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

[Joel Fernandes <joel@joelfernandes.org>]

On Sat, Jan 19, 2019 at 12:43:35PM -0500, Daniel Colascione wrote:
> On Sat, Jan 19, 2019 at 11:27 AM Joel Fernandes <joel@joelfernandes.org> wrote:
> >
> > On Sat, Jan 19, 2019 at 09:25:32AM +0100, Greg KH wrote:
> > > On Fri, Jan 18, 2019 at 05:55:43PM -0500, Joel Fernandes wrote:
> > > > --- /dev/null
> > > > +++ b/kernel/kheaders.c
> 
> Thanks a ton for this work. It'll make it much easier to do cool
> things with BPF.

You're welcome, thanks.

> One question: I can imagine wanting to probe
> structures that are defined, not in headers, but in random
> implementation files. Would it be possible to optionally include *all*
> kernel source files?

That would be prohibitively too large to justify keeping it in memory, even
compressed. Arguably, such structures should be moved into include/ if
modules or whatever is extending the kernel like eBPF needs them.

> If not, what about a hash, so we could at least
> do precise correlation between a candidate local tree and what's
> actually on device?

That would make a tool too difficult to write wouldn't it, since they you have to
correlate every possible hash and keep updating eBPF tools with new hashes -
probably not scalable. I think what you want is to use the kernel version to
assume what such internal structures look like although that's still not
robust.

> BTW, I'm not sure that the magic constants you've defined are long
> enough.  I'd feel more comfortable with two UUIDs (16 bytes each).

Ok, I'll expand it.

> I'd also strongly consider LZMA compression: xz -9 on the kernel
> headers (with comments) brings the size down to 5MB, compared to the
> 7MB I get for gzip -9. Considering that this feature is optional, I
> think it's okay to introduce a dependency on widespread modern
> compression tools. (For comparison, bzip2 -9 gets us 6MB.)

Ok, I'll look into LZMA. Thanks for checking the compression sizes.

- Joel