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

[Jiri Olsa <olsajiri@gmail.com>]

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

jirka

> 
> Note that this patch is only effective after enabling pahole [0]
> and kernel [1] changes are merged.
> 
> [0]: https://lore.kernel.org/bpf/0f25134ec999e368478c4ca993b3b729c2a03383.1706491733.git.dxu@dxuuu.xyz/
> [1]: https://lore.kernel.org/bpf/cover.1706491398.git.dxu@dxuuu.xyz/
> 
> Signed-off-by: Daniel Xu <dxu@dxuuu.xyz>
> ---
>  tools/bpf/bpftool/btf.c | 29 +++++++++++++++++++++++++++++
>  1 file changed, 29 insertions(+)
> 
> diff --git a/tools/bpf/bpftool/btf.c b/tools/bpf/bpftool/btf.c
> index 91fcb75babe3..9ab26ed12733 100644
> --- a/tools/bpf/bpftool/btf.c
> +++ b/tools/bpf/bpftool/btf.c
> @@ -20,6 +20,8 @@
>  #include "json_writer.h"
>  #include "main.h"
>  
> +#define KFUNC_DECL_TAG		"bpf_kfunc"
> +
>  static const char * const btf_kind_str[NR_BTF_KINDS] = {
>  	[BTF_KIND_UNKN]		= "UNKNOWN",
>  	[BTF_KIND_INT]		= "INT",
> @@ -454,6 +456,28 @@ static int dump_btf_raw(const struct btf *btf,
>  	return 0;
>  }
>  
> +static void dump_btf_kfuncs(const struct btf *btf)
> +{
> +	int cnt = btf__type_cnt(btf);
> +	int i;
> +
> +	for (i = 1; i < cnt; i++) {
> +		const struct btf_type *t = btf__type_by_id(btf, i);
> +		char kfunc_sig[1024];
> +		const char *name;
> +
> +		if (!btf_is_decl_tag(t))
> +			continue;
> +
> +		name = btf__name_by_offset(btf, t->name_off);
> +		if (strncmp(name, KFUNC_DECL_TAG, sizeof(KFUNC_DECL_TAG)))
> +			continue;
> +
> +		btf_dumper_type_only(btf, t->type, kfunc_sig, sizeof(kfunc_sig));
> +		printf("extern %s __ksym;\n\n", kfunc_sig);


> +	}
> +}
> +
>  static void __printf(2, 0) btf_dump_printf(void *ctx,
>  					   const char *fmt, va_list args)
>  {
> @@ -476,6 +500,9 @@ static int dump_btf_c(const struct btf *btf,
>  	printf("#ifndef BPF_NO_PRESERVE_ACCESS_INDEX\n");
>  	printf("#pragma clang attribute push (__attribute__((preserve_access_index)), apply_to = record)\n");
>  	printf("#endif\n\n");
> +	printf("#ifndef __ksym\n");
> +	printf("#define __ksym __attribute__((section(\".ksyms\")))\n");
> +	printf("#endif\n\n");
>  
>  	if (root_type_cnt) {
>  		for (i = 0; i < root_type_cnt; i++) {
> @@ -491,6 +518,8 @@ static int dump_btf_c(const struct btf *btf,
>  			if (err)
>  				goto done;
>  		}
> +
> +		dump_btf_kfuncs(btf);
>  	}
>  
>  	printf("#ifndef BPF_NO_PRESERVE_ACCESS_INDEX\n");
> -- 
> 2.42.1
>