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

[hpa@zytor.com]

On January 25, 2019 11:15:52 AM PST, Daniel Colascione <dancol@google.com> wrote:
>On Fri, Jan 25, 2019 at 11:01 AM <hpa@zytor.com> wrote:
>>
>> On January 24, 2019 12:59:29 PM PST, Joel Fernandes
><joel@joelfernandes.org> 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.
-- 
Sent from my Android device with K-9 Mail. Please excuse my brevity.