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? [...]
