Message-ID: <20190122133901.GA189736@google.com>
Date:   Tue, 22 Jan 2019 08:39:01 -0500
From:   Joel Fernandes <joel@...lfernandes.org>
To:     hpa@...or.com
Cc:     Greg KH <gregkh@...uxfoundation.org>,
        Christoph Hellwig <hch@...radead.org>,
        linux-kernel@...r.kernel.org,
        Andrew Morton <akpm@...ux-foundation.org>, ast@...nel.org,
        atishp04@...il.com, Borislav Petkov <bp@...en8.de>,
        dancol@...gle.com, Ingo Molnar <mingo@...hat.com>,
        Jan Kara <jack@...e.cz>, Jonathan Corbet <corbet@....net>,
        karim.yaghmour@...rsys.com, Kees Cook <keescook@...omium.org>,
        kernel-team@...roid.com, linux-doc@...r.kernel.org,
        Manoj Rao <linux@...ojrajarao.com>,
        Masahiro Yamada <yamada.masahiro@...ionext.com>,
        paulmck@...ux.vnet.ibm.com,
        "Peter Zijlstra (Intel)" <peterz@...radead.org>,
        rdunlap@...radead.org, rostedt@...dmis.org,
        Thomas Gleixner <tglx@...utronix.de>,
        "maintainer:X86 ARCHITECTURE (32-BIT AND 64-BIT)" <x86@...nel.org>,
        yhs@...com
Subject: Re: [RFC] Provide in-kernel headers for making it easy to extend the
 kernel

Hi Hans,

Thanks for the discussion and sorry for my late reply due to the holidays.
I replied inline below:

On Sun, Jan 20, 2019 at 06:49:56PM -0800, hpa@...or.com wrote:
> On January 20, 2019 5:45:53 PM PST, Joel Fernandes <joel@...lfernandes.org> wrote:
> >On Sun, Jan 20, 2019 at 01:58:15PM -0800, hpa@...or.com wrote:
> >> On January 20, 2019 8:10:03 AM PST, Joel Fernandes
> ><joel@...lfernandes.org> wrote:
> >> >On Sat, Jan 19, 2019 at 11:01:13PM -0800, hpa@...or.com wrote:
> >> >> On January 19, 2019 2:36:06 AM PST, Greg KH
> >> ><gregkh@...uxfoundation.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.

Agreed. Although there are many ways distros can misconfigure a kernel and
shoot themselves in the foot.

> I am *not* disagreeing with the problem statement!

Ok, that's great to know. Thanks for all the discussions!

> 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.)

My thought is for the tools needing the module to try to load them, if they
need the headers. And then unload the module once they are done with it.

> Heck, we could even make it a squashfs image that can just be mounted.

That would add another dependency on a kernel configuration though.

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

Sandeep answered this in the other thread, modprobe is there in Android.
About storing sources on the filesystem, I already went down this route (this
was my first effort) and there is quite some resistance to ship sources on
the filesystem of an Android device due to Licensing reasons. A kernel module
though is something that's a binary artifact and is not something
"distributed" by Google.

The other issue is with enforcing different OEM vendors that they should
store kernel-header sources on their Android products. That's much harder to
enforce, however we can easily enforce kernel config options be enabled,
since we already enforce a kernel configuration options that are needed for
Android to work correctly:
https://source.android.com/devices/architecture/kernel/config. So this config
option can just be another one.

By the way, we can easily write a script to just extract the .ko directly -
if the whole "load it as a module" thing bothers you. The kheaders.ko can
just be thought of as a tarball. There's already a script to extract
/proc/config.gz in the same/similar way: scripts/extract-ikconfig

thanks,

 - Joel

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