linux-kernel - Re: [PATCH bpf-next v2 1/2] bpf: Have bpf_rdonly

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Message-ID: <CAEf4BzZ-Jxw0r=34Jf5nScW8kaowXh5Kah7hYkZ-o6oqSVGnaw@mail.gmail.com>
Date: Tue, 6 Feb 2024 09:32:02 -0800
From: Andrii Nakryiko <andrii.nakryiko@...il.com>
To: Daniel Xu <dxu@...uu.xyz>
Cc: Jiri Olsa <olsajiri@...il.com>, andrii@...nel.org, daniel@...earbox.net, 
	ast@...nel.org, quentin@...valent.com, alan.maguire@...cle.com, 
	martin.lau@...ux.dev, eddyz87@...il.com, song@...nel.org, 
	yonghong.song@...ux.dev, john.fastabend@...il.com, kpsingh@...nel.org, 
	sdf@...gle.com, haoluo@...gle.com, bpf@...r.kernel.org, 
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH bpf-next v2 1/2] bpf: Have bpf_rdonly_cast() take a const pointer

On Tue, Feb 6, 2024 at 7:44 AM Daniel Xu <dxu@...uu.xyz> wrote:
>
> Hi Jiri,
>
> On Tue, Feb 06, 2024 at 02:42:22PM +0100, Jiri Olsa wrote:
> > On Sun, Feb 04, 2024 at 02:06:34PM -0700, Daniel Xu wrote:
> > > Since 20d59ee55172 ("libbpf: add bpf_core_cast() macro"), libbpf is now
> > > exporting a const arg version of bpf_rdonly_cast(). This causes the
> > > following conflicting type error when generating kfunc prototypes from
> > > BTF:
> > >
> > > In file included from skeleton/pid_iter.bpf.c:5:
> > > /home/dxu/dev/linux/tools/bpf/bpftool/bootstrap/libbpf/include/bpf/bpf_core_read.h:297:14: error: conflicting types for 'bpf_rdonly_cast'
> > > extern void *bpf_rdonly_cast(const void *obj__ign, __u32 btf_id__k) __ksym __weak;
> > >              ^
> > > ./vmlinux.h:135625:14: note: previous declaration is here
> > > extern void *bpf_rdonly_cast(void *obj__ign, u32 btf_id__k) __weak __ksym;
> >
> > hi,
> > I'm hiting more of these when compiling bpf selftests (attached),
> > it looks like some kfuncs declarations in bpf_kfuncs.h might be in conflict
>
> Yep, I was actually going to put that as an office hours topic on how we
> want to handle that for selftests. Marking kfuncs in bpf_kfuncs.h and
> bpf_experimental.h as __weak is an option. ifdef is another option.
> Final option I can think of is bumping required pahole version up and
> simply deleting all the kfunc definitions.

I think we should mark all kfuncs in selftests as __weak. And then
audit all of them and add consts where it makes sense and make sure
that all definitions are compatible between vmlinux.h and manual
declarations. I'd rather avoid #ifdef'ing anything, it should work as
is.


>
> But given that pahole changes come with the feature flag, I don't see
> this as a pressing issue. So I was planning on getting to that after
> current outstanding patchsets (just so there's less stuff for me to
> juggle).
>
> [...]
>
> Thanks,
> Daniel