Re: [PATCH v4 bpf-next 00/11] bpf: support resilient split BTF

[Alan Maguire <alan.maguire@oracle.com>]

On 21/05/2024 23:01, Andrii Nakryiko wrote:
> On Tue, May 21, 2024 at 12:08 PM Eduard Zingerman <eddyz87@gmail.com> wrote:
>>
>> On Tue, 2024-05-21 at 11:54 -0700, Andrii Nakryiko wrote:
>>
>> [...]
>>
>>> I'm probably leaning towards not doing automatic relocations in
>>> btf__parse(), tbh. Distilled BTF is a rather special kernel-specific
>>> feature, if we need to teach resolve_btfids and bpftool to do
>>> something extra for that case (i.e., call another API for relocation,
>>> if necessary), then it's fine, doesn't seems like a problem.
>>
>> My point is that with current implementation it does not even make
>> sense to call btf__parse() for an ELF with distilled base,
>> because it would fail.
> 
> True (unless application loaded .BTF.base as stand-alone BTF first,
> but it's pretty advanced scenario)
> 
>>
>> And selecting BTF encoding based on a few retries seems like a kludge
>> if there is a simple way to check if distilled base has to be used
>> (presence of the .BTF.base section).
> 
> agreed
> 
>>
>>> Much worse is having to do some workarounds to prevent an API from
>>> doing some undesired extra steps (like in resolve_btfids not wanting a
>>> relocation). Orthogonality FTW, IMO.
>>
>> For resolve_btfids it is a bit different, imo.
>> It does want some base: for in-tree modules it wants vmlinux,
>> for out-of-tree it wants distilled base.
>> So it has to be adjusted either way.
> 
> Ok, so I read some more code and re-read your discussion w/ Alan. I
> agree with your proposal, I think it's logical (even if relocation
> does feel a bit "extra" for "parse"-like API, but ok, whatever).
> 
> I see what you are saying about resolve_btfids needing the changes
> either way, and that's true. But instead of adding (unnecessary, IMO)
> -R argument, resolve_btfids should be able to detect .BTF.base section
> presence and infer that this is distilled BTF case, and thus proceed
> with ignoring `-B <vmlinux>` argument (we can even complain that `-B
> vmlinux` is specified if distilled BTF is used, not sure.

That's a good idea; we already have ELF parsing in resolve_btfids, so as
we can detect the .BTF.base presence there easily, no need for a
parameter. I put together a quick proof-of-concept and it works well.

This simplifies things considerably; we can eliminate the bpftool
patches completely since ELF parsing does the right thing already, and
the resolve_btfids change is small, with no user interface-visible
changes there.

Thanks!

Alan