feature: support to get type information from btf

[dalehamel]

It looks like BTF has been merged as of 4.18 into the linux kernel https://github.com/iovisor/bcc/blob/master/docs/kernel-versions.md

What would it take for bpftrace to be able to use type information from BTF instead of parsing headers? I suppose this support should be added to iovisor/bcc first, and then leverage it from there?

I imagine that a lot of the code in clang_parser would need to be replaced with calls to get the information from btf instead of relying on clang to parse headers for us.

For issues like #329, I think that this would allow for a more reliable struct navigation of the task_struct, and this (should) effectively allow bpftrace to handle structs with #ifdefs on members, where we can otherwise not correctly/reliably calculate field offsets.

Should this issue be filed against bcc instead? Is anyone aware of work going on in this area?

[brendangregg]

Yes, I believe @yonghong-song is working on it for bcc, and once we have it all working on bcc we can adopt it in bpftrace too.

[fbs]

Fixed by #734