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:
[...]

> Changes since v3[3]:
> 
> - distill now checks for duplicate-named struct/unions and records
>   them as a sized struct/union to help identify which of the
>   multiple base BTF structs/unions it refers to (Eduard, patch 1)

Hi Alan,

Sorry, a little bit more on this topic.
- In patch #1 two kinds of structs get BTF_KIND_STRUCT declaration
  with size: those embedded and those with ambiguous name.
- In patch #7 btf_relocate_map_distilled_base() unconditionally
  requires size field for BTF_KIND_STRUCT to match.

This might hinder portability in the following scenario:
- base type is referred to only by pointer;
- base type has ambiguous name (in the old kernel used for base BTF
  generation);
- base type size is different in the new kernel when module is loaded.

There is also a scenario when type name is not ambiguous in the old
kernel, but becomes ambiguous in the new kernel.

So, what I had in mind when commented in v3 was:
- if distilled base has FWD for a structure and an ambiguous name,
  fail to relocate;
- if distilled base has STRUCT for a structure, find a unique pair
  name/size or fail to relocate.

This covers scenario #1 but ignores scenario #2 and requires minimal
changes for v3 design.

An alternative would be to e.g. keep STRUCT with size for all
structures in the base BTF and to compute "embedded" flag during
relocation:
- if distilled base STRUCT is embedded, search for a unique pair
  name/size or fail to relocate;
- if distilled base STRUCT is not embedded, search for a uniquely
  named struct, if that fails search for a unique pair name/size,
  or fail to relocate.

If we consider above to much of a hassle, I think v3 design + size
check for STRUCT is better because it is a bit simpler.

Wdyt?

[...]