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

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Message-ID: <3986a6b863be2ec62820ea5d2cf471f7e233fac0.camel@gmail.com>
Date: Tue, 04 Nov 2025 16:16:21 -0800
From: Eduard Zingerman <eddyz87@...il.com>
To: Andrii Nakryiko <andrii.nakryiko@...il.com>, Donglin Peng
	 <dolinux.peng@...il.com>
Cc: ast@...nel.org, linux-kernel@...r.kernel.org, bpf@...r.kernel.org, Alan
 Maguire <alan.maguire@...cle.com>, Song Liu <song@...nel.org>, pengdonglin
 <pengdonglin@...omi.com>
Subject: Re: [RFC PATCH v4 2/7] libbpf: Add BTF permutation support for type
 reordering

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.