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

[Eduard Zingerman <eddyz87@gmail.com>]

On Tue, 2024-05-21 at 10:15 +0100, Alan Maguire wrote:

[...]

> This is a neat approach, and as you say it eliminates the need to modify
> bpftool to handle distilled base BTF and relocation.  The only wrinkle
> is resolve_btfids; we call resolve_btfids for modules with a "-B
> vmlinux" argument, so in that case we'd be calling btf_parse_elf() with
> both a split and base BTF. According to the approach outlined above,
> we'd relocate split BTF - originally relative to .BTF.base - to be
> relative to vmlinux BTF, but in the case of resolve_btfids we don't want
> that relocation. We want the BTF ids to reflect the distilled base BTF
> ids since they will be relocated later on module load along with the
> split BTF references themselves.

You are correct, I missed this detail, resolve_btfids needs distilled
base instead of vmlinux for out of tree modules.

> We can handle this by having a -R flag to skip relocation; it would
> simply ensure we first try calling btf__parse(), falling back to
> btf__parse_split(); we need the fallback logic as it is possible the
> pahole version didn't add .BTF.base sections. This logic would only be
> activated for out-of-tree module builds so seems acceptable to me. If
> that makes sense, with your permission I can rework the series to
> include your BTF parsing patch.

Makes sense to me, but I'm curious whether you and Andrii consider
this a good interface, compared to _opts version.

Thanks,
Eduard