Re: [RFC PATCH v4 2/7] libbpf: Add BTF permutation support for type reordering

[Eduard Zingerman <eddyz87@gmail.com>]

On Tue, 2025-11-04 at 17:04 -0800, Andrii Nakryiko wrote:
> On Tue, Nov 4, 2025 at 4:16 PM Eduard Zingerman <eddyz87@gmail.com> wrote:
> > 
> > On Tue, 2025-11-04 at 16:11 -0800, Andrii Nakryiko wrote:
> > 
> > [...]
> > 
> > > > +static int btf_permute_remap_type_id(__u32 *type_id, void *ctx)
> > > > +{
> > > > +       struct btf_permute *p = ctx;
> > > > +       __u32 new_type_id = *type_id;
> > > > +
> > > > +       /* skip references that point into the base BTF */
> > > > +       if (new_type_id < p->btf->start_id)
> > > > +               return 0;
> > > > +
> > > > +       new_type_id = p->map[*type_id - p->btf->start_id];
> > > 
> > > I'm actually confused, I thought p->ids would be the mapping from
> > > original type ID (minus start_id, of course) to a new desired ID, but
> > > it looks to be the other way? ids is a desired resulting *sequence* of
> > > types identified by their original ID. I find it quite confusing. I
> > > think about permutation as a mapping from original type ID to a new
> > > type ID, am I confused?
> > 
> > Yes, it is a desired sequence, not mapping.
> > I guess its a bit simpler to use for sorting use-case, as you can just
> > swap ids while sorting.
> 
> The question is really what makes most sense as an interface. Because
> for sorting cases it's just the matter of a two-line for() loop to
> create ID mapping once types are sorted.
> 
> I have slight preference for id_map approach because it is easy to
> extend to the case of selectively dropping some types. We can just
> define that such IDs should be mapped to zero. This will work as a
> natural extension. With the desired end sequence of IDs, it's less
> natural and will require more work to determine which IDs are missing
> from the sequence.
> 
> So unless there is some really good and strong reason, shall we go
> with the ID mapping approach?

If the interface is extended with types_cnt, as you suggest, deleting
types is trivial with sequence interface as well. At-least the way it
is implemented by this patch, you just copy elements from 'ids' one by
one.