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

[Eduard Zingerman <eddyz87@gmail.com>]

On Fri, 2024-05-17 at 11:22 +0100, Alan Maguire wrote:

(Also, please note that CI fails for this series).

[...]

> Also explored Eduard's suggestion of doing an implicit fallback
> to checking for .BTF.base section in btf__parse() when it is
> called to get base BTF.  However while it is doable, it turned
> out to be difficult operationally.  Since fallback is implicit
> we do not know the source of the BTF - was it from .BTF or
> .BTF.base? In bpftool, we want to try first standalone BTF,
> then split, then split with distilled base.  Having a way
> to explicitly request .BTF.base via btf__parse_opts() fits
> that model better.

I don't think this is the case. Here is what I mean:
https://github.com/eddyz87/bpf/tree/distilled-base-alternative-parse-elf

The branch above is a modification for btf_parse_elf() and a few
reverts on top of this patch-set.

I modified btf_parse_elf() to follow the logic below:

| base_btf   | .BTF.base | Effect                                      |
| specified? | present?  |                                             |
|------------+-----------+---------------------------------------------|
| no         | no        | load btf from .BTF                          |
|------------+-----------+---------------------------------------------|
| yes        | no        | load btf from .BTF using base_btf as base   |
|            |           |                                             |
|------------+-----------+---------------------------------------------|
| no         | yes       | load btf from .BTF using .BTF.base as base  |
|            |           |                                             |
|------------+-----------+---------------------------------------------|
| yes        | yes       | load btf from .BTF using .BTF.base as base, |
|            |           | relocate btf against base_btf               |

When organized like that, there is no need to modify libbpf clients to
work with split BTF.

The `bpftool btf dump file ./btf_testmod.ko` would print non-relocated BTF.
The `bpftool btf -B ../../../vmlinux dump file ./btf_testmod.ko` would
print relocated BTF, no need for separate -R flag.
Imo, loading split BTF w/o relocation when .BTF.base is present
is interesting only for debug purposes and could be handled separately
as all building blocks are present in the library.

[...]