[Question] attributes encoding in BTF

[Alexei Starovoitov]

On Thu, Jun 5, 2025 at 12:35 AM Alexis Lothoré
<alexis.lothore at bootlin.com> wrote:
>
> Hi Ihor,
>
> On Wed Jun 4, 2025 at 7:31 PM CEST, Ihor Solodrai wrote:
> > On 6/4/25 2:02 AM, Alexis Lothoré wrote:
>
> [...]
>
> >> Could I be missing something obvious ? Or did I misunderstand the actual
> >> attribute encoding feature ?
> >
> > Hi Alexis.
> >
> > The changes recently landed in pahole and libbpf re attributes had a
> > very narrow goal: passing through particular attributes for some BPF
> > kfuncs from the kernel source to vmlinux.h
> >
> > BTF now has a way of encoding any attribute (as opposed to only bpf
> > type/decl tags) by setting type/decl tag kind flag [1]. So it is
> > possible to represent attributes like packed and aligned in BTF.
> >
> > However, the BTF tags need to be generated by something, in case of
> > vmlinux by pahole. Pahole generates BTF by parsing DWARF. And, as far as
> > I understand, attributes are not (can not be?) represented in DWARF in a
> > generic way, it really depends on specifics of the attribute.
> >
> > In order to support packed/aligned, pahole needs to know how to figure
> > them out from DWARF input and add the tags to BTF. And this does not
> > happen right now, which is why you don't see anything in bpftool output.
> >
> > [1]
> > https://lore.kernel.org/bpf/20250130201239.1429648-1-ihor.solodrai@linux.dev/
>
> Thanks for the details ! I have missed this possibility, as I have been
> assuming that DWARF info was exposing the needed info. I'll take a look at
> it, but if those attributes can not be represented by DWARF, I'll have to
> find another way of getting those packing/alignment modifications on data
> type (eg: re-use/share btf__align_of from libbpf, as suggested by Andrii,
> but it may not able to cover all cases).

Not sure all the trouble is worth it.
I feel it's a corner case. Something we don't need to fix.