Re: [RFC] Provide in-kernel headers for making it easy to extend the kernel

[Joel Fernandes <joel@joelfernandes.org>]

On Thu, Jan 24, 2019 at 07:57:26PM +0100, Karim Yaghmour wrote:
> 
> On 1/23/19 11:37 PM, Daniel Colascione wrote:
[..]
> > > Personally I advocated a more aggressive approach with Joel in private:
> > > just put the darn headers straight into the kernel image, it's the
> > > *only* artifact we're sure will follow the Android device whatever
> > > happens to it (like built-in ftrace).
> > 
> > I was thinking along similar lines. Ordinarily, we make loadable
> > kernel modules. What we kind of want here is a non-loadable kernel
> > module --- or a non-loadable section in the kernel image proper. I'm
> > not familiar with early-stage kernel loader operation: I know it's
> > possible to crease discardable sections in the kernel image, but can
> > we create sections that are never slurped into memory in the first
> > place? If not, maybe loading and immediately discarding the header
> > section is good enough.
> 
> Interesting. Maybe just append it to the image but have it not loaded and
> have a kernel parameter than enables a "/proc/kheaders" driver to know where
> the fetch the appended headers from storage at runtime. There would be no
> RAM loading whatsoever of the headers, just some sort of
> "kheaders=/dev/foobar:offset:size" parameter. If you turn the option on, you
> get a fatter kernel image size to store on permanent storage, but no impact
> on what's loaded at boot time.

Embedding anything into the kernel image does impact boot time though because
it increase the time spent by bootloader. A module OTOH would not have such
overhead.

Also a kernel can be booted in any number of ways other than mass storage so
it is not a generic Linux-wide solution to have a kheaders= option like that.
If the option is forgotten, then the running system can't use the feature.
The other issue is it requires a kernel command line option / bootloader
changes for that which adds more configuration burden, which not be needed
with a module.

> > Would such a thing really do better than LZMA? LZMA already has very
> > clever techniques for eliminating long-range redundancies in
> > compressible text, including redundancies at the sub-byte level. I can
> > certainly understand the benefit of stripping comments, since removing
> > comments really does decrease the total amount of information the
> > compressor has to preserve, but I'm not sure how much the encoding
> > scheme you propose below would help, since it reminds me of the
> > encoding scheme that LZMA would discover automatically.
> 
> I'm no compression algorithm expert. If you say LZMA would do the
> same/better than what I suggested then I have no reason to contest that. My
> goal is to see the headers as part of the kernel image that's distributed on
> devices so that they don't have to be chased around. I'm just trying to make
> it as palatable as possible.

I believe LZMA is really good at that sort of thing too.

Also at 3.3MB of module size, I think we are really good size-wise. But Dan
is helping look at possibly reducing further if he gets time. Many modules in
my experience are much bigger. amdgpu.ko on my Linux machine is 6.1MB.

I really think making it a module is the best way to make sure this is
bundled with the kernel on the widest number of Android and other Linux
systems, without incurring boot time overhead, or any other command line
configuration burden.

I spoke to so many people at LPC personally with other kernel contributors,
and many folks told me one word - MODULE :D.  Even though I hesitated at
first, now it seems the right solution.

If no one seriously objects, I'll clean this up and post a v2 and with the
RFC tag taken off. Thank you!

 - Joel