To support more robust split BTF, adding supplemental context for the
base BTF type ids that split BTF refers to is required.  Without such
references, a simple shuffling of base BTF type ids (without any other
significant change) invalidates the split BTF.  Here the attempt is made
to store additional context to make split BTF more robust.

This context comes in the form of distilled base BTF providing minimal
information (name and - in some cases - size) for base INTs, FLOATs,
STRUCTs, UNIONs, ENUMs and ENUM64s along with modified split BTF that
points at that base and contains any additional types needed (such as
TYPEDEF, PTR and anonymous STRUCT/UNION declarations).  This
information constitutes the minimal BTF representation needed to
disambiguate or remove split BTF references to base BTF.  The rules
are as follows:

- INT, FLOAT, FWD are recorded in full.
- if a named base BTF STRUCT or UNION is referred to from split BTF, it
  will be encoded as a zero-member sized STRUCT/UNION (preserving
  size for later relocation checks).  Only base BTF STRUCT/UNIONs
  that are either embedded in split BTF STRUCT/UNIONs or that have
  multiple STRUCT/UNION instances of the same name will _need_ size
  checks at relocation time, but as it is possible a different set of
  types will be duplicates in the later to-be-resolved base BTF,
  we preserve size information for all named STRUCT/UNIONs.
- if an ENUM[64] is named, a ENUM forward representation (an ENUM
  with no values) of the same size is used.
- in all other cases, the type is added to the new split BTF.

Avoiding struct/union/enum/enum64 expansion is important to keep the
distilled base BTF representation to a minimum size.

When successful, new representations of the distilled base BTF and new
split BTF that refers to it are returned.  Both need to be freed by the
caller.

So to take a simple example, with split BTF with a type referring
to "struct sk_buff", we will generate distilled base BTF with a
0-member STRUCT sk_buff of the appropriate size, and the split BTF
will refer to it instead.

Tools like pahole can utilize such split BTF to populate the .BTF
section (split BTF) and an additional .BTF.base section.  Then
when the split BTF is loaded, the distilled base BTF can be used
to relocate split BTF to reference the current (and possibly changed)
base BTF.

So for example if "struct sk_buff" was id 502 when the split BTF was
originally generated,  we can use the distilled base BTF to see that
id 502 refers to a "struct sk_buff" and replace instances of id 502
with the current (relocated) base BTF sk_buff type id.

Distilled base BTF is small; when building a kernel with all modules
using distilled base BTF as a test, overall module size grew by only
5.3Mb total across ~2700 modules.
Signed-off-by: Alan Maguire <alan.maguire@oracle.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Acked-by: Eduard Zingerman <eddyz87@gmail.com>
Link: https://lore.kernel.org/bpf/20240613095014.357981-2-alan.maguire@oracle.com