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

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Date: Tue, 6 Feb 2024 08:44:18 -0700
From: Daniel Xu <dxu@...uu.xyz>
To: Jiri Olsa <olsajiri@...il.com>
Cc: 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

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.

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