have a tarball file, but it is a user's choice.

OK, now I understand it.

> > > There isn't a compressed in-ram filesystem right
> > > now that I'm aware off that can achieve the kind of high compression ratio
> > > this patchset does.
> > 
> > I think if linux can support something like tarfs(or compressed initramfs)
> > in kernel, it gives linux an improvement not only a hack. :-)
> 
> Agreed, that sounds like a good idea. I will consider doing it once the
> series in its current form can be accepted. I am saying so since this series
> is simple, and I can do that as a next step since that idea will take a lot
> of time to implement. But I am keen on doing it.

I look forward to it :-)

Thank you!

-- 
Masami Hiramatsu <mhiramat@kernel.org>