Re: eBPF CO-RE cross-compilation for 32-bit ARM platforms

[Russell King - ARM Linux admin <linux@armlinux.org.uk>]

On Fri, Aug 07, 2020 at 07:23:53PM +0200, Jean-Philippe Brucker wrote:
> On Fri, Aug 07, 2020 at 04:20:58PM +0200, Jakov Petrina wrote:
> > Hi everyone,
> > 
> > recently we have begun extensive research into eBPF and related
> > technologies. Seeking an easier development process, we have switched over
> > to using the eBPF CO-RE [0] approach internally which has enabled us to
> > simplify most aspects of eBPF development, especially those related to
> > cross-compilation.

For those of us not familiar with what CO-RE is, this doesn't help.
I assume the [0] was a reference to something that explained it,
but that isn't included.

> > However, as part of these efforts we have stumbled upon several problems
> > that we feel would benefit from a community discussion where we may share
> > our solutions and discuss alternatives moving forward.
> > 
> > As a reference point, we have started researching and modifying several eBPF
> > CO-RE samples that have been developed or migrated from existing `bcc`
> > tooling. Most notable examples are those present in `bcc`'s `libbpf-tools`
> > directory [1]. Some of these samples have just recently been converted to
> > respective eBPF CO-RE variants, of which the `tcpconnect` tracing sample has
> > proven to be very interesting.
> > 
> > First showstopper for cross-compiling aforementioned example on the ARM
> > 32-bit platform has been with regards to generation of the required
> > `vmlinux.h` kernel header from the BTF information. More specifically, our
> > initial approach to have e.g. a compilation target dependency which would
> > invoke `bpftool` at configure time was not appropriate due to several
> > issues: a) CO-RE requires host kernel to have been compiled in such a way to
> > expose BTF information which may not available, and b) the generated

What is "BTF information"?  Google suggests it's something to do with
the British Thyroid Foundation.

Please don't use three letter abbreviations unless they are widely
understood, or if you wish to, please ensure that you explain them.
TLAs otherwise are an exclusion mechanism.

> > `vmlinux.h` was actually architecture-specific.

What is this "vmlinux.h" ?  It isn't something that the kernel provides
afaics.  It doesn't seem to be present on my existing x86 Debian system.
I've seen it on Fedora systems in the dim and distant past.

Where do you think it comes from?  Where are you finding it?
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