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

[Sandeep Patil <sspatil@android.com>]

On Sun, Jan 20, 2019 at 06:49:56PM -0800, hpa@zytor.com wrote:
> On January 20, 2019 5:45:53 PM PST, Joel Fernandes <joel@joelfernandes.org> wrote:
> >On Sun, Jan 20, 2019 at 01:58:15PM -0800, hpa@zytor.com wrote:
> >> On January 20, 2019 8:10:03 AM PST, Joel Fernandes
> ><joel@joelfernandes.org> wrote:
> >> >On Sat, Jan 19, 2019 at 11:01:13PM -0800, hpa@zytor.com wrote:
> >> >> On January 19, 2019 2:36:06 AM PST, Greg KH
> >> ><gregkh@linuxfoundation.org> 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.
> >> >> >
> >> >> >> 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.
> >> >> >
> >> >> >thanks,
> >> >> >
> >> >> >greg k-h
> >> >> 
> >> >> Yeah, well... but it is kind of a losing game... the more
> >in-kernel
> >> >stuff there is the less smiley are things to actually be supported.
> >> >
> >> >It is better than nothing, and if this makes things a bit easier and
> >> >solves
> >> >real-world issues people have been having, and is optional, then I
> >> >don't see
> >> >why not.
> >> >
> >> >> Modularizing is it in some ways even crazier in the sense that at
> >> >that point you are relying on the filesystem containing the module,
> >> >which has to be loaded into the kernel by a root user. One could
> >even
> >> >wonder if a better way to do this would be to have "make
> >> >modules_install" park an archive file – or even a directory as
> >opposed
> >> >to a symlink – with this stuff in /lib/modules. We could even
> >provide a
> >> >tmpfs shim which autoloads such an archive via the firmware loader;
> >> >this might even be generically useful, who knows.
> >> >
> >> >All this seems to assume where the modules are located. In Android,
> >we
> >> >don't
> >> >have /lib/modules. This patch generically fits into the grand scheme
> >> >things
> >> >and I think is just better made a part of the kernel since it is not
> >> >that
> >> >huge once compressed, as Dan also pointed. The more complex, and the
> >> >more
> >> >assumptions we make, the less likely people writing tools will get
> >it
> >> >right
> >> >and be able to easily use it.
> >> >
> >> >> 
> >> >> Note also that initramfs contents can be built into the kernel.
> >> >Extracting such content into a single-instance tmpfs would again be
> >a
> >> >possibility
> >> >
> >> >Such an approach would bloat the kernel image size though, which may
> >> >not work
> >> >for everyone. The module based approach, on the other hand, gives an
> >> >option
> >> >to the user to enable the feature, but not have it loaded into
> >memory
> >> >or used
> >> >until it is really needed.
> >> >
> >> >thanks,
> >> >
> >> > - Joel
> >> 
> >> Well, where are the modules? They must exist in the filesystem.
> >
> >The scheme of loading a module doesn't depend on _where_ the module is
> >on the
> >filesystem. As long as a distro knows how to load a module in its own
> >way (by
> >looking into whichever paths it cares about), that's all that matters. 
> >And
> >the module contains compressed headers which saves space, vs storing it
> >uncompressed on the file system.
> >
> >To remove complete reliance on the filesystem, there is an option of
> >not
> >building it as a module, and making it as a built-in.
> >
> >I think I see your point now - you're saying if its built-in, then it
> >becomes kernel memory that is lost and unswappable. Did I get that
> >right?
> >I am saying that if that's a major concern, then:
> >1. Don't make it a built-in, make it a module.
> >2. Don't enable it at for your distro, and use a linux-headers package
> >or
> >whatever else you have been using so far that works for you.
> >
> >thanks,
> >
> > - Joel
> 
> My point is that if we're going to actually solve a problem, we need to make it so that the distro won't just disable it anyway, and it ought to be something scalable; otherwise nothing is gained.
> 
> I am *not* disagreeing with the problem statement!
> 
> Now, /proc isn't something that will autoload modules. A filesystem *will*, although you need to be able to mount it; furthermore, it makes it trivially to extend it (and the firmware interface provides an . easy way to feed the data to such a filesystem without having to muck with anything magic.)
> 
> Heck, we could even make it a squashfs image that can just be mounted.
> 
> So, first of all, where does Android keep its modules, and what is actually included? Is /sbin/modprobe used to load the modules, as is normal? We might even be able to address this with some fairly trivial enhancements to modprobe; specifically to search in the module paths for something that isn't a module per se.
> 

FWIW, 'modprobe' does exist on Android. Although most of the times,
Init's builtin insmod function[1] gets used.

The module locations are /{system, vendor,odm}/lib/modules and yes,
modprobe can be made to load modules from there too. [2]

- ssp

> The best scenario would be if we could simply have the tools find the location equivalent of /lib/modules/$version/source...


[1] https://android.googlesource.com/platform/system/core/+/master/init/builtins.cpp#230
[2] https://source.android.com/devices/architecture/kernel/modular-kernels

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> 
>