Hi Joel,

On Thu, 28 Feb 2019 10:00:54 -0500
Joel Fernandes [off-list ref] wrote:

quoted

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?

quoted

quoted

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...

Yes, that's true. Check the other thread with Masahiro that we are discussing
this point on and let us continue discussing there:
https://lore.kernel.org/patchwork/patch/1046307/#1238223
https://lore.kernel.org/patchwork/patch/1046307/#1238491

quoted

quoted

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.

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. :-)

Thank you,


-- 
Masami Hiramatsu [off-list ref]