linux-kernel - Re: [PATCH bpf-next] bpftool: Support dumping kfunc prototypes from BTF

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Date: Mon, 29 Jan 2024 09:13:13 -0700
From: Daniel Xu <dxu@...uu.xyz>
To: Jiri Olsa <olsajiri@...il.com>
Cc: quentin@...valent.com, daniel@...earbox.net, ast@...nel.org, 
	andrii@...nel.org, alexei.starovoitov@...il.com, alan.maguire@...cle.com, 
	memxor@...il.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] bpftool: Support dumping kfunc prototypes from
 BTF

Hi Jiri,

On Mon, Jan 29, 2024 at 01:11:17PM +0100, Jiri Olsa wrote:
> On Sun, Jan 28, 2024 at 06:35:33PM -0700, Daniel Xu wrote:
> > This patch enables dumping kfunc prototypes from bpftool. This is useful
> > b/c with this patch, end users will no longer have to manually define
> > kfunc prototypes. For the kernel tree, this also means we can drop
> > kfunc prototypes from:
> > 
> >         tools/testing/selftests/bpf/bpf_kfuncs.h
> >         tools/testing/selftests/bpf/bpf_experimental.h
> > 
> > Example usage:
> > 
> >         $ make PAHOLE=/home/dxu/dev/pahole/build/pahole -j30 vmlinux
> > 
> >         $ ./tools/bpf/bpftool/bpftool btf dump file ./vmlinux format c | rg "__ksym;" | head -3
> >         extern void cgroup_rstat_updated(struct cgroup * cgrp, int cpu) __ksym;
> >         extern void cgroup_rstat_flush(struct cgroup * cgrp) __ksym;
> >         extern struct bpf_key * bpf_lookup_user_key(u32 serial, u64 flags) __ksym;
> 
> hi,
> I'm getting following declaration for bpf_rbtree_add_impl:
> 
> 	extern int bpf_rbtree_add_impl(struct bpf_rb_root * root, struct bpf_rb_node * node, bool (struct bpf_rb_node *, const struct bpf_rb_node *)* less, void * meta__ign, u64 off) __ksym; 
> 
> and it fails to compile with:
> 
> 	In file included from skeleton/pid_iter.bpf.c:3:
> 	./vmlinux.h:164511:141: error: expected ')'
> 	 164511 | extern int bpf_rbtree_add_impl(struct bpf_rb_root * root, struct bpf_rb_node * node, bool (struct bpf_rb_node *, const struct bpf_rb_node *)* less, void * meta__ign, u64 off) __ksym;
> 		|                                                                                                                                             ^
> 	./vmlinux.h:164511:31: note: to match this '('
> 	 164511 | extern int bpf_rbtree_add_impl(struct bpf_rb_root * root, struct bpf_rb_node * node, bool (struct bpf_rb_node *, const struct bpf_rb_node *)* less, void * meta__ign, u64 off) __ksym;
> 
> looks like the btf_dumper_type_only won't dump function pointer argument
> properly.. I guess we should fix that, but looking at the other stuff in
> vmlinux.h like *_ops struct we can print function pointers properly, so
> perhaps another way around is to use btf_dumper interface instead

Ah, crap, looks like between all the branch switching I didn't build
vmlinux with kfunc annotations. Having fixed that, I can repro this
build failure.

I'll take a look and see what the best way to fix this is.

Given that end to end the whole flow basically works, should we start
working on merging patches?

Thanks,
Daniel

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