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

[Joel Fernandes]

On Fri, Jan 25, 2019 at 12:34:52PM -0800, Daniel Colascione wrote:
> On Fri, Jan 25, 2019 at 12:28 PM <hpa@xxxxxxxxx> wrote:
> >
> > On January 25, 2019 11:15:52 AM PST, Daniel Colascione <dancol@xxxxxxxxxx> wrote:
> > >On Fri, Jan 25, 2019 at 11:01 AM <hpa@xxxxxxxxx> wrote:
> > >>
> > >> On January 24, 2019 12:59:29 PM PST, Joel Fernandes
> > ><joel@xxxxxxxxxxxxxxxxx> wrote:
> > >> >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
> > >>
> > >> So let me throw in a different notion.
> > >>
> > >> A kernel module really is nothing other than a kernel build system
> > >artifact stored in the filesystem.
> > >>
> > >> I really don't at any reason whatsoever why this is direct from just
> > >producing an archive and putting it in the module directory, except
> > >that the latter is far simpler.
> > >>
> > >> I see literally *no* problem, social or technical, you are solvin by
> > >actually making it a kernel ELF object.
> > >
> > >Joel does have a point. Suppose we're on Android and we're testing a
> > >random local kernel we've built. We can command the device to boot
> > >into the bootloader, then send our locally-built kernel to the device
> > >with "fastboot boot mykernelz". Having booted the device this way,
> > >there's no on-disk artifact corresponding to mykernelz: we just sent
> > >the boot kernel directly to the device's memory. Now, suppose I want
> > >to attach DCTV or some other fancy ftrace-based analysis tool to the
> > >device in order to study how mykernelz acts in some scenario I care
> > >about. With Joel's approach, DCTV would be able to grab the kernel
> > >headers from the running kernel, compile whatever kprobe or BPF
> > >incantations needed, and have everything Just Work. If we put the
> > >headers only on disk without any way to retrieve them at runtime, we'd
> > >need a different path for kernel self-description in the case where a
> > >running kernel doesn't have an on-disk representation, and that adds a
> > >lot of complexity for everyone everywhere. By providing
> > >guaranteed-correct kernel headers via some runtime interface, we make
> > >a lot of things Just Work, and that has value.
> >
> > Joel specifically is talking about using a module, which *does* have to be in the filesystem.
> >
> > You can't have it both ways, unfortunately.

You can have it whichever way is convenient to you, do all Linux users have
the same CONFIG options?

For me its module, for Dan its built-in. So what's your point? :)

> In general, whatever we support in module form, we also support as
> part of the kernel image itself.

Exactly.

thanks,

 - Joel