linux-kernel - Re: [PATCH bpf-next v3 2/3] bpf: btf: Add BTF_KFUNCS

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Date: Mon, 8 Jan 2024 10:59:53 -0700
From: Daniel Xu <dxu@...uu.xyz>
To: Lorenz Bauer <lorenz.bauer@...valent.com>
Cc: andrii@...nel.org, ast@...nel.org, daniel@...earbox.net, 
	martin.lau@...ux.dev, alexei.starovoitov@...il.com, olsajiri@...il.com, 
	quentin@...valent.com, alan.maguire@...cle.com, memxor@...il.com, song@...nel.org, 
	yonghong.song@...ux.dev, john.fastabend@...il.com, kpsingh@...nel.org, sdf@...gle.com, 
	haoluo@...gle.com, jolsa@...nel.org, bpf@...r.kernel.org, 
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH bpf-next v3 2/3] bpf: btf: Add BTF_KFUNCS_START/END macro
 pair

On Mon, Jan 08, 2024 at 10:14:13AM +0100, Lorenz Bauer wrote:
> On Sat, Jan 6, 2024 at 7:25 PM Daniel Xu <dxu@...uu.xyz> wrote:
> >
> > This macro pair is functionally equivalent to BTF_SET8_START/END, except
> > with BTF_SET8_KFUNCS flag set in the btf_id_set8 flags field. The next
> > commit will codemod all kfunc set8s to this new variant such that all
> > kfuncs are tagged as such in .BTF_ids section.
> >
> > Signed-off-by: Daniel Xu <dxu@...uu.xyz>
> > ---
> >  include/linux/btf_ids.h | 11 +++++++++++
> >  1 file changed, 11 insertions(+)
> >
> > diff --git a/include/linux/btf_ids.h b/include/linux/btf_ids.h
> > index dca09b7f21dc..0fe4f1cd1918 100644
> > --- a/include/linux/btf_ids.h
> > +++ b/include/linux/btf_ids.h
> > @@ -8,6 +8,9 @@ struct btf_id_set {
> >         u32 ids[];
> >  };
> >
> > +/* This flag implies BTF_SET8 holds kfunc(s) */
> > +#define BTF_SET8_KFUNCS                (1 << 0)
> 
> Nit: could this be an enum so that the flag is discoverable via BTF?

Sure, makes sense.

> Also, isn't this UAPI if pahole interprets this flag?

Not sure. I guess it'd fall under same category as any of the structs
the kernel lays out in .BTF_ids, like `struct btf_id_set8`. IMO it's
not, as that's kinda confusing to call anything in ELF uapi. Eg I don't
think people would consider layout of `.data..percpu` section uapi.

Thanks,
Daniel