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Subject: Re: [RFC] Provide in-kernel headers for making it easy to extend the kernel
To:     Joel Fernandes <joel@joelfernandes.org>
CC:     Greg KH <gregkh@linuxfoundation.org>,
        Christoph Hellwig <hch@infradead.org>,
        linux-kernel@vger.kernel.org,
        Andrew Morton <akpm@linux-foundation.org>, ast@kernel.org,
        atishp04@gmail.com, Borislav Petkov <bp@alien8.de>,
        dancol@google.com, Ingo Molnar <mingo@redhat.com>,
        Jan Kara <jack@suse.cz>, Jonathan Corbet <corbet@lwn.net>,
        karim.yaghmour@opersys.com, Kees Cook <keescook@chromium.org>,
        kernel-team@android.com, linux-doc@vger.kernel.org,
        Manoj Rao <linux@manojrajarao.com>,
        Masahiro Yamada <yamada.masahiro@socionext.com>,
        paulmck@linux.vnet.ibm.com,
        "Peter Zijlstra (Intel)" <peterz@infradead.org>,
        rdunlap@infradead.org, rostedt@goodmis.org,
        Thomas Gleixner <tglx@linutronix.de>,
        "maintainer:X86 ARCHITECTURE (32-BIT AND 64-BIT)" <x86@kernel.org>,
        yhs@fb.com
From:   hpa@zytor.com
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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.

The best scenario would be if we could simply have the tools find the location equivalent of /lib/modules/$version/source...
-- 
Sent from my Android device with K-9 Mail. Please excuse my brevity.