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

[Joel Fernandes <joel@joelfernandes.org>]

On Sat, Jan 19, 2019 at 11:36:06AM +0100, Greg KH wrote:
> On Sat, Jan 19, 2019 at 02:28:00AM -0800, Christoph Hellwig wrote:
> > This seems like a pretty horrible idea and waste of kernel memory.
> 
> It's only a waste if you want it to be a waste, i.e. if you load the
> kernel module.
> 
> This really isn't any different from how /proc/config.gz works.

Yes, that's right, it can be built as a module and loaded and unloaded on
demand to free the kernel memory. Also such a module can be made a part of
the initrd image itself (like other kernel modules that are built into an
initrd), so this solution is already initrd-ready if anyone really wants it
that way.

> > Just add support to kbuild to store a compressed archive in initramfs
> > and unpack it in the right place.
> 
> I think the issue is that some devices do not use initramfs, or switch
> away from it after init happens or something like that.  Joel has all of
> the looney details that he can provide.

Initially I had written this as an image embedded into an init section,
similar to the initramfs. On boot if a boot argument was passed, then the
archive embedded was allocated additional non-init kernel memory to store it for
retrieval through /proc. After that, the image that's embedded would be
dropped entirely as its init memory. If no boot argument was passed, then no
additional memory is used at all. Even though I got that working, I decided
not to go with it because it was clunky because: 1. It required modification
of the linker scripts in the kernel to embed the image into the init section,
and also required a boot argument to be passed which makes the decision on
whether to have headers as a boot-time decision than a post-boot decision.
For these reasons, it is much cleaner to just have a loadable module and make
it on-demand, as I am doing in this patch similar to what /proc/config.gz
does.

thanks!

 - Joel