Re: [PATCH dwarves v5 2/2] pahole: Inject kfunc decl tags into BTF

[Jiri Olsa <olsajiri@gmail.com>]

On Wed, Mar 20, 2024 at 12:54:11PM +0000, Alan Maguire wrote:
> On 15/03/2024 19:48, Daniel Xu wrote:
> > This commit teaches pahole to parse symbols in .BTF_ids section in
> > vmlinux and discover exported kfuncs. Pahole then takes the list of
> > kfuncs and injects a BTF_KIND_DECL_TAG for each kfunc.
> > 
> > Example of encoding:
> > 
> >         $ bpftool btf dump file .tmp_vmlinux.btf | rg "DECL_TAG 'bpf_kfunc'" | wc -l
> >         121
> > 
> >         $ bpftool btf dump file .tmp_vmlinux.btf | rg 56337
> >         [56337] FUNC 'bpf_ct_change_timeout' type_id=56336 linkage=static
> >         [127861] DECL_TAG 'bpf_kfunc' type_id=56337 component_idx=-1
> > 
> > This enables downstream users and tools to dynamically discover which
> > kfuncs are available on a system by parsing vmlinux or module BTF, both
> > available in /sys/kernel/btf.
> > 
> > This feature is enabled with --btf_features=decl_tag,decl_tag_kfuncs.
> > 
> > Signed-off-by: Daniel Xu <dxu@dxuuu.xyz>
> 
> This is great work; a lot of steps needed to collect this info, but it's
> really valuable!
> 
> Reviewed-by: Alan Maguire <alan.maguire@oracle.com>
> Tested-by: Alan Maguire <alan.maguire@oracle.com>
> 
> BTW we need something like the attached patch to switch to using
> --btf_features for pahole 1.26 and later; will I send it officially or
> do you have something that does the same that you want to roll into your
> bpf-next series? Let me know what works from your side. Thanks!

SNIP

> 
> From 2ca37cfa1e9be4f4d9717fc3dc346cdd7c617e32 Mon Sep 17 00:00:00 2001
> From: Alan Maguire <alan.maguire@oracle.com>
> Date: Mon, 11 Mar 2024 10:17:47 +0000
> Subject: [PATCH bpf-next] kbuild,bpf: switch to using --btf_features for
>  pahole v1.26 and later
> 
> The btf_features list can be used for pahole v1.26 and later -
> it is useful because if a feature is not yet implemented it will
> not exit with a failure message.  This will allow us to add feature
> requests to the pahole options without having to check pahole versions
> in future; if the version of pahole supports the feature it will be
> added.
> 
> Signed-off-by: Alan Maguire <alan.maguire@oracle.com>
> ---
>  scripts/Makefile.btf | 7 +++++--
>  1 file changed, 5 insertions(+), 2 deletions(-)
> 
> diff --git a/scripts/Makefile.btf b/scripts/Makefile.btf
> index 82377e470aed..9694ca3c5252 100644
> --- a/scripts/Makefile.btf
> +++ b/scripts/Makefile.btf
> @@ -12,8 +12,11 @@ pahole-flags-$(call test-ge, $(pahole-ver), 121)	+= --btf_gen_floats
>  
>  pahole-flags-$(call test-ge, $(pahole-ver), 122)	+= -j
>  
> -pahole-flags-$(CONFIG_PAHOLE_HAS_LANG_EXCLUDE)		+= --lang_exclude=rust
> -
>  pahole-flags-$(call test-ge, $(pahole-ver), 125)	+= --skip_encoding_btf_inconsistent_proto --btf_gen_optimized
>  
> +# Switch to using --btf_features for v1.26 and later.
> +pahole-flags-$(call test-ge, $(pahole-ver), 126)	= -j --btf_features=encode_force,var,float,emum64,decl_tag,type_tag,optimized_func,consistent_func

I was testing with '--btf_features=all' but I guess it's better to be specific, lgtm

jirka

> +
> +pahole-flags-$(CONFIG_PAHOLE_HAS_LANG_EXCLUDE)		+= --lang_exclude=rust
> +
>  export PAHOLE_FLAGS := $(pahole-flags-y)
> -- 
> 2.39.3
>