Re: [PATCH v8 bpf-next 03/10] libbpf: use kind layout to compute an unknown kind size

[Eduard Zingerman <eddyz87@gmail.com>]

On Tue, 2025-12-16 at 16:30 -0800, Andrii Nakryiko wrote:

[...]

> > > One interesting question is what to do about libbpf's BTF
> > > sanitization? Should we still try to replace unknown types with
> > > something that byte-size-wise is compatible? It might not work in all
> > > cases, depending on the semantics of unknown KIND, but it should work
> > > in practice if we are careful about adding new kinds "responsibly".
> > > WDYT?
> > 
> > The question here is to how to compute the size for the unknown.
> > It is possible to have a flag specifying if btf_type->size is a true
> > size. But computation is more sophisticated for e.g. arrays.
> > On the other hand, if member of some structure has unknown kind,
> > it can be safely deleted, as struct has size field and offsets for all
> > members. So, sanitization by deleting types of unknown kind is
> > possible to some extent.
> 
> I think it's unlikely we'll add some kind that will be directly
> embeddable into struct except for some modifiers. For modifiers (which
> I'm arguing we should add a flag stating that this kind is used as a
> modifier and its type field is actually a type ID field), we can
> replace them either with typedef or const and preserve layout and most
> of semantics. And for optional stuff like decl_tag, they are usually
> stand-alone pointing to types (rather than having types pointed to
> them), so just replacing them with something that is compatible in
> terms of byte size in BTF data should be sufficient.
> 
> I think BTF sanitization will have to be best effort, but if we keep
> sanitization in mind, we can ensure reasonable behavior.
> 
> Ultimately, though, you should always strive to use the very latest
> libbpf with your BPF object files. So maybe no sanitization of unknown
> kinds is the right (and simple) answer here: just update libbpf and it
> will take care of sanitization of *known* kinds.

Idk, this would matter if we decide to change clang to emit new BTF
kinds for BPF programs. Maybe postpone sanitization implementation
until requirements are better understood?