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

[Donglin Peng]

On Thu, Nov 6, 2025 at 2:23 AM Andrii Nakryiko
<andrii.nakryiko@xxxxxxxxx> wrote:
>
> On Tue, Nov 4, 2025 at 5:20 PM Eduard Zingerman <eddyz87@xxxxxxxxx> wrote:
> >
> > On Tue, 2025-11-04 at 17:04 -0800, Andrii Nakryiko wrote:
> > > On Tue, Nov 4, 2025 at 4:16 PM Eduard Zingerman <eddyz87@xxxxxxxxx> 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.
>
> But it is way less explicit and obvious way to delete element. With ID
> map it is obvious, that type will be mapped to zero. With list of IDs,
> you effectively search for elements that are missing, which IMO is way
> less optimal an interface.
>
> So I still favor the ID map approach.

Hi Andrii,

I've submitted v5 implementing the sequence-based approach, and I plan
to introduce
the ID map approach in v6. However, I have a few remaining questions that need
clarification:

1. ID Map Array Semantics:

   -  When the ID map array specifies `[2] = 4`, does this indicate
that the original type
      at `start_id + 2` should be remapped to position `start_id + 4`?
Should the following
      mapping attempts be rejected:
      a) If the target index `4` exceeds the total number of types (`nr_types`)?
      b) If multiple source types map to the same target location
(e.g., both `[1] = 3`
          and `[2] = 3`)?

   - If [3] = 0, does this indicate that the type at start_id + 3 should
     be dropped?

   - Does this also imply that the VOID type (ID 0) cannot be remapped
     and must always remain unchanged?


2. ID Map Array Size:

   - Must the ID map array size  <=  the number of BTF types? If the array
     is smaller, should any missing types be automatically dropped?